How to set limits (ulimit) for services run by systemd

Environment

• Red Hat Enterprise Linux 7
• Red Hat Enterprise Linux 8
• Red Hat Enterprise Linux 9

Issue

• How to set limits for services run by systemd?
• configure Apache 2.4 to have maximum control open file - ulimit
• I want to configure Apache 2.4 to have maximum control open file.

Resolution

1. Find the desired limit directive in the systemd.exec(5) man page
   The following is the relevant excerpt:

   
    LimitCPU=, LimitFSIZE=, LimitDATA=, LimitSTACK=, LimitCORE=, LimitRSS=, LimitNOFILE=, LimitAS=, LimitNPROC=,
    LimitMEMLOCK=, LimitLOCKS=, LimitSIGPENDING=, LimitMSGQUEUE=, LimitNICE=, LimitRTPRIO=, LimitRTTIME=
       These settings set both soft and hard limits of various resources for executed processes.
       See setrlimit(2) for details. Use the string infinity to configure no limit on a specific resource.
   
       Table 1. Limit directives and their equivalent with ulimit
       ┌────────────────┬───────────────────┐
       │Directive       │ ulimit equivalent │
       ├────────────────┼───────────────────┤
       │LimitCPU        │ ulimit -t         │
       ├────────────────┼───────────────────┤
       │LimitFSIZE      │ ulimit -f         │
       ├────────────────┼───────────────────┤
       │LimitDATA       │ ulimit -d         │
       ├────────────────┼───────────────────┤
       │LimitSTACK      │ ulimit -s         │
       ├────────────────┼───────────────────┤
       │LimitCORE       │ ulimit -c         │
       ├────────────────┼───────────────────┤
       │LimitRSS        │ ulimit -m         │
       ├────────────────┼───────────────────┤
       │LimitNOFILE     │ ulimit -n         │
       ├────────────────┼───────────────────┤
       │LimitAS         │ ulimit -v         │
       ├────────────────┼───────────────────┤
       │LimitNPROC      │ ulimit -u         │
       ├────────────────┼───────────────────┤
       │LimitMEMLOCK    │ ulimit -l         │
       ├────────────────┼───────────────────┤
       │LimitLOCKS      │ ulimit -x         │
       ├────────────────┼───────────────────┤
       │LimitSIGPENDING │ ulimit -i         │
       ├────────────────┼───────────────────┤
       │LimitMSGQUEUE   │ ulimit -q         │
       ├────────────────┼───────────────────┤
       │LimitNICE       │ ulimit -e         │
       ├────────────────┼───────────────────┤
       │LimitRTPRIO     │ ulimit -r         │
       ├────────────────┼───────────────────┤
       │LimitRTTIME     │ No equivalent     │
       └────────────────┴───────────────────┘
    

2. Execute systemctl edit <unit name>

For example:

# systemctl edit httpd.service

You can switch editor by setting SYSTEMD_EDITOR variable.

# export SYSTEMD_EDITOR=/bin/vim
# systemctl edit httpd.service

3. Enter the Service section as follows using the editor that starts automatically

For example:

[Service]
LimitNOFILE=8000
LimitNPROC=32000

4. Execute systemctl daemon-reload

5. Execute systemctl restart <unit>

6. Confirm that new limits were applied by checking the /proc/<PID>/limits file or output of systemctl show <unit> | grep -i limit

Full example demonstration:

# systemctl show httpd -p LimitNPROC
LimitNPROC=15084

# eval $(systemctl show httpd -p MainPID)
# egrep 'process|open files' /proc/$MainPID/limits
Max processes             15084                15084                processes 
Max open files            1024                 4096                 files     

# systemctl edit httpd.service
[Service]
LimitNOFILE=8000
LimitNPROC=32000

# cat /etc/systemd/system/httpd.service.d/override.conf
[Service]
LimitNOFILE=8000
LimitNPROC=32000

# systemctl daemon-reload
# systemctl restart httpd

# systemctl show httpd | grep -ie nproc -e nofile
LimitNOFILE=8000
LimitNPROC=32000

# eval $(systemctl show httpd -p MainPID)
# egrep 'process|open files' /proc/$MainPID/limits
Max processes             32000                32000                processes
Max open files            8000                 8000                 files

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