Change Soft/Hard limits and file descriptor limits on Amazon Linux 2(systemd)
Motivation
When I migrate Amazon Linux to Amazon Linux2, I investigate how to change file descriptor limits and number of process per user on Linux server working with systemd.
This post is technical memo for myself.
I try the following procedure after booting the official Amazon Linux 2 AMI as it is.
Soft limits and Hard limits
Let me review what Soft limits and Hard limits are.
Linux limits the number of process per user. There are two kinds of limits named Soft limits and Hard limits. Soft limits mean number of the process for the current user, and Hard limits mean upper limit of Soft limits that can be changed by the user.
Setting for Linux login user
Edit limits.conf
The configurations in /etc/security/limits.conf are applied to Linux login user only login with pluggable authentication module(PAM).
The pam_limits module in Ansible edits this file.
cat /etc/security/limits.conf
# /etc/security/limits.conf
#
#This file sets the resource limits for the users logged in via PAM.
#It does not affect resource limits of the system services.
#
#Also note that configuration files in /etc/security/limits.d directory,
#which are read in alphabetical order, override the settings in this
#file in case the domain is the same or more specific.
#That means for example that setting a limit for wildcard domain here
#can be overriden with a wildcard setting in a config file in the
#subdirectory, but a user specific setting here can be overriden only
#with a user specific setting in the subdirectory.
#
#Each line describes a limit for a user in the form:
#
#<domain> <type> <item> <value>
#
#Where:
#<domain> can be:
# - a user name
# - a group name, with @group syntax
# - the wildcard *, for default entry
# - the wildcard %, can be also used with %group syntax,
# for maxlogin limit
#
#<type> can have the two values:
# - "soft" for enforcing the soft limits
# - "hard" for enforcing hard limits
#
#<item> can be one of the following:
# - core - limits the core file size (KB)
# - data - max data size (KB)
# - fsize - maximum filesize (KB)
# - memlock - max locked-in-memory address space (KB)
# - nofile - max number of open file descriptors
# - rss - max resident set size (KB)
# - stack - max stack size (KB)
# - cpu - max CPU time (MIN)
# - nproc - max number of processes
# - as - address space limit (KB)
# - maxlogins - max number of logins for this user
# - maxsyslogins - max number of logins on the system
# - priority - the priority to run user process with
# - locks - max number of file locks the user can hold
# - sigpending - max number of pending signals
# - msgqueue - max memory used by POSIX message queues (bytes)
# - nice - max nice priority allowed to raise to values: [-20, 19]
# - rtprio - max realtime priority
#
#<domain> <type> <item> <value>
#
#* soft core 0
#* hard rss 10000
#@student hard nproc 20
#@faculty soft nproc 20
#@faculty hard nproc 50
#ftp hard nproc 0
#@student - maxlogins 4
# End of fileOverriding limits.conf
A part of comments in /etc/security/limits.conf explains the priority of settings as bellow.
Also note that configuration files in /etc/security/limits.d directory, which are read in alphabetical order, override the settings in this file in case the domain is the same or more specific.
There is a default Soft limits settings in /etc/security/limits.d/20-nproc.conf on Amazon Linux 2.
cat /etc/security/limits.d/20-nproc.conf
> # Default limit for number of user's processes to prevent
> # accidental fork bombs.
> # See rhbz #432903 for reasoning.
>
> * soft nproc 4096
> root soft nproc unlimitedCheck current configuration
ulimit -a command lists available system resources for current user.
ulimit command sets Hard limits with -H option, and Soft limits with -S option.
ulimit -a
> core file size (blocks, -c) 0
> data seg size (kbytes, -d) unlimited
> scheduling priority (-e) 0
> file size (blocks, -f) unlimited
> pending signals (-i) 3828
> max locked memory (kbytes, -l) 64
> max memory size (kbytes, -m) unlimited
> open files (-n) 1024
> pipe size (512 bytes, -p) 8
> POSIX message queues (bytes, -q) 819200
> real-time priority (-r) 0
> stack size (kbytes, -s) 8192
> cpu time (seconds, -t) unlimited
> max user processes (-u) 3828
> virtual memory (kbytes, -v) unlimited
> file locks (-x) unlimitedConfigures daemon
Now limits the system resources for daemon process.
The daemon process in systemd uses the configuration in /etc/security/limits.conf, but
/etc/systemd/system.conf and /etc/systemd/user.conf,/etc/systemd/<systemd_unit>/override.conf.
Change overall default settings
To change default settings for process controlled by systemd, edit /etc/systemd/system.conf.
For instance, changing the file descriptor limits and upper limits of the process is as bellow.
1[Manager]
2DefaultLimitNOFILE=65536
3DefaultLimitNPROC=65536Settings per daemon
When system administrators give services in systemd appropriate system resources,
they creates /etc/systemd/system/(service name).service and add [Service] block.
Changing only system resource despite of /etc/systemd/system/（service name）.service file already exists, override with creating /etc/systemd/system/（service name）.service.d/override.conf.
In addition, the setting of override.conf takes precedence over (service name) .service.
Changing the number of file descriptor limits is as below.
1[Service]
2LimitNOFILE=40000Now restart daemon.
systemctl daemon-reload
systemctl stop (service)
systemctl start (service)Check the configuration.
cat /proc/${process ID}/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 0 unlimited bytes
Max resident set unlimited unlimited bytes
Max processes 65536 65536 processes
Max open files 40000 40000 files
Max locked memory 65536 65536 bytes
Max address space unlimited unlimited bytes
Max file locks unlimited unlimited locks
Max pending signals 29779 29779 signals
Max msgqueue size 819200 819200 bytes
Max nice priority 0 0
Max realtime priority 0 0
Max realtime timeout unlimited unlimited usThe file descriptor (Max open files) is 40000.
Conclusion
It is available to …
- Change Soft limits for current login user
- Change Hard limits for current login user
- Change default settings for daemon process managed by systemd
