Show Process Limits Using /proc Filesystem

Show process limits with /proc filesystem

I think I mentioned the special /proc filesystem before, it’s available in Linux distros and helps you obtain system and process information via normal files created in a special structure. Today I’d like to show you another cool trick /proc has.

Show Process Info Using /proc

Just to remind you, here’s what I mean: on my Red Hat PC I have this sshd daemon process running:

root@redhat:/ # ps -aef | grep [o]penssh
root 5130 1 0 Oct03 ? 00:00:00 /usr/sbin/sshd -D -oCiphers=aes256-gcm@openssh.com,chacha20-poly1305@openssh.com,aes256-ctr,aes256-cbc,aes128-gcm@openssh.com,aes128-ctr,aes128-cbc -oMACs=hmac-sha2-256-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha2-256,hmac-sha1,umac-128@openssh.com,hmac-sha2-512 -oGSSAPIKexAlgorithms=gss-gex-sha1-,gss-group14-sha1- -oKexAlgorithms=curve25519-sha256@libssh.org,ecdh-sha2-nistp256,ecdh-sha2-nistp384,ecdh-sha2-nistp521,diffie-hellman-group-exchange-sha256,diffie-hellman-group14-sha256,diffie-hellman-group16-sha512,diffie-hellman-group18-sha512,diffie-hellman-group-exchange-sha1,diffie-hellman-group14-sha1 -oHostKeyAlgorithms=rsa-sha2-256,ecdsa-sha2-nistp256,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384,ecdsa-sha2-nistp384-cert-v01@openssh.com,rsa-sha2-512,ecdsa-sha2-nistp521,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,ssh-rsa,ssh-rsa-cert-v01@openssh.com -oPubkeyAcceptedKeyTypes=rsa-sha2-256,ecdsa-sha2-nistp256,ecdsa-sha2-nistp256-cert-v01@openssh.com,ecdsa-sha2-nistp384,ecdsa-sha2-nistp384-cert-v01@openssh.com,rsa-sha2-512,ecdsa-sha2-nistp521,ecdsa-sha2-nistp521-cert-v01@openssh.com,ssh-ed25519,ssh-ed25519-cert-v01@openssh.com,ssh-rsa,ssh-rsa-cert-v01@openssh.com

So the sshd process ID (PID) is 5130. That means I can use /proc filesystem to learn quite a bit about the process:

root@redhat:/ # cd /proc/5130
root@redhat:/proc/5130 # ls
 attr        cmdline          environ  io         mem         ns             pagemap      sched      smaps_rollup  syscall        wchan
 autogroup   comm             exe      limits     mountinfo   numa_maps      patch_state  schedstat  stack         task
 auxv        coredump_filter  fd       loginuid   mounts      oom_adj        personality  sessionid  stat          timers
 cgroup      cpuset           fdinfo   map_files  mountstats  oom_score      projid_map   setgroups  statm         timerslack_ns
 clear_refs  cwd              gid_map  maps       net         oom_score_adj  root         smaps      status        uid_map

Each file or directory in this /proc/5130 location shows some information specific to this PID 5130.

For instance, if we list files in the fd directory there, we’ll see all the files and sockets open by sshd at the moment:

root@redhat:/proc/5130 # ls -al fd/*
lr-x------. 1 root root 64 Oct 3 14:10 fd/0 -> /dev/null
lrwx------. 1 root root 64 Oct 3 14:10 fd/1 -> 'socket:[39555]'
lrwx------. 1 root root 64 Oct 3 14:10 fd/2 -> 'socket:[39555]'
lr-x------. 1 root root 64 Oct 3 14:10 fd/3 -> /dev/urandom
lr-x------. 1 root root 64 Oct 3 14:10 fd/4 -> /var/lib/sss/mc/passwd
lrwx------. 1 root root 64 Oct 3 14:10 fd/5 -> 'socket:[45446]'
lrwx------. 1 root root 64 Oct 3 14:10 fd/6 -> 'socket:[45450]'
lr-x------. 1 root root 64 Oct 3 14:10 fd/7 -> /var/lib/sss/mc/group
lrwx------. 1 root root 64 Oct 3 14:10 fd/8 -> 'socket:[45452]'

TODO: I’ll be sure to write a separate post on the /proc filesystem with more thorough walkthrough.

Show Process Limits Using /proc

One of the files in /proc subdirectories is file called limits, and it’s super useful for confirming the current OS limits applied to the process in question.

So for the sshd process with PID 5130, here’s what we can see:

root@redhat:/proc/5130 # cat limits
Limit Soft Limit Hard Limit Units
Max cpu time unlimited unlimited seconds
Max file size unlimited unlimited bytes
Max data size unlimited unlimited bytes
Max stack size 8388608 unlimited bytes
Max core file size unlimited unlimited bytes
Max resident set unlimited unlimited bytes
Max processes 127372 127372 processes
Max open files 1024 4096 files
Max locked memory 16777216 16777216 bytes
Max address space unlimited unlimited bytes
Max file locks unlimited unlimited locks
Max pending signals 127372 127372 signals
Max msgqueue size 819200 819200 bytes
Max nice priority 0 0
Max realtime priority 0 0
Max realtime timeout unlimited unlimited us

Basically, this confirms that I haven’t fine-tuned anything on this new desktop just yet – open files count of 1024 is small cause it’s not a server that requires serving multiple files simultaneously.

Hope you find this useful!

See Also