The /proc Filesystem

/proc/sys	Terrehon Bowden <terrehon@pacbell.net>, Bodo Bauer <bb@ricochet.net>	October 7 1999
2.4.x update	Jorge Nerin <comandante@zaralinux.com>	November 14 2000
move /proc/sys	Shen Feng <shen@cn.fujitsu.com>	April 1 2009
fixes/update part 1.1	Stefani Seibold <stefani@seibold.net>	June 9 2009

Preface

0.1 Introduction/Credits

This documentation is part of a soon (or so we hope) to be released book on the SuSE Linux distribution. As there is no complete documentation for the /proc file system and we’ve used many freely available sources to write these chapters, it seems only fair to give the work back to the Linux community. This work is based on the 2.2.* kernel version and the upcoming 2.4.*. I’m afraid it’s still far from complete, but we hope it will be useful. As far as we know, it is the first ‘all-in-one’ document about the /proc file system. It is focused on the Intel x86 hardware, so if you are looking for PPC, ARM, SPARC, AXP, etc., features, you probably won’t find what you are looking for. It also only covers IPv4 networking, not IPv6 nor other protocols - sorry. But additions and patches are welcome and will be added to this document if you mail them to Bodo.

We’d like to thank Alan Cox, Rik van Riel, and Alexey Kuznetsov and a lot of other people for help compiling this documentation. We’d also like to extend a special thank you to Andi Kleen for documentation, which we relied on heavily to create this document, as well as the additional information he provided. Thanks to everybody else who contributed source or docs to the Linux kernel and helped create a great piece of software… :)

If you have any comments, corrections or additions, please don’t hesitate to contact Bodo Bauer at bb@ricochet.net. We’ll be happy to add them to this document.

The latest version of this document is available online at http://tldp.org/LDP/Linux-Filesystem-Hierarchy/html/proc.html

If the above direction does not works for you, you could try the kernel mailing list at linux-kernel@vger.kernel.org and/or try to reach me at comandante@zaralinux.com.

0.2 Legal Stuff

We don’t guarantee the correctness of this document, and if you come to us complaining about how you screwed up your system because of incorrect documentation, we won’t feel responsible…

Chapter 1: Collecting System Information

In This Chapter

• Investigating the properties of the pseudo file system /proc and its ability to provide information on the running Linux system

• Examining /proc’s structure

• Uncovering various information about the kernel and the processes running on the system

---

The proc file system acts as an interface to internal data structures in the kernel. It can be used to obtain information about the system and to change certain kernel parameters at runtime (sysctl).

First, we’ll take a look at the read-only parts of /proc. In Chapter 2, we show you how you can use /proc/sys to change settings.

1.1 Process-Specific Subdirectories

The directory /proc contains (among other things) one subdirectory for each process running on the system, which is named after the process ID (PID).

The link ‘self’ points to the process reading the file system. Each process subdirectory has the entries listed in Table 1-1.

Note that an open file descriptor to /proc/<pid> or to any of its contained files or subdirectories does not prevent <pid> being reused for some other process in the event that <pid> exits. Operations on open /proc/<pid> file descriptors corresponding to dead processes never act on any new process that the kernel may, through chance, have also assigned the process ID <pid>. Instead, operations on these FDs usually fail with ESRCH.

Table 1-1: Process specific entries in /proc

File	Content
clear_refs	Clears page referenced bits shown in smaps output
cmdline	Command line arguments
cpu	Current and last cpu in which it was executed (2.4)(smp)
cwd	Link to the current working directory
environ	Values of environment variables
exe	Link to the executable of this process
fd	Directory, which contains all file descriptors
maps	Memory maps to executables and library files (2.4)
mem	Memory held by this process
root	Link to the root directory of this process
stat	Process status
statm	Process memory status information
status	Process status in human readable form
wchan	Present with CONFIG_KALLSYMS=y: it shows the kernel function symbol the task is blocked in - or “0” if not blocked.
pagemap	Page table
stack	Report full stack trace, enable via CONFIG_STACKTRACE
smaps	An extension based on maps, showing the memory consumption of each mapping and flags associated with it
smaps_rollup	Accumulated smaps stats for all mappings of the process. This can be derived from smaps, but is faster and more convenient
numa_maps	An extension based on maps, showing the memory locality and binding policy as well as mem usage (in pages) of each mapping.

For example, to get the status information of a process, all you have to do is read the file /proc/PID/status:

>cat /proc/self/status
Name:   cat
State:  R (running)
Tgid:   5452
Pid:    5452
PPid:   743
TracerPid:      0                                             (2.4)
Uid:    501     501     501     501
Gid:    100     100     100     100
FDSize: 256
Groups: 100 14 16
VmPeak:     5004 kB
VmSize:     5004 kB
VmLck:         0 kB
VmHWM:       476 kB
VmRSS:       476 kB
RssAnon:             352 kB
RssFile:             120 kB
RssShmem:              4 kB
VmData:      156 kB
VmStk:        88 kB
VmExe:        68 kB
VmLib:      1412 kB
VmPTE:        20 kb
VmSwap:        0 kB
HugetlbPages:          0 kB
CoreDumping:    0
THP_enabled:    1
Threads:        1
SigQ:   0/28578
SigPnd: 0000000000000000
ShdPnd: 0000000000000000
SigBlk: 0000000000000000
SigIgn: 0000000000000000
SigCgt: 0000000000000000
CapInh: 00000000fffffeff
CapPrm: 0000000000000000
CapEff: 0000000000000000
CapBnd: ffffffffffffffff
CapAmb: 0000000000000000
NoNewPrivs:     0
Seccomp:        0
Speculation_Store_Bypass:       thread vulnerable
SpeculationIndirectBranch:      conditional enabled
voluntary_ctxt_switches:        0
nonvoluntary_ctxt_switches:     1

This shows you nearly the same information you would get if you viewed it with the ps command. In fact, ps uses the proc file system to obtain its information. But you get a more detailed view of the process by reading the file /proc/PID/status. It fields are described in table 1-2.

The statm file contains more detailed information about the process memory usage. Its seven fields are explained in Table 1-3. The stat file contains detailed information about the process itself. Its fields are explained in Table 1-4.

(for SMP CONFIG users)

For making accounting scalable, RSS related information are handled in an asynchronous manner and the value may not be very precise. To see a precise snapshot of a moment, you can see /proc/<pid>/smaps file and scan page table. It’s slow but very precise.

Table 1-2: Contents of the status files (as of 4.19)

Field	Content
Name	filename of the executable
Umask	file mode creation mask
State	state (R is running, S is sleeping, D is sleeping in an uninterruptible wait, Z is zombie, T is traced or stopped)
Tgid	thread group ID
Ngid	NUMA group ID (0 if none)
Pid	process id
PPid	process id of the parent process
TracerPid	PID of process tracing this process (0 if not)
Uid	Real, effective, saved set, and file system UIDs
Gid	Real, effective, saved set, and file system GIDs
FDSize	number of file descriptor slots currently allocated
Groups	supplementary group list
NStgid	descendant namespace thread group ID hierarchy
NSpid	descendant namespace process ID hierarchy
NSpgid	descendant namespace process group ID hierarchy
NSsid	descendant namespace session ID hierarchy
VmPeak	peak virtual memory size
VmSize	total program size
VmLck	locked memory size
VmPin	pinned memory size
VmHWM	peak resident set size (“high water mark”)
VmRSS	size of memory portions. It contains the three following parts (VmRSS = RssAnon + RssFile + RssShmem)
RssAnon	size of resident anonymous memory
RssFile	size of resident file mappings
RssShmem	size of resident shmem memory (includes SysV shm, mapping of tmpfs and shared anonymous mappings)
VmData	size of private data segments
VmStk	size of stack segments
VmExe	size of text segment
VmLib	size of shared library code
VmPTE	size of page table entries
VmSwap	amount of swap used by anonymous private data (shmem swap usage is not included)
HugetlbPages	size of hugetlb memory portions
CoreDumping	process’s memory is currently being dumped (killing the process may lead to a corrupted core)
THP_enabled	process is allowed to use THP (returns 0 when PR_SET_THP_DISABLE is set on the process
Threads	number of threads
SigQ	number of signals queued/max. number for queue
SigPnd	bitmap of pending signals for the thread
ShdPnd	bitmap of shared pending signals for the process
SigBlk	bitmap of blocked signals
SigIgn	bitmap of ignored signals
SigCgt	bitmap of caught signals
CapInh	bitmap of inheritable capabilities
CapPrm	bitmap of permitted capabilities
CapEff	bitmap of effective capabilities
CapBnd	bitmap of capabilities bounding set
CapAmb	bitmap of ambient capabilities
NoNewPrivs	no_new_privs, like prctl(PR_GET_NO_NEW_PRIV, …)
Seccomp	seccomp mode, like prctl(PR_GET_SECCOMP, …)
Speculation_Store_Bypass	speculative store bypass mitigation status
SpeculationIndirectBranch	indirect branch speculation mode
Cpus_allowed	mask of CPUs on which this process may run
Cpus_allowed_list	Same as previous, but in “list format”
Mems_allowed	mask of memory nodes allowed to this process
Mems_allowed_list	Same as previous, but in “list format”
voluntary_ctxt_switches	number of voluntary context switches
nonvoluntary_ctxt_switches	number of non voluntary context switches

Table 1-3: Contents of the statm files (as of 2.6.8-rc3)

Field	Content
size	total program size (pages)	(same as VmSize in status)
resident	size of memory portions (pages)	(same as VmRSS in status)
shared	number of pages that are shared	(i.e. backed by a file, same as RssFile+RssShmem in status)
trs	number of pages that are ‘code’	(not including libs; broken, includes data segment)
lrs	number of pages of library	(always 0 on 2.6)
drs	number of pages of data/stack	(including libs; broken, includes library text)
dt	number of dirty pages	(always 0 on 2.6)

