How can I find the correct ulimit values for a user account or process on Linux systems?
For proper operation, we must ensure that the correct ulimit values set after installing various software. The Linux system provides means of restricting the number of resources that can be used. Limits set for each Linux user account. However, system limits are applied separately to each process that is running for that user too. For example, if certain thresholds are too low, the system might not be able to server web pages using Nginx/Apache or PHP/Python app. System resource limits viewed or set with the NA command. Let us see how to use the ulimit that provides control over the resources available to the shell and processes.
How to find ulimit for user on Linux
The ulimit Linux command sets or displays user process resource limits. Typically, limits are defined in /etc/security/limits.conf file or systemd units.
Two types of limits
All Linux limits are categorized as either soft or hard:
- Soft limit – All users can change soft limits, up to max set by the hard limits. Pass the -S option to the ulimit.
- Hard limit – Only root users allowed to change esource hard limits. Pass the -H option to the ulimit.
Viewing ulimit for Linux user account
The syntax is as follows to view all soft and hard limits for the current user:ulimit -Sa ## Show soft limit ##
ulimit -Ha ## Show hard limit ##
core file size (blocks, -c) unlimited data seg size (kbytes, -d) unlimited scheduling priority (-e) 0 file size (blocks, -f) unlimited pending signals (-i) 126787 max locked memory (kbytes, -l) 65536 max memory size (kbytes, -m) unlimited open files (-n) 1048576 pipe size (512 bytes, -p) 8 POSIX message queues (bytes, -q) 819200 real-time priority (-r) 0 stack size (kbytes, -s) unlimited cpu time (seconds, -t) unlimited max user processes (-u) 126787 virtual memory (kbytes, -v) unlimited file locks (-x) unlimited
List all hard ulimit for user named ‘tom’
You must run the following command as root user or at least have access to that account via sudo/su:su - tom -c "ulimit -Ha"
su - tom --shell /bin/bash -c "ulimit -Ha"
## You can use the sudo command ##
sudo -u tom bash -c "ulimit -Ha"
sudo -u tom sh -c "ulimit -Ha"
Find all soft ulimit for user named ‘jerry’
Again, run it as root either using the su command or sudo command:su - jerry -c "ulimit -Sa"
su - jerry --shell /bin/sh -c "ulimit -Sa"
## You can use the sudo command ##
sudo -u jerry bash -c "ulimit -Sa"
sudo -u jerry sh -c "ulimit -Sa"
Related tip: How to run multiple commands in sudo under Linux or Unix
Finding ‘ulimit -a’ for process user
A Linux process is nothing but a running instance of a program. For example, when you start the Firefox app, you created a process. However, some process runs in the background for a longer time. Typically server process works in the background, and you can not use the sudo command or su command to find their limits. In this example, nginx is running as www-data user on Debian Linux, but shell access to www-data user account blocked by default for security reasons. In other words, the following su or sudo command would fail 100%:$ su - www-data -c "ulimit -Sa"
This account is currently not available.
How to find ulimit for a process
The syntax is:cat /proc/PID/limits
First find PID (process ID) for nginx, run ps command along with the grep command:ps aux | grep nginx
Sample outputs:
root 8868 0.0 0.0 127044 24048 ? Ss May23 0:00 nginx: master process /usr/sbin/nginx -g daemon on; master_process on; www-data 18074 0.0 0.0 127276 24716 ? S May23 0:09 nginx: worker process www-data 18075 0.0 0.0 127276 22284 ? S May23 0:00 nginx: worker process
Now run the following cat command:cat /proc/8868/limits
Sample outputs:
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 0 unlimited bytes
Max resident set unlimited unlimited bytes
Max processes unlimited unlimited processes
Max open files 1024 1048576 files
Max locked memory 65536 65536 bytes
Max address space unlimited unlimited bytes
Max file locks unlimited unlimited locks
Max pending signals 385944 385944 signals
Max msgqueue size 819200 819200 bytes
Max nice priority 0 0
Max realtime priority 0 0
Max realtime timeout unlimited unlimited us
Most developers and sysadmin need to know the max number of processes and open files per nginx process. In a nutshell, we find the PID using the “ps aux | grep appname“, then look at that PID’s “limits” file in /proc/ directory. Then you will know for sure what values apply to that process to tweak for performance.
Understanding ulimit flags
| Option | Description |
|---|---|
| -S | use the `soft’ resource limit |
| -H | use the `hard’ resource limit |
| -a | all current limits are reported |
| -b | the socket buffer size |
| -c | the maximum size of core files created |
| -d | the maximum size of a process’s data segment |
| -e | the maximum scheduling priority (`nice’) |
| -f | the maximum size of files written by the shell and its children |
| -i | the maximum number of pending signals |
| -k | the maximum number of kqueues allocated for this process |
| -l | the maximum size a process may lock into memory |
| -m | the maximum resident set size |
| -n | the maximum number of open file descriptors |
| -p | the pipe buffer size |
| -q | the maximum number of bytes in POSIX message queues |
| -r | the maximum real-time scheduling priority |
| -s | the maximum stack size |
| -t | the maximum amount of cpu time in seconds |
| -u | the maximum number of user processes |
| -v | the size of virtual memory |
| -x | the maximum number of file locks |
| -P | the maximum number of pseudoterminals |
| -T | the maximum number of threads |
help ulimit
man bash
Conclusion
In this tutorial, You learned to find ulimit values of currently running process and user account on Linux using the ‘ulimit -a’ builtin command and /proc/${PID}/limits file.