Table 1-4: Contents of the stat files (as of 2.6.30-rc7)

Field	Content
pid	process id
tcomm	filename of the executable
state	state (R is running, S is sleeping, D is sleeping in an uninterruptible wait, Z is zombie, T is traced or stopped)
ppid	process id of the parent process
pgrp	pgrp of the process
sid	session id
tty_nr	tty the process uses
tty_pgrp	pgrp of the tty
flags	task flags
min_flt	number of minor faults
cmin_flt	number of minor faults with child’s
maj_flt	number of major faults
cmaj_flt	number of major faults with child’s
utime	user mode jiffies
stime	kernel mode jiffies
cutime	user mode jiffies with child’s
cstime	kernel mode jiffies with child’s
priority	priority level
nice	nice level
num_threads	number of threads
it_real_value	(obsolete, always 0)
start_time	time the process started after system boot
vsize	virtual memory size
rss	resident set memory size
rsslim	current limit in bytes on the rss
start_code	address above which program text can run
end_code	address below which program text can run
start_stack	address of the start of the main process stack
esp	current value of ESP
eip	current value of EIP
pending	bitmap of pending signals
blocked	bitmap of blocked signals
sigign	bitmap of ignored signals
sigcatch	bitmap of caught signals
0	(place holder, used to be the wchan address, use /proc/PID/wchan instead)
0	(place holder)
0	(place holder)
exit_signal	signal to send to parent thread on exit
task_cpu	which CPU the task is scheduled on
rt_priority	realtime priority
policy	scheduling policy (man sched_setscheduler)
blkio_ticks	time spent waiting for block IO
gtime	guest time of the task in jiffies
cgtime	guest time of the task children in jiffies
start_data	address above which program data+bss is placed
end_data	address below which program data+bss is placed
start_brk	address above which program heap can be expanded with brk()
arg_start	address above which program command line is placed
arg_end	address below which program command line is placed
env_start	address above which program environment is placed
env_end	address below which program environment is placed
exit_code	the thread’s exit_code in the form reported by the waitpid system call

The /proc/PID/maps file contains the currently mapped memory regions and their access permissions.

The format is:

address           perms offset  dev   inode      pathname

08048000-08049000 r-xp 00000000 03:00 8312       /opt/test
08049000-0804a000 rw-p 00001000 03:00 8312       /opt/test
0804a000-0806b000 rw-p 00000000 00:00 0          [heap]
a7cb1000-a7cb2000 ---p 00000000 00:00 0
a7cb2000-a7eb2000 rw-p 00000000 00:00 0
a7eb2000-a7eb3000 ---p 00000000 00:00 0
a7eb3000-a7ed5000 rw-p 00000000 00:00 0
a7ed5000-a8008000 r-xp 00000000 03:00 4222       /lib/libc.so.6
a8008000-a800a000 r--p 00133000 03:00 4222       /lib/libc.so.6
a800a000-a800b000 rw-p 00135000 03:00 4222       /lib/libc.so.6
a800b000-a800e000 rw-p 00000000 00:00 0
a800e000-a8022000 r-xp 00000000 03:00 14462      /lib/libpthread.so.0
a8022000-a8023000 r--p 00013000 03:00 14462      /lib/libpthread.so.0
a8023000-a8024000 rw-p 00014000 03:00 14462      /lib/libpthread.so.0
a8024000-a8027000 rw-p 00000000 00:00 0
a8027000-a8043000 r-xp 00000000 03:00 8317       /lib/ld-linux.so.2
a8043000-a8044000 r--p 0001b000 03:00 8317       /lib/ld-linux.so.2
a8044000-a8045000 rw-p 0001c000 03:00 8317       /lib/ld-linux.so.2
aff35000-aff4a000 rw-p 00000000 00:00 0          [stack]
ffffe000-fffff000 r-xp 00000000 00:00 0          [vdso]

where “address” is the address space in the process that it occupies, “perms” is a set of permissions:

r = read
w = write
x = execute
s = shared
p = private (copy on write)

“offset” is the offset into the mapping, “dev” is the device (major:minor), and “inode” is the inode on that device. 0 indicates that no inode is associated with the memory region, as the case would be with BSS (uninitialized data). The “pathname” shows the name associated file for this mapping. If the mapping is not associated with a file:

[heap]	the heap of the program
[stack]	the stack of the main process
[vdso]	the “virtual dynamic shared object”, the kernel system call handler

or if empty, the mapping is anonymous.

The /proc/PID/smaps is an extension based on maps, showing the memory consumption for each of the process’s mappings. For each mapping (aka Virtual Memory Area, or VMA) there is a series of lines such as the following:

08048000-080bc000 r-xp 00000000 03:02 13130      /bin/bash

Size:               1084 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Rss:                 892 kB
Pss:                 374 kB
Shared_Clean:        892 kB
Shared_Dirty:          0 kB
Private_Clean:         0 kB
Private_Dirty:         0 kB
Referenced:          892 kB
Anonymous:             0 kB
LazyFree:              0 kB
AnonHugePages:         0 kB
ShmemPmdMapped:        0 kB
Shared_Hugetlb:        0 kB
Private_Hugetlb:       0 kB
Swap:                  0 kB
SwapPss:               0 kB
KernelPageSize:        4 kB
MMUPageSize:           4 kB
Locked:                0 kB
THPeligible:           0
VmFlags: rd ex mr mw me dw

The first of these lines shows the same information as is displayed for the mapping in /proc/PID/maps. Following lines show the size of the mapping (size); the size of each page allocated when backing a VMA (KernelPageSize), which is usually the same as the size in the page table entries; the page size used by the MMU when backing a VMA (in most cases, the same as KernelPageSize); the amount of the mapping that is currently resident in RAM (RSS); the process’ proportional share of this mapping (PSS); and the number of clean and dirty shared and private pages in the mapping.

The “proportional set size” (PSS) of a process is the count of pages it has in memory, where each page is divided by the number of processes sharing it. So if a process has 1000 pages all to itself, and 1000 shared with one other process, its PSS will be 1500.

Note that even a page which is part of a MAP_SHARED mapping, but has only a single pte mapped, i.e. is currently used by only one process, is accounted as private and not as shared.

“Referenced” indicates the amount of memory currently marked as referenced or accessed.

“Anonymous” shows the amount of memory that does not belong to any file. Even a mapping associated with a file may contain anonymous pages: when MAP_PRIVATE and a page is modified, the file page is replaced by a private anonymous copy.

“LazyFree” shows the amount of memory which is marked by madvise(MADV_FREE). The memory isn’t freed immediately with madvise(). It’s freed in memory pressure if the memory is clean. Please note that the printed value might be lower than the real value due to optimizations used in the current implementation. If this is not desirable please file a bug report.

“AnonHugePages” shows the ammount of memory backed by transparent hugepage.

“ShmemPmdMapped” shows the ammount of shared (shmem/tmpfs) memory backed by huge pages.

“Shared_Hugetlb” and “Private_Hugetlb” show the ammounts of memory backed by hugetlbfs page which is not counted in “RSS” or “PSS” field for historical reasons. And these are not included in {Shared,Private}_{Clean,Dirty} field.

“Swap” shows how much would-be-anonymous memory is also used, but out on swap.

For shmem mappings, “Swap” includes also the size of the mapped (and not replaced by copy-on-write) part of the underlying shmem object out on swap. “SwapPss” shows proportional swap share of this mapping. Unlike “Swap”, this does not take into account swapped out page of underlying shmem objects. “Locked” indicates whether the mapping is locked in memory or not. “THPeligible” indicates whether the mapping is eligible for allocating THP pages - 1 if true, 0 otherwise. It just shows the current status.

“VmFlags” field deserves a separate description. This member represents the kernel flags associated with the particular virtual memory area in two letter encoded manner. The codes are the following:

rd	readable
wr	writeable
ex	executable
sh	shared
mr	may read
mw	may write
me	may execute
ms	may share
gd	stack segment growns down
pf	pure PFN range
dw	disabled write to the mapped file
lo	pages are locked in memory
io	memory mapped I/O area
sr	sequential read advise provided
rr	random read advise provided
dc	do not copy area on fork
de	do not expand area on remapping
ac	area is accountable
nr	swap space is not reserved for the area
ht	area uses huge tlb pages
sf	synchronous page fault
ar	architecture specific flag
wf	wipe on fork
dd	do not include area into core dump
sd	soft dirty flag
mm	mixed map area
hg	huge page advise flag
nh	no huge page advise flag
mg	mergable advise flag
bt	arm64 BTI guarded page
mt	arm64 MTE allocation tags are enabled
um	userfaultfd missing tracking
uw	userfaultfd wr-protect tracking

Note that there is no guarantee that every flag and associated mnemonic will be present in all further kernel releases. Things get changed, the flags may be vanished or the reverse – new added. Interpretation of their meaning might change in future as well. So each consumer of these flags has to follow each specific kernel version for the exact semantic.

This file is only present if the CONFIG_MMU kernel configuration option is enabled.

Note: reading /proc/PID/maps or /proc/PID/smaps is inherently racy (consistent output can be achieved only in the single read call).

This typically manifests when doing partial reads of these files while the memory map is being modified. Despite the races, we do provide the following guarantees:

1. The mapped addresses never go backwards, which implies no two regions will ever overlap.

2. If there is something at a given vaddr during the entirety of the life of the smaps/maps walk, there will be some output for it.

The /proc/PID/smaps_rollup file includes the same fields as /proc/PID/smaps, but their values are the sums of the corresponding values for all mappings of the process. Additionally, it contains these fields:

• Pss_Anon

• Pss_File

• Pss_Shmem

They represent the proportional shares of anonymous, file, and shmem pages, as described for smaps above. These fields are omitted in smaps since each mapping identifies the type (anon, file, or shmem) of all pages it contains. Thus all information in smaps_rollup can be derived from smaps, but at a significantly higher cost.

The /proc/PID/clear_refs is used to reset the PG_Referenced and ACCESSED/YOUNG bits on both physical and virtual pages associated with a process, and the soft-dirty bit on pte (see Soft-Dirty PTEs for details). To clear the bits for all the pages associated with the process:

> echo 1 > /proc/PID/clear_refs

To clear the bits for the anonymous pages associated with the process:

> echo 2 > /proc/PID/clear_refs

To clear the bits for the file mapped pages associated with the process:

> echo 3 > /proc/PID/clear_refs

To clear the soft-dirty bit:

> echo 4 > /proc/PID/clear_refs

To reset the peak resident set size (“high water mark”) to the process’s current value:

> echo 5 > /proc/PID/clear_refs

Any other value written to /proc/PID/clear_refs will have no effect.

The /proc/pid/pagemap gives the PFN, which can be used to find the pageflags using /proc/kpageflags and number of times a page is mapped using /proc/kpagecount. For detailed explanation, see Examining Process Page Tables.

The /proc/pid/numa_maps is an extension based on maps, showing the memory locality and binding policy, as well as the memory usage (in pages) of each mapping. The output follows a general format where mapping details get summarized separated by blank spaces, one mapping per each file line:

address   policy    mapping details

00400000 default file=/usr/local/bin/app mapped=1 active=0 N3=1 kernelpagesize_kB=4
00600000 default file=/usr/local/bin/app anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206000000 default file=/lib64/ld-2.12.so mapped=26 mapmax=6 N0=24 N3=2 kernelpagesize_kB=4
320621f000 default file=/lib64/ld-2.12.so anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206220000 default file=/lib64/ld-2.12.so anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206221000 default anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206800000 default file=/lib64/libc-2.12.so mapped=59 mapmax=21 active=55 N0=41 N3=18 kernelpagesize_kB=4
320698b000 default file=/lib64/libc-2.12.so
3206b8a000 default file=/lib64/libc-2.12.so anon=2 dirty=2 N3=2 kernelpagesize_kB=4
3206b8e000 default file=/lib64/libc-2.12.so anon=1 dirty=1 N3=1 kernelpagesize_kB=4
3206b8f000 default anon=3 dirty=3 active=1 N3=3 kernelpagesize_kB=4
7f4dc10a2000 default anon=3 dirty=3 N3=3 kernelpagesize_kB=4
7f4dc10b4000 default anon=2 dirty=2 active=1 N3=2 kernelpagesize_kB=4
7f4dc1200000 default file=/anon_hugepage\040(deleted) huge anon=1 dirty=1 N3=1 kernelpagesize_kB=2048
7fff335f0000 default stack anon=3 dirty=3 N3=3 kernelpagesize_kB=4
7fff3369d000 default mapped=1 mapmax=35 active=0 N3=1 kernelpagesize_kB=4

Where:

“address” is the starting address for the mapping;

“policy” reports the NUMA memory policy set for the mapping (see NUMA Memory Policy);

“mapping details” summarizes mapping data such as mapping type, page usage counters, node locality page counters (N0 == node0, N1 == node1, …) and the kernel page size, in KB, that is backing the mapping up.

1.2 Kernel data

Similar to the process entries, the kernel data files give information about the running kernel. The files used to obtain this information are contained in /proc and are listed in Table 1-5. Not all of these will be present in your system. It depends on the kernel configuration and the loaded modules, which files are there, and which are missing.

Table 1-5: Kernel info in /proc

File	Content
apm	Advanced power management info
buddyinfo	Kernel memory allocator information (see text) (2.5)
bus	Directory containing bus specific information
cmdline	Kernel command line
cpuinfo	Info about the CPU
devices	Available devices (block and character)
dma	Used DMS channels
filesystems	Supported filesystems
driver	Various drivers grouped here, currently rtc (2.4)
execdomains	Execdomains, related to security (2.4)
fb	Frame Buffer devices (2.4)
fs	File system parameters, currently nfs/exports (2.4)
ide	Directory containing info about the IDE subsystem
interrupts	Interrupt usage
iomem	Memory map (2.4)
ioports	I/O port usage
irq	Masks for irq to cpu affinity (2.4)(smp?)
isapnp	ISA PnP (Plug&Play) Info (2.4)
kcore	Kernel core image (can be ELF or A.OUT(deprecated in 2.4))
kmsg	Kernel messages
ksyms	Kernel symbol table
loadavg	Load average of last 1, 5 & 15 minutes; number of processes currently runnable (running or on ready queue); total number of processes in system; last pid created. All fields are separated by one space except “number of processes currently runnable” and “total number of processes in system”, which are separated by a slash (‘/’). Example: 0.61 0.61 0.55 3/828 22084
locks	Kernel locks
meminfo	Memory info
misc	Miscellaneous
modules	List of loaded modules
mounts	Mounted filesystems
net	Networking info (see text)
pagetypeinfo	Additional page allocator information (see text) (2.5)
partitions	Table of partitions known to the system
pci	Deprecated info of PCI bus (new way -> /proc/bus/pci/, decoupled by lspci (2.4)
rtc	Real time clock
scsi	SCSI info (see text)
slabinfo	Slab pool info
softirqs	softirq usage
stat	Overall statistics
swaps	Swap space utilization
sys	See chapter 2
sysvipc	Info of SysVIPC Resources (msg, sem, shm) (2.4)
tty	Info of tty drivers
uptime	Wall clock since boot, combined idle time of all cpus
version	Kernel version
video	bttv info of video resources (2.4)
vmallocinfo	Show vmalloced areas

You can, for example, check which interrupts are currently in use and what they are used for by looking in the file /proc/interrupts:

> cat /proc/interrupts
           CPU0
  0:    8728810          XT-PIC  timer
  1:        895          XT-PIC  keyboard
  2:          0          XT-PIC  cascade
  3:     531695          XT-PIC  aha152x
  4:    2014133          XT-PIC  serial
  5:      44401          XT-PIC  pcnet_cs
  8:          2          XT-PIC  rtc
 11:          8          XT-PIC  i82365
 12:     182918          XT-PIC  PS/2 Mouse
 13:          1          XT-PIC  fpu
 14:    1232265          XT-PIC  ide0
 15:          7          XT-PIC  ide1
NMI:          0

In 2.4.* a couple of lines where added to this file LOC & ERR (this time is the output of a SMP machine):

> cat /proc/interrupts

           CPU0       CPU1
  0:    1243498    1214548    IO-APIC-edge  timer
  1:       8949       8958    IO-APIC-edge  keyboard
  2:          0          0          XT-PIC  cascade
  5:      11286      10161    IO-APIC-edge  soundblaster
  8:          1          0    IO-APIC-edge  rtc
  9:      27422      27407    IO-APIC-edge  3c503
 12:     113645     113873    IO-APIC-edge  PS/2 Mouse
 13:          0          0          XT-PIC  fpu
 14:      22491      24012    IO-APIC-edge  ide0
 15:       2183       2415    IO-APIC-edge  ide1
 17:      30564      30414   IO-APIC-level  eth0
 18:        177        164   IO-APIC-level  bttv
NMI:    2457961    2457959
LOC:    2457882    2457881
ERR:       2155

NMI is incremented in this case because every timer interrupt generates a NMI (Non Maskable Interrupt) which is used by the NMI Watchdog to detect lockups.

LOC is the local interrupt counter of the internal APIC of every CPU.

ERR is incremented in the case of errors in the IO-APIC bus (the bus that connects the CPUs in a SMP system. This means that an error has been detected, the IO-APIC automatically retry the transmission, so it should not be a big problem, but you should read the SMP-FAQ.

In 2.6.2* /proc/interrupts was expanded again. This time the goal was for /proc/interrupts to display every IRQ vector in use by the system, not just those considered ‘most important’. The new vectors are:

THR

interrupt raised when a machine check threshold counter (typically counting ECC corrected errors of memory or cache) exceeds a configurable threshold. Only available on some systems.

TRM

a thermal event interrupt occurs when a temperature threshold has been exceeded for the CPU. This interrupt may also be generated when the temperature drops back to normal.

SPU

a spurious interrupt is some interrupt that was raised then lowered by some IO device before it could be fully processed by the APIC. Hence the APIC sees the interrupt but does not know what device it came from. For this case the APIC will generate the interrupt with a IRQ vector of 0xff. This might also be generated by chipset bugs.

RES, CAL, TLB

rescheduling, call and TLB flush interrupts are sent from one CPU to another per the needs of the OS. Typically, their statistics are used by kernel developers and interested users to determine the occurrence of interrupts of the given type.

The above IRQ vectors are displayed only when relevant. For example, the threshold vector does not exist on x86_64 platforms. Others are suppressed when the system is a uniprocessor. As of this writing, only i386 and x86_64 platforms support the new IRQ vector displays.

Of some interest is the introduction of the /proc/irq directory to 2.4. It could be used to set IRQ to CPU affinity. This means that you can “hook” an IRQ to only one CPU, or to exclude a CPU of handling IRQs. The contents of the irq subdir is one subdir for each IRQ, and two files; default_smp_affinity and prof_cpu_mask.

For example:

> ls /proc/irq/
0  10  12  14  16  18  2  4  6  8  prof_cpu_mask
1  11  13  15  17  19  3  5  7  9  default_smp_affinity
> ls /proc/irq/0/
smp_affinity

smp_affinity is a bitmask, in which you can specify which CPUs can handle the IRQ. You can set it by doing:

> echo 1 > /proc/irq/10/smp_affinity

This means that only the first CPU will handle the IRQ, but you can also echo 5 which means that only the first and third CPU can handle the IRQ.

The contents of each smp_affinity file is the same by default:

> cat /proc/irq/0/smp_affinity
ffffffff

There is an alternate interface, smp_affinity_list which allows specifying a CPU range instead of a bitmask:

> cat /proc/irq/0/smp_affinity_list
1024-1031

The default_smp_affinity mask applies to all non-active IRQs, which are the IRQs which have not yet been allocated/activated, and hence which lack a /proc/irq/[0-9]* directory.

The node file on an SMP system shows the node to which the device using the IRQ reports itself as being attached. This hardware locality information does not include information about any possible driver locality preference.

prof_cpu_mask specifies which CPUs are to be profiled by the system wide profiler. Default value is ffffffff (all CPUs if there are only 32 of them).

The way IRQs are routed is handled by the IO-APIC, and it’s Round Robin between all the CPUs which are allowed to handle it. As usual the kernel has more info than you and does a better job than you, so the defaults are the best choice for almost everyone. [Note this applies only to those IO-APIC’s that support “Round Robin” interrupt distribution.]

There are three more important subdirectories in /proc: net, scsi, and sys. The general rule is that the contents, or even the existence of these directories, depend on your kernel configuration. If SCSI is not enabled, the directory scsi may not exist. The same is true with the net, which is there only when networking support is present in the running kernel.

The slabinfo file gives information about memory usage at the slab level. Linux uses slab pools for memory management above page level in version 2.2. Commonly used objects have their own slab pool (such as network buffers, directory cache, and so on).

> cat /proc/buddyinfo

Node 0, zone      DMA      0      4      5      4      4      3 ...
Node 0, zone   Normal      1      0      0      1    101      8 ...
Node 0, zone  HighMem      2      0      0      1      1      0 ...

External fragmentation is a problem under some workloads, and buddyinfo is a useful tool for helping diagnose these problems. Buddyinfo will give you a clue as to how big an area you can safely allocate, or why a previous allocation failed.

Each column represents the number of pages of a certain order which are available. In this case, there are 0 chunks of 2^0*PAGE_SIZE available in ZONE_DMA, 4 chunks of 2^1*PAGE_SIZE in ZONE_DMA, 101 chunks of 2^4*PAGE_SIZE available in ZONE_NORMAL, etc…

More information relevant to external fragmentation can be found in pagetypeinfo:

> cat /proc/pagetypeinfo
Page block order: 9
Pages per block:  512

Free pages count per migrate type at order       0      1      2      3      4      5      6      7      8      9     10
Node    0, zone      DMA, type    Unmovable      0      0      0      1      1      1      1      1      1      1      0
Node    0, zone      DMA, type  Reclaimable      0      0      0      0      0      0      0      0      0      0      0
Node    0, zone      DMA, type      Movable      1      1      2      1      2      1      1      0      1      0      2
Node    0, zone      DMA, type      Reserve      0      0      0      0      0      0      0      0      0      1      0
Node    0, zone      DMA, type      Isolate      0      0      0      0      0      0      0      0      0      0      0
Node    0, zone    DMA32, type    Unmovable    103     54     77      1      1      1     11      8      7      1      9
Node    0, zone    DMA32, type  Reclaimable      0      0      2      1      0      0      0      0      1      0      0
Node    0, zone    DMA32, type      Movable    169    152    113     91     77     54     39     13      6      1    452
Node    0, zone    DMA32, type      Reserve      1      2      2      2      2      0      1      1      1      1      0
Node    0, zone    DMA32, type      Isolate      0      0      0      0      0      0      0      0      0      0      0

Number of blocks type     Unmovable  Reclaimable      Movable      Reserve      Isolate
Node 0, zone      DMA            2            0            5            1            0
Node 0, zone    DMA32           41            6          967            2            0

Fragmentation avoidance in the kernel works by grouping pages of different migrate types into the same contiguous regions of memory called page blocks. A page block is typically the size of the default hugepage size, e.g. 2MB on X86-64. By keeping pages grouped based on their ability to move, the kernel can reclaim pages within a page block to satisfy a high-order allocation.

The pagetypinfo begins with information on the size of a page block. It then gives the same type of information as buddyinfo except broken down by migrate-type and finishes with details on how many page blocks of each type exist.

If min_free_kbytes has been tuned correctly (recommendations made by hugeadm from libhugetlbfs https://github.com/libhugetlbfs/libhugetlbfs/), one can make an estimate of the likely number of huge pages that can be allocated at a given point in time. All the “Movable” blocks should be allocatable unless memory has been mlock()’d. Some of the Reclaimable blocks should also be allocatable although a lot of filesystem metadata may have to be reclaimed to achieve this.

meminfo

Provides information about distribution and utilization of memory. This varies by architecture and compile options. Some of the counters reported here overlap. The memory reported by the non overlapping counters may not add up to the overall memory usage and the difference for some workloads can be substantial. In many cases there are other means to find out additional memory using subsystem specific interfaces, for instance /proc/net/sockstat for TCP memory allocations.

The following is from a 16GB PIII, which has highmem enabled. You may not have all of these fields.

> cat /proc/meminfo

MemTotal:     16344972 kB
MemFree:      13634064 kB
MemAvailable: 14836172 kB
Buffers:          3656 kB
Cached:        1195708 kB
SwapCached:          0 kB
Active:         891636 kB
Inactive:      1077224 kB
HighTotal:    15597528 kB
HighFree:     13629632 kB
LowTotal:       747444 kB
LowFree:          4432 kB
SwapTotal:           0 kB
SwapFree:            0 kB
Dirty:             968 kB
Writeback:           0 kB
AnonPages:      861800 kB
Mapped:         280372 kB
Shmem:             644 kB
KReclaimable:   168048 kB
Slab:           284364 kB
SReclaimable:   159856 kB
SUnreclaim:     124508 kB
PageTables:      24448 kB
NFS_Unstable:        0 kB
Bounce:              0 kB
WritebackTmp:        0 kB
CommitLimit:   7669796 kB
Committed_AS:   100056 kB
VmallocTotal:   112216 kB
VmallocUsed:       428 kB
VmallocChunk:   111088 kB
Percpu:          62080 kB
HardwareCorrupted:   0 kB
AnonHugePages:   49152 kB
ShmemHugePages:      0 kB
ShmemPmdMapped:      0 kB

MemTotal

Total usable RAM (i.e. physical RAM minus a few reserved bits and the kernel binary code)

MemFree

The sum of LowFree+HighFree

MemAvailable

An estimate of how much memory is available for starting new applications, without swapping. Calculated from MemFree, SReclaimable, the size of the file LRU lists, and the low watermarks in each zone. The estimate takes into account that the system needs some page cache to function well, and that not all reclaimable slab will be reclaimable, due to items being in use. The impact of those factors will vary from system to system.

Buffers

Relatively temporary storage for raw disk blocks shouldn’t get tremendously large (20MB or so)

Cached

in-memory cache for files read from the disk (the pagecache). Doesn’t include SwapCached

SwapCached

Memory that once was swapped out, is swapped back in but still also is in the swapfile (if memory is needed it doesn’t need to be swapped out AGAIN because it is already in the swapfile. This saves I/O)

Active

Memory that has been used more recently and usually not reclaimed unless absolutely necessary.

Inactive

Memory which has been less recently used. It is more eligible to be reclaimed for other purposes

HighTotal, HighFree

Highmem is all memory above ~860MB of physical memory. Highmem areas are for use by userspace programs, or for the pagecache. The kernel must use tricks to access this memory, making it slower to access than lowmem.

LowTotal, LowFree

Lowmem is memory which can be used for everything that highmem can be used for, but it is also available for the kernel’s use for its own data structures. Among many other things, it is where everything from the Slab is allocated. Bad things happen when you’re out of lowmem.

SwapTotal

total amount of swap space available

SwapFree

Memory which has been evicted from RAM, and is temporarily on the disk

Dirty

Memory which is waiting to get written back to the disk

Writeback

Memory which is actively being written back to the disk

AnonPages

Non-file backed pages mapped into userspace page tables

HardwareCorrupted

The amount of RAM/memory in KB, the kernel identifies as corrupted.

AnonHugePages

Non-file backed huge pages mapped into userspace page tables

Mapped

files which have been mmaped, such as libraries

Shmem

Total memory used by shared memory (shmem) and tmpfs

ShmemHugePages

Memory used by shared memory (shmem) and tmpfs allocated with huge pages

ShmemPmdMapped

Shared memory mapped into userspace with huge pages

KReclaimable

Kernel allocations that the kernel will attempt to reclaim under memory pressure. Includes SReclaimable (below), and other direct allocations with a shrinker.

Slab

in-kernel data structures cache

SReclaimable

Part of Slab, that might be reclaimed, such as caches

SUnreclaim

Part of Slab, that cannot be reclaimed on memory pressure

PageTables

amount of memory dedicated to the lowest level of page tables.

NFS_Unstable

Always zero. Previous counted pages which had been written to the server, but has not been committed to stable storage.

Bounce

Memory used for block device “bounce buffers”

WritebackTmp

Memory used by FUSE for temporary writeback buffers

CommitLimit

Based on the overcommit ratio (‘vm.overcommit_ratio’), this is the total amount of memory currently available to be allocated on the system. This limit is only adhered to if strict overcommit accounting is enabled (mode 2 in ‘vm.overcommit_memory’).

The CommitLimit is calculated with the following formula:

CommitLimit = ([total RAM pages] - [total huge TLB pages]) *
               overcommit_ratio / 100 + [total swap pages]

For example, on a system with 1G of physical RAM and 7G of swap with a vm.overcommit_ratio of 30 it would yield a CommitLimit of 7.3G.

For more details, see the memory overcommit documentation in vm/overcommit-accounting.

Committed_AS

The amount of memory presently allocated on the system. The committed memory is a sum of all of the memory which has been allocated by processes, even if it has not been “used” by them as of yet. A process which malloc()’s 1G of memory, but only touches 300M of it will show up as using 1G. This 1G is memory which has been “committed” to by the VM and can be used at any time by the allocating application. With strict overcommit enabled on the system (mode 2 in ‘vm.overcommit_memory’), allocations which would exceed the CommitLimit (detailed above) will not be permitted. This is useful if one needs to guarantee that processes will not fail due to lack of memory once that memory has been successfully allocated.

VmallocTotal

total size of vmalloc memory area

VmallocUsed

amount of vmalloc area which is used

VmallocChunk

largest contiguous block of vmalloc area which is free

Percpu

Memory allocated to the percpu allocator used to back percpu allocations. This stat excludes the cost of metadata.

vmallocinfo

Provides information about vmalloced/vmaped areas. One line per area, containing the virtual address range of the area, size in bytes, caller information of the creator, and optional information depending on the kind of area:

pages=nr	number of pages
phys=addr	if a physical address was specified
ioremap	I/O mapping (ioremap() and friends)
vmalloc	vmalloc() area
vmap	vmap()ed pages
user	VM_USERMAP area
vpages	buffer for pages pointers was vmalloced (huge area)
N<node>=nr	(Only on NUMA kernels) Number of pages allocated on memory node <node>

> cat /proc/vmallocinfo
0xffffc20000000000-0xffffc20000201000 2101248 alloc_large_system_hash+0x204 ...
/0x2c0 pages=512 vmalloc N0=128 N1=128 N2=128 N3=128
0xffffc20000201000-0xffffc20000302000 1052672 alloc_large_system_hash+0x204 ...
/0x2c0 pages=256 vmalloc N0=64 N1=64 N2=64 N3=64
0xffffc20000302000-0xffffc20000304000    8192 acpi_tb_verify_table+0x21/0x4f...
phys=7fee8000 ioremap
0xffffc20000304000-0xffffc20000307000   12288 acpi_tb_verify_table+0x21/0x4f...
phys=7fee7000 ioremap
0xffffc2000031d000-0xffffc2000031f000    8192 init_vdso_vars+0x112/0x210
0xffffc2000031f000-0xffffc2000032b000   49152 cramfs_uncompress_init+0x2e ...
/0x80 pages=11 vmalloc N0=3 N1=3 N2=2 N3=3
0xffffc2000033a000-0xffffc2000033d000   12288 sys_swapon+0x640/0xac0      ...
pages=2 vmalloc N1=2
0xffffc20000347000-0xffffc2000034c000   20480 xt_alloc_table_info+0xfe ...
/0x130 [x_tables] pages=4 vmalloc N0=4
0xffffffffa0000000-0xffffffffa000f000   61440 sys_init_module+0xc27/0x1d00 ...
pages=14 vmalloc N2=14
0xffffffffa000f000-0xffffffffa0014000   20480 sys_init_module+0xc27/0x1d00 ...
pages=4 vmalloc N1=4
0xffffffffa0014000-0xffffffffa0017000   12288 sys_init_module+0xc27/0x1d00 ...
pages=2 vmalloc N1=2
0xffffffffa0017000-0xffffffffa0022000   45056 sys_init_module+0xc27/0x1d00 ...
pages=10 vmalloc N0=10

softirqs

Provides counts of softirq handlers serviced since boot time, for each CPU.

> cat /proc/softirqs
              CPU0       CPU1       CPU2       CPU3
    HI:          0          0          0          0
TIMER:       27166      27120      27097      27034
NET_TX:          0          0          0         17
NET_RX:         42          0          0         39
BLOCK:           0          0        107       1121
TASKLET:         0          0          0        290
SCHED:       27035      26983      26971      26746
HRTIMER:         0          0          0          0
    RCU:      1678       1769       2178       2250

1.3 IDE devices in /proc/ide

The subdirectory /proc/ide contains information about all IDE devices of which the kernel is aware. There is one subdirectory for each IDE controller, the file drivers and a link for each IDE device, pointing to the device directory in the controller specific subtree.

The file ‘drivers’ contains general information about the drivers used for the IDE devices:

> cat /proc/ide/drivers
ide-cdrom version 4.53
ide-disk version 1.08

More detailed information can be found in the controller specific subdirectories. These are named ide0, ide1 and so on. Each of these directories contains the files shown in table 1-6.

Table 1-6: IDE controller info in /proc/ide/ide?

File	Content
channel	IDE channel (0 or 1)
config	Configuration (only for PCI/IDE bridge)
mate	Mate name
model	Type/Chipset of IDE controller

Each device connected to a controller has a separate subdirectory in the controllers directory. The files listed in table 1-7 are contained in these directories.

Table 1-7: IDE device information

File	Content
cache	The cache
capacity	Capacity of the medium (in 512Byte blocks)
driver	driver and version
geometry	physical and logical geometry
identify	device identify block
media	media type
model	device identifier
settings	device setup
smart_thresholds	IDE disk management thresholds
smart_values	IDE disk management values

The most interesting file is settings. This file contains a nice overview of the drive parameters:

# cat /proc/ide/ide0/hda/settings
name                    value           min             max             mode
----                    -----           ---             ---             ----
bios_cyl                526             0               65535           rw
bios_head               255             0               255             rw
bios_sect               63              0               63              rw
breada_readahead        4               0               127             rw
bswap                   0               0               1               r
file_readahead          72              0               2097151         rw
io_32bit                0               0               3               rw
keepsettings            0               0               1               rw
max_kb_per_request      122             1               127             rw
multcount               0               0               8               rw
nice1                   1               0               1               rw
nowerr                  0               0               1               rw
pio_mode                write-only      0               255             w
slow                    0               0               1               rw
unmaskirq               0               0               1               rw
using_dma               0               0               1               rw

1.4 Networking info in /proc/net

The subdirectory /proc/net follows the usual pattern. Table 1-8 shows the additional values you get for IP version 6 if you configure the kernel to support this. Table 1-9 lists the files and their meaning.

Table 1-8: IPv6 info in /proc/net

File	Content
udp6	UDP sockets (IPv6)
tcp6	TCP sockets (IPv6)
raw6	Raw device statistics (IPv6)
igmp6	IP multicast addresses, which this host joined (IPv6)
if_inet6	List of IPv6 interface addresses
ipv6_route	Kernel routing table for IPv6
rt6_stats	Global IPv6 routing tables statistics
sockstat6	Socket statistics (IPv6)
snmp6	Snmp data (IPv6)

Table 1-9: Network info in /proc/net

File	Content
arp	Kernel ARP table
dev	network devices with statistics
dev_mcast	the Layer2 multicast groups a device is listening too (interface index, label, number of references, number of bound addresses).
dev_stat	network device status
ip_fwchains	Firewall chain linkage
ip_fwnames	Firewall chain names
ip_masq	Directory containing the masquerading tables
ip_masquerade	Major masquerading table
netstat	Network statistics
raw	raw device statistics
route	Kernel routing table
rpc	Directory containing rpc info
rt_cache	Routing cache
snmp	SNMP data
sockstat	Socket statistics
tcp	TCP sockets
udp	UDP sockets
unix	UNIX domain sockets
wireless	Wireless interface data (Wavelan etc)
igmp	IP multicast addresses, which this host joined
psched	Global packet scheduler parameters.
netlink	List of PF_NETLINK sockets
ip_mr_vifs	List of multicast virtual interfaces
ip_mr_cache	List of multicast routing cache

You can use this information to see which network devices are available in your system and how much traffic was routed over those devices:

> cat /proc/net/dev
Inter-|Receive                                                   |[...
 face |bytes    packets errs drop fifo frame compressed multicast|[...
    lo:  908188   5596     0    0    0     0          0         0 [...
  ppp0:15475140  20721   410    0    0   410          0         0 [...
  eth0:  614530   7085     0    0    0     0          0         1 [...

...] Transmit
...] bytes    packets errs drop fifo colls carrier compressed
...]  908188     5596    0    0    0     0       0          0
...] 1375103    17405    0    0    0     0       0          0
...] 1703981     5535    0    0    0     3       0          0

In addition, each Channel Bond interface has its own directory. For example, the bond0 device will have a directory called /proc/net/bond0/. It will contain information that is specific to that bond, such as the current slaves of the bond, the link status of the slaves, and how many times the slaves link has failed.

1.5 SCSI info

If you have a SCSI host adapter in your system, you’ll find a subdirectory named after the driver for this adapter in /proc/scsi. You’ll also see a list of all recognized SCSI devices in /proc/scsi:

>cat /proc/scsi/scsi
Attached devices:
Host: scsi0 Channel: 00 Id: 00 Lun: 00
  Vendor: IBM      Model: DGHS09U          Rev: 03E0
  Type:   Direct-Access                    ANSI SCSI revision: 03
Host: scsi0 Channel: 00 Id: 06 Lun: 00
  Vendor: PIONEER  Model: CD-ROM DR-U06S   Rev: 1.04
  Type:   CD-ROM                           ANSI SCSI revision: 02

The directory named after the driver has one file for each adapter found in the system. These files contain information about the controller, including the used IRQ and the IO address range. The amount of information shown is dependent on the adapter you use. The example shows the output for an Adaptec AHA-2940 SCSI adapter:

> cat /proc/scsi/aic7xxx/0

Adaptec AIC7xxx driver version: 5.1.19/3.2.4
Compile Options:
  TCQ Enabled By Default : Disabled
  AIC7XXX_PROC_STATS     : Disabled
  AIC7XXX_RESET_DELAY    : 5
Adapter Configuration:
           SCSI Adapter: Adaptec AHA-294X Ultra SCSI host adapter
                           Ultra Wide Controller
    PCI MMAPed I/O Base: 0xeb001000
 Adapter SEEPROM Config: SEEPROM found and used.
      Adaptec SCSI BIOS: Enabled
                    IRQ: 10
                   SCBs: Active 0, Max Active 2,
                         Allocated 15, HW 16, Page 255
             Interrupts: 160328
      BIOS Control Word: 0x18b6
   Adapter Control Word: 0x005b
   Extended Translation: Enabled
Disconnect Enable Flags: 0xffff
     Ultra Enable Flags: 0x0001
 Tag Queue Enable Flags: 0x0000
Ordered Queue Tag Flags: 0x0000
Default Tag Queue Depth: 8
    Tagged Queue By Device array for aic7xxx host instance 0:
      {255,255,255,255,255,255,255,255,255,255,255,255,255,255,255,255}
    Actual queue depth per device for aic7xxx host instance 0:
      {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1}
Statistics:
(scsi0:0:0:0)
  Device using Wide/Sync transfers at 40.0 MByte/sec, offset 8
  Transinfo settings: current(12/8/1/0), goal(12/8/1/0), user(12/15/1/0)
  Total transfers 160151 (74577 reads and 85574 writes)
(scsi0:0:6:0)
  Device using Narrow/Sync transfers at 5.0 MByte/sec, offset 15
  Transinfo settings: current(50/15/0/0), goal(50/15/0/0), user(50/15/0/0)
  Total transfers 0 (0 reads and 0 writes)

1.6 Parallel port info in /proc/parport

The directory /proc/parport contains information about the parallel ports of your system. It has one subdirectory for each port, named after the port number (0,1,2,…).

These directories contain the four files shown in Table 1-10.

Table 1-10: Files in /proc/parport

File	Content
autoprobe	Any IEEE-1284 device ID information that has been acquired.
devices	list of the device drivers using that port. A + will appear by the name of the device currently using the port (it might not appear against any).
hardware	Parallel port’s base address, IRQ line and DMA channel.
irq	IRQ that parport is using for that port. This is in a separate file to allow you to alter it by writing a new value in (IRQ number or none).

1.7 TTY info in /proc/tty

Information about the available and actually used tty’s can be found in the directory /proc/tty. You’ll find entries for drivers and line disciplines in this directory, as shown in Table 1-11.

Table 1-11: Files in /proc/tty

File	Content
drivers	list of drivers and their usage
ldiscs	registered line disciplines
driver/serial	usage statistic and status of single tty lines

To see which tty’s are currently in use, you can simply look into the file /proc/tty/drivers:

> cat /proc/tty/drivers
pty_slave            /dev/pts      136   0-255 pty:slave
pty_master           /dev/ptm      128   0-255 pty:master
pty_slave            /dev/ttyp       3   0-255 pty:slave
pty_master           /dev/pty        2   0-255 pty:master
serial               /dev/cua        5   64-67 serial:callout
serial               /dev/ttyS       4   64-67 serial
/dev/tty0            /dev/tty0       4       0 system:vtmaster
/dev/ptmx            /dev/ptmx       5       2 system
/dev/console         /dev/console    5       1 system:console
/dev/tty             /dev/tty        5       0 system:/dev/tty
unknown              /dev/tty        4    1-63 console

1.8 Miscellaneous kernel statistics in /proc/stat

Various pieces of information about kernel activity are available in the /proc/stat file. All of the numbers reported in this file are aggregates since the system first booted. For a quick look, simply cat the file:

> cat /proc/stat
cpu  2255 34 2290 22625563 6290 127 456 0 0 0
cpu0 1132 34 1441 11311718 3675 127 438 0 0 0
cpu1 1123 0 849 11313845 2614 0 18 0 0 0
intr 114930548 113199788 3 0 5 263 0 4 [... lots more numbers ...]
ctxt 1990473
btime 1062191376
processes 2915
procs_running 1
procs_blocked 0
softirq 183433 0 21755 12 39 1137 231 21459 2263

The very first “cpu” line aggregates the numbers in all of the other “cpuN” lines. These numbers identify the amount of time the CPU has spent performing different kinds of work. Time units are in USER_HZ (typically hundredths of a second). The meanings of the columns are as follows, from left to right:

• user: normal processes executing in user mode

• nice: niced processes executing in user mode

• system: processes executing in kernel mode

• idle: twiddling thumbs

• iowait: In a word, iowait stands for waiting for I/O to complete. But there are several problems:

  1. CPU will not wait for I/O to complete, iowait is the time that a task is waiting for I/O to complete. When CPU goes into idle state for outstanding task I/O, another task will be scheduled on this CPU.

  2. In a multi-core CPU, the task waiting for I/O to complete is not running on any CPU, so the iowait of each CPU is difficult to calculate.

  3. The value of iowait field in /proc/stat will decrease in certain conditions.

  So, the iowait is not reliable by reading from /proc/stat.

• irq: servicing interrupts

• softirq: servicing softirqs

• steal: involuntary wait

• guest: running a normal guest

• guest_nice: running a niced guest

The “intr” line gives counts of interrupts serviced since boot time, for each of the possible system interrupts. The first column is the total of all interrupts serviced including unnumbered architecture specific interrupts; each subsequent column is the total for that particular numbered interrupt. Unnumbered interrupts are not shown, only summed into the total.

The “ctxt” line gives the total number of context switches across all CPUs.

The “btime” line gives the time at which the system booted, in seconds since the Unix epoch.

The “processes” line gives the number of processes and threads created, which includes (but is not limited to) those created by calls to the fork() and clone() system calls.

The “procs_running” line gives the total number of threads that are running or ready to run (i.e., the total number of runnable threads).

The “procs_blocked” line gives the number of processes currently blocked, waiting for I/O to complete.

The “softirq” line gives counts of softirqs serviced since boot time, for each of the possible system softirqs. The first column is the total of all softirqs serviced; each subsequent column is the total for that particular softirq.

1.9 Ext4 file system parameters

Information about mounted ext4 file systems can be found in /proc/fs/ext4. Each mounted filesystem will have a directory in /proc/fs/ext4 based on its device name (i.e., /proc/fs/ext4/hdc or /proc/fs/ext4/dm-0). The files in each per-device directory are shown in Table 1-12, below.

Table 1-12: Files in /proc/fs/ext4/<devname>

File	Content
mb_groups	details of multiblock allocator buddy cache of free blocks

1.10 /proc/consoles

Shows registered system console lines.

To see which character device lines are currently used for the system console /dev/console, you may simply look into the file /proc/consoles:

> cat /proc/consoles
tty0                 -WU (ECp)       4:7
ttyS0                -W- (Ep)        4:64

The columns are:

device	name of the device
operations	R = can do read operations W = can do write operations U = can do unblank
flags	E = it is enabled C = it is preferred console B = it is primary boot console p = it is used for printk buffer b = it is not a TTY but a Braille device a = it is safe to use when cpu is offline
major:minor	major and minor number of the device separated by a colon

Summary

The /proc file system serves information about the running system. It not only allows access to process data but also allows you to request the kernel status by reading files in the hierarchy.

The directory structure of /proc reflects the types of information and makes it easy, if not obvious, where to look for specific data.

Chapter 2: Modifying System Parameters

In This Chapter

• Modifying kernel parameters by writing into files found in /proc/sys

• Exploring the files which modify certain parameters

• Review of the /proc/sys file tree

---

A very interesting part of /proc is the directory /proc/sys. This is not only a source of information, it also allows you to change parameters within the kernel. Be very careful when attempting this. You can optimize your system, but you can also cause it to crash. Never alter kernel parameters on a production system. Set up a development machine and test to make sure that everything works the way you want it to. You may have no alternative but to reboot the machine once an error has been made.

To change a value, simply echo the new value into the file. You need to be root to do this. You can create your own boot script to perform this every time your system boots.

The files in /proc/sys can be used to fine tune and monitor miscellaneous and general things in the operation of the Linux kernel. Since some of the files can inadvertently disrupt your system, it is advisable to read both documentation and source before actually making adjustments. In any case, be very careful when writing to any of these files. The entries in /proc may change slightly between the 2.1.* and the 2.2 kernel, so if there is any doubt review the kernel documentation in the directory /usr/src/linux/Documentation. This chapter is heavily based on the documentation included in the pre 2.2 kernels, and became part of it in version 2.2.1 of the Linux kernel.

Please see: Documentation/admin-guide/sysctl/ directory for descriptions of these entries.

Summary

Certain aspects of kernel behavior can be modified at runtime, without the need to recompile the kernel, or even to reboot the system. The files in the /proc/sys tree can not only be read, but also modified. You can use the echo command to write value into these files, thereby changing the default settings of the kernel.

Chapter 3: Per-process Parameters

3.1 /proc/<pid>/oom_adj & /proc/<pid>/oom_score_adj- Adjust the oom-killer score

These files can be used to adjust the badness heuristic used to select which process gets killed in out of memory (oom) conditions.

The badness heuristic assigns a value to each candidate task ranging from 0 (never kill) to 1000 (always kill) to determine which process is targeted. The units are roughly a proportion along that range of allowed memory the process may allocate from based on an estimation of its current memory and swap use. For example, if a task is using all allowed memory, its badness score will be 1000. If it is using half of its allowed memory, its score will be 500.

The amount of “allowed” memory depends on the context in which the oom killer was called. If it is due to the memory assigned to the allocating task’s cpuset being exhausted, the allowed memory represents the set of mems assigned to that cpuset. If it is due to a mempolicy’s node(s) being exhausted, the allowed memory represents the set of mempolicy nodes. If it is due to a memory limit (or swap limit) being reached, the allowed memory is that configured limit. Finally, if it is due to the entire system being out of memory, the allowed memory represents all allocatable resources.

The value of /proc/<pid>/oom_score_adj is added to the badness score before it is used to determine which task to kill. Acceptable values range from -1000 (OOM_SCORE_ADJ_MIN) to +1000 (OOM_SCORE_ADJ_MAX). This allows userspace to polarize the preference for oom killing either by always preferring a certain task or completely disabling it. The lowest possible value, -1000, is equivalent to disabling oom killing entirely for that task since it will always report a badness score of 0.

Consequently, it is very simple for userspace to define the amount of memory to consider for each task. Setting a /proc/<pid>/oom_score_adj value of +500, for example, is roughly equivalent to allowing the remainder of tasks sharing the same system, cpuset, mempolicy, or memory controller resources to use at least 50% more memory. A value of -500, on the other hand, would be roughly equivalent to discounting 50% of the task’s allowed memory from being considered as scoring against the task.

For backwards compatibility with previous kernels, /proc/<pid>/oom_adj may also be used to tune the badness score. Its acceptable values range from -16 (OOM_ADJUST_MIN) to +15 (OOM_ADJUST_MAX) and a special value of -17 (OOM_DISABLE) to disable oom killing entirely for that task. Its value is scaled linearly with /proc/<pid>/oom_score_adj.

The value of /proc/<pid>/oom_score_adj may be reduced no lower than the last value set by a CAP_SYS_RESOURCE process. To reduce the value any lower requires CAP_SYS_RESOURCE.

3.2 /proc/<pid>/oom_score - Display current oom-killer score

This file can be used to check the current score used by the oom-killer for any given <pid>. Use it together with /proc/<pid>/oom_score_adj to tune which process should be killed in an out-of-memory situation.

Please note that the exported value includes oom_score_adj so it is effectively in range [0,2000].

3.3 /proc/<pid>/io - Display the IO accounting fields

This file contains IO statistics for each running process.

Example

test:/tmp # dd if=/dev/zero of=/tmp/test.dat &
[1] 3828

test:/tmp # cat /proc/3828/io
rchar: 323934931
wchar: 323929600
syscr: 632687
syscw: 632675
read_bytes: 0
write_bytes: 323932160
cancelled_write_bytes: 0

Description

rchar

I/O counter: chars read The number of bytes which this task has caused to be read from storage. This is simply the sum of bytes which this process passed to read() and pread(). It includes things like tty IO and it is unaffected by whether or not actual physical disk IO was required (the read might have been satisfied from pagecache).

wchar

I/O counter: chars written The number of bytes which this task has caused, or shall cause to be written to disk. Similar caveats apply here as with rchar.

syscr

I/O counter: read syscalls Attempt to count the number of read I/O operations, i.e. syscalls like read() and pread().

syscw

I/O counter: write syscalls Attempt to count the number of write I/O operations, i.e. syscalls like write() and pwrite().

read_bytes

I/O counter: bytes read Attempt to count the number of bytes which this process really did cause to be fetched from the storage layer. Done at the submit_bio() level, so it is accurate for block-backed filesystems. <please add status regarding NFS and CIFS at a later time>

write_bytes

I/O counter: bytes written Attempt to count the number of bytes which this process caused to be sent to the storage layer. This is done at page-dirtying time.

cancelled_write_bytes

The big inaccuracy here is truncate. If a process writes 1MB to a file and then deletes the file, it will in fact perform no writeout. But it will have been accounted as having caused 1MB of write. In other words: The number of bytes which this process caused to not happen, by truncating pagecache. A task can cause “negative” IO too. If this task truncates some dirty pagecache, some IO which another task has been accounted for (in its write_bytes) will not be happening. We _could_ just subtract that from the truncating task’s write_bytes, but there is information loss in doing that.

Note

At its current implementation state, this is a bit racy on 32-bit machines: if process A reads process B’s /proc/pid/io while process B is updating one of those 64-bit counters, process A could see an intermediate result.

More information about this can be found within the taskstats documentation in Documentation/accounting.

3.4 /proc/<pid>/coredump_filter - Core dump filtering settings

When a process is dumped, all anonymous memory is written to a core file as long as the size of the core file isn’t limited. But sometimes we don’t want to dump some memory segments, for example, huge shared memory or DAX. Conversely, sometimes we want to save file-backed memory segments into a core file, not only the individual files.

/proc/<pid>/coredump_filter allows you to customize which memory segments will be dumped when the <pid> process is dumped. coredump_filter is a bitmask of memory types. If a bit of the bitmask is set, memory segments of the corresponding memory type are dumped, otherwise they are not dumped.

The following 9 memory types are supported:

• (bit 0) anonymous private memory

• (bit 1) anonymous shared memory

• (bit 2) file-backed private memory

• (bit 3) file-backed shared memory

• (bit 4) ELF header pages in file-backed private memory areas (it is effective only if the bit 2 is cleared)

• (bit 5) hugetlb private memory

• (bit 6) hugetlb shared memory

• (bit 7) DAX private memory

• (bit 8) DAX shared memory

Note that MMIO pages such as frame buffer are never dumped and vDSO pages are always dumped regardless of the bitmask status.

Note that bits 0-4 don’t affect hugetlb or DAX memory. hugetlb memory is only affected by bit 5-6, and DAX is only affected by bits 7-8.

The default value of coredump_filter is 0x33; this means all anonymous memory segments, ELF header pages and hugetlb private memory are dumped.

If you don’t want to dump all shared memory segments attached to pid 1234, write 0x31 to the process’s proc file:

$ echo 0x31 > /proc/1234/coredump_filter

When a new process is created, the process inherits the bitmask status from its parent. It is useful to set up coredump_filter before the program runs. For example:

$ echo 0x7 > /proc/self/coredump_filter
$ ./some_program

3.5 /proc/<pid>/mountinfo - Information about mounts

This file contains lines of the form:

36 35 98:0 /mnt1 /mnt2 rw,noatime master:1 - ext3 /dev/root rw,errors=continue
(1)(2)(3)   (4)   (5)      (6)     (n…m) (m+1)(m+2) (m+3)         (m+4)

(1)   mount ID:        unique identifier of the mount (may be reused after umount)
(2)   parent ID:       ID of parent (or of self for the top of the mount tree)
(3)   major:minor:     value of st_dev for files on filesystem
(4)   root:            root of the mount within the filesystem
(5)   mount point:     mount point relative to the process's root
(6)   mount options:   per mount options
(n…m) optional fields: zero or more fields of the form "tag[:value]"
(m+1) separator:       marks the end of the optional fields
(m+2) filesystem type: name of filesystem of the form "type[.subtype]"
(m+3) mount source:    filesystem specific information or "none"
(m+4) super options:   per super block options

Parsers should ignore all unrecognised optional fields. Currently the possible optional fields are:

shared:X	mount is shared in peer group X
master:X	mount is slave to peer group X
propagate_from:X	mount is slave and receives propagation from peer group X 1
unbindable	mount is unbindable

1

X is the closest dominant peer group under the process’s root. If X is the immediate master of the mount, or if there’s no dominant peer group under the same root, then only the “master:X” field is present and not the “propagate_from:X” field.

For more information on mount propagation see:

Shared Subtrees

3.6 /proc/<pid>/comm & /proc/<pid>/task/<tid>/comm

These files provide a method to access a task’s comm value. It also allows for a task to set its own or one of its thread siblings comm value. The comm value is limited in size compared to the cmdline value, so writing anything longer then the kernel’s TASK_COMM_LEN (currently 16 chars) will result in a truncated comm value.

3.7 /proc/<pid>/task/<tid>/children - Information about task children

This file provides a fast way to retrieve first level children pids of a task pointed by <pid>/<tid> pair. The format is a space separated stream of pids.

Note the “first level” here – if a child has its own children they will not be listed here; one needs to read /proc/<children-pid>/task/<tid>/children to obtain the descendants.

Since this interface is intended to be fast and cheap it doesn’t guarantee to provide precise results and some children might be skipped, especially if they’ve exited right after we printed their pids, so one needs to either stop or freeze processes being inspected if precise results are needed.

3.8 /proc/<pid>/fdinfo/<fd> - Information about opened file

This file provides information associated with an opened file. The regular files have at least four fields – ‘pos’, ‘flags’, ‘mnt_id’ and ‘ino’. The ‘pos’ represents the current offset of the opened file in decimal form [see lseek(2) for details], ‘flags’ denotes the octal O_xxx mask the file has been created with [see open(2) for details] and ‘mnt_id’ represents mount ID of the file system containing the opened file [see 3.5 /proc/<pid>/mountinfo for details]. ‘ino’ represents the inode number of the file.

A typical output is:

pos:    0
flags:  0100002
mnt_id: 19
ino:    63107

All locks associated with a file descriptor are shown in its fdinfo too:

lock:       1: FLOCK  ADVISORY  WRITE 359 00:13:11691 0 EOF

The files such as eventfd, fsnotify, signalfd, epoll among the regular pos/flags pair provide additional information particular to the objects they represent.

Eventfd files

pos:    0
flags:  04002
mnt_id: 9
ino:    63107
eventfd-count:  5a

where ‘eventfd-count’ is hex value of a counter.

Signalfd files

pos:    0
flags:  04002
mnt_id: 9
ino:    63107
sigmask:        0000000000000200

where ‘sigmask’ is hex value of the signal mask associated with a file.

Epoll files

pos:    0
flags:  02
mnt_id: 9
ino:    63107
tfd:        5 events:       1d data: ffffffffffffffff pos:0 ino:61af sdev:7

where ‘tfd’ is a target file descriptor number in decimal form, ‘events’ is events mask being watched and the ‘data’ is data associated with a target [see epoll(7) for more details].

The ‘pos’ is current offset of the target file in decimal form [see lseek(2)], ‘ino’ and ‘sdev’ are inode and device numbers where target file resides, all in hex format.

Fsnotify files

For inotify files the format is the following:

pos:    0
flags:  02000000
mnt_id: 9
ino:    63107
inotify wd:3 ino:9e7e sdev:800013 mask:800afce ignored_mask:0 fhandle-bytes:8 fhandle-type:1 f_handle:7e9e0000640d1b6d

where ‘wd’ is a watch descriptor in decimal form, i.e. a target file descriptor number, ‘ino’ and ‘sdev’ are inode and device where the target file resides and the ‘mask’ is the mask of events, all in hex form [see inotify(7) for more details].

If the kernel was built with exportfs support, the path to the target file is encoded as a file handle. The file handle is provided by three fields ‘fhandle-bytes’, ‘fhandle-type’ and ‘f_handle’, all in hex format.

If the kernel is built without exportfs support the file handle won’t be printed out.

If there is no inotify mark attached yet the ‘inotify’ line will be omitted.

For fanotify files the format is:

pos:    0
flags:  02
mnt_id: 9
ino:    63107
fanotify flags:10 event-flags:0
fanotify mnt_id:12 mflags:40 mask:38 ignored_mask:40000003
fanotify ino:4f969 sdev:800013 mflags:0 mask:3b ignored_mask:40000000 fhandle-bytes:8 fhandle-type:1 f_handle:69f90400c275b5b4

where fanotify ‘flags’ and ‘event-flags’ are values used in fanotify_init call, ‘mnt_id’ is the mount point identifier, ‘mflags’ is the value of flags associated with mark which are tracked separately from events mask. ‘ino’ and ‘sdev’ are target inode and device, ‘mask’ is the events mask and ‘ignored_mask’ is the mask of events which are to be ignored. All are in hex format. Incorporation of ‘mflags’, ‘mask’ and ‘ignored_mask’ provide information about flags and mask used in fanotify_mark call [see fsnotify manpage for details].

While the first three lines are mandatory and always printed, the rest is optional and may be omitted if no marks created yet.

Timerfd files

pos:    0
flags:  02
mnt_id: 9
ino:    63107
clockid: 0
ticks: 0
settime flags: 01
it_value: (0, 49406829)
it_interval: (1, 0)

where ‘clockid’ is the clock type and ‘ticks’ is the number of the timer expirations that have occurred [see timerfd_create(2) for details]. ‘settime flags’ are flags in octal form been used to setup the timer [see timerfd_settime(2) for details]. ‘it_value’ is remaining time until the timer expiration. ‘it_interval’ is the interval for the timer. Note the timer might be set up with TIMER_ABSTIME option which will be shown in ‘settime flags’, but ‘it_value’ still exhibits timer’s remaining time.

DMA Buffer files

pos:    0
flags:  04002
mnt_id: 9
ino:    63107
size:   32768
count:  2
exp_name:  system-heap

where ‘size’ is the size of the DMA buffer in bytes. ‘count’ is the file count of the DMA buffer file. ‘exp_name’ is the name of the DMA buffer exporter.

3.9 /proc/<pid>/map_files - Information about memory mapped files

This directory contains symbolic links which represent memory mapped files the process is maintaining. Example output:

| lr-------- 1 root root 64 Jan 27 11:24 333c600000-333c620000 -> /usr/lib64/ld-2.18.so
| lr-------- 1 root root 64 Jan 27 11:24 333c81f000-333c820000 -> /usr/lib64/ld-2.18.so
| lr-------- 1 root root 64 Jan 27 11:24 333c820000-333c821000 -> /usr/lib64/ld-2.18.so
| ...
| lr-------- 1 root root 64 Jan 27 11:24 35d0421000-35d0422000 -> /usr/lib64/libselinux.so.1
| lr-------- 1 root root 64 Jan 27 11:24 400000-41a000 -> /usr/bin/ls

The name of a link represents the virtual memory bounds of a mapping, i.e. vm_area_struct::vm_start-vm_area_struct::vm_end.

The main purpose of the map_files is to retrieve a set of memory mapped files in a fast way instead of parsing /proc/<pid>/maps or /proc/<pid>/smaps, both of which contain many more records. At the same time one can open(2) mappings from the listings of two processes and comparing their inode numbers to figure out which anonymous memory areas are actually shared.

3.10 /proc/<pid>/timerslack_ns - Task timerslack value

This file provides the value of the task’s timerslack value in nanoseconds. This value specifies an amount of time that normal timers may be deferred in order to coalesce timers and avoid unnecessary wakeups.

This allows a task’s interactivity vs power consumption tradeoff to be adjusted.

Writing 0 to the file will set the task’s timerslack to the default value.

Valid values are from 0 - ULLONG_MAX

An application setting the value must have PTRACE_MODE_ATTACH_FSCREDS level permissions on the task specified to change its timerslack_ns value.

3.11 /proc/<pid>/patch_state - Livepatch patch operation state

When CONFIG_LIVEPATCH is enabled, this file displays the value of the patch state for the task.

A value of ‘-1’ indicates that no patch is in transition.

A value of ‘0’ indicates that a patch is in transition and the task is unpatched. If the patch is being enabled, then the task hasn’t been patched yet. If the patch is being disabled, then the task has already been unpatched.

A value of ‘1’ indicates that a patch is in transition and the task is patched. If the patch is being enabled, then the task has already been patched. If the patch is being disabled, then the task hasn’t been unpatched yet.

3.12 /proc/<pid>/arch_status - task architecture specific status

When CONFIG_PROC_PID_ARCH_STATUS is enabled, this file displays the architecture specific status of the task.

Example

$ cat /proc/6753/arch_status
AVX512_elapsed_ms:      8

Description

x86 specific entries

AVX512_elapsed_ms

If AVX512 is supported on the machine, this entry shows the milliseconds elapsed since the last time AVX512 usage was recorded. The recording happens on a best effort basis when a task is scheduled out. This means that the value depends on two factors:

1. The time which the task spent on the CPU without being scheduled out. With CPU isolation and a single runnable task this can take several seconds.

2. The time since the task was scheduled out last. Depending on the reason for being scheduled out (time slice exhausted, syscall …) this can be arbitrary long time.

As a consequence the value cannot be considered precise and authoritative information. The application which uses this information has to be aware of the overall scenario on the system in order to determine whether a task is a real AVX512 user or not. Precise information can be obtained with performance counters.

A special value of ‘-1’ indicates that no AVX512 usage was recorded, thus the task is unlikely an AVX512 user, but depends on the workload and the scheduling scenario, it also could be a false negative mentioned above.

Chapter 4: Configuring procfs

4.1 Mount options

The following mount options are supported:

hidepid=	Set /proc/<pid>/ access mode.
gid=	Set the group authorized to learn processes information.
subset=	Show only the specified subset of procfs.

hidepid=off or hidepid=0 means classic mode - everybody may access all /proc/<pid>/ directories (default).

hidepid=noaccess or hidepid=1 means users may not access any /proc/<pid>/ directories but their own. Sensitive files like cmdline, sched*, status are now protected against other users. This makes it impossible to learn whether any user runs specific program (given the program doesn’t reveal itself by its behaviour). As an additional bonus, as /proc/<pid>/cmdline is unaccessible for other users, poorly written programs passing sensitive information via program arguments are now protected against local eavesdroppers.

hidepid=invisible or hidepid=2 means hidepid=1 plus all /proc/<pid>/ will be fully invisible to other users. It doesn’t mean that it hides a fact whether a process with a specific pid value exists (it can be learned by other means, e.g. by “kill -0 $PID”), but it hides process’ uid and gid, which may be learned by stat()’ing /proc/<pid>/ otherwise. It greatly complicates an intruder’s task of gathering information about running processes, whether some daemon runs with elevated privileges, whether other user runs some sensitive program, whether other users run any program at all, etc.

hidepid=ptraceable or hidepid=4 means that procfs should only contain /proc/<pid>/ directories that the caller can ptrace.

gid= defines a group authorized to learn processes information otherwise prohibited by hidepid=. If you use some daemon like identd which needs to learn information about processes information, just add identd to this group.

subset=pid hides all top level files and directories in the procfs that are not related to tasks.

Chapter 5: Filesystem behavior

Originally, before the advent of pid namepsace, procfs was a global file system. It means that there was only one procfs instance in the system.

When pid namespace was added, a separate procfs instance was mounted in each pid namespace. So, procfs mount options are global among all mountpoints within the same namespace:

# grep ^proc /proc/mounts
proc /proc proc rw,relatime,hidepid=2 0 0

# strace -e mount mount -o hidepid=1 -t proc proc /tmp/proc
mount("proc", "/tmp/proc", "proc", 0, "hidepid=1") = 0
+++ exited with 0 +++

# grep ^proc /proc/mounts
proc /proc proc rw,relatime,hidepid=2 0 0
proc /tmp/proc proc rw,relatime,hidepid=2 0 0

and only after remounting procfs mount options will change at all mountpoints:

# mount -o remount,hidepid=1 -t proc proc /tmp/proc

# grep ^proc /proc/mounts
proc /proc proc rw,relatime,hidepid=1 0 0
proc /tmp/proc proc rw,relatime,hidepid=1 0 0

This behavior is different from the behavior of other filesystems.

The new procfs behavior is more like other filesystems. Each procfs mount creates a new procfs instance. Mount options affect own procfs instance. It means that it became possible to have several procfs instances displaying tasks with different filtering options in one pid namespace:

# mount -o hidepid=invisible -t proc proc /proc
# mount -o hidepid=noaccess -t proc proc /tmp/proc
# grep ^proc /proc/mounts
proc /proc proc rw,relatime,hidepid=invisible 0 0
proc /tmp/proc proc rw,relatime,hidepid=noaccess 0 0