Number of Files Opened

Linux allows you to control how many opened file descriptors a user can have at any one time.

System-wide settings

To show the number of files opened on a system, run:

Description	Command
Read the current file descriptor numbers. 3 values are: Current number of opened files Number of free allocated file descriptors Maximum number of file descriptors for the whole system	# cat /proc/sys/fs/file-nr 7552 0 9223372036854775807
Maximum number of file descriptiors.	# cat /proc/sys/fs/file-max 9223372036854775807
Set the number of file descriptors	## Via /proc # cat 99999 > /proc/sys/fs/file-nr ## Via sysctl # sysctl -w fs.file-max=99999 ## Edit /etc/sysctl.conf so it applies on boot # echo "fs.file-max = 99999" >> /etc/sysctl.conf

User session setting

On typical Linux distributions using PAM, user's session's limits are controlled by editing /etc/security/limits.conf and are applied by the pam_limits.so module (Enabled when using session required pam_limits.so in the system-auth config)

A user can check their current session's limits with ulimit on the bash shell.

Description	What
Max number of files opened	# ulimit -n 1024 ## Soft limit # ulimit -Sn 1024 ## Hard limit # ulimit -Hn 1024 This tells you that the maximum number of files you can have in this session is 1024 files.
Temporarily set a new limit	# ulimit -n 65536 Sets both the hard and soft limit for the shell's session. Cannot exceed the limits defined in /etc/security/limits.conf.

To change a user's soft and hard file limits, add the following lines to /etc/security/limits.conf:

## In /etc/security/limits.conf
username soft nofile 16384
username hard nofile 32768

The user will have to re-login for these new limits to be applied.

Systemd service

Services that are started by systemd will not adhere to the limits specified in the /etc/security/limits.conf file (because those are set by PAM?). You will have to change the service file and specify the limits there instead.

For example:

[Unit]
Description=Galaxy Web Service
After=network.target

[Service]
Type=simple
User=galaxy
ExecStart=/galaxy/run.sh
# ExecStop=/galaxy/run.sh stop
# ExecReload=/galaxy/run.sh restart
Restart=always

LimitNOFILE=65536

[Install]
WantedBy=multi-user.target

Checking if limits are set properly

Each process's limits are available under /proc/$pid/limits. You can read this file to check whether a max file limit value is being set properly.

Other limit directives

Directive        ulimit equivalent     Unit
LimitCPU=        ulimit -t             Seconds      
LimitFSIZE=      ulimit -f             Bytes
LimitDATA=       ulimit -d             Bytes
LimitSTACK=      ulimit -s             Bytes
LimitCORE=       ulimit -c             Bytes
LimitRSS=        ulimit -m             Bytes
LimitNOFILE=     ulimit -n             Number of File Descriptors 
LimitAS=         ulimit -v             Bytes
LimitNPROC=      ulimit -u             Number of Processes 
LimitMEMLOCK=    ulimit -l             Bytes
LimitLOCKS=      ulimit -x             Number of Locks 
LimitSIGPENDING= ulimit -i             Number of Queued Signals 
LimitMSGQUEUE=   ulimit -q             Bytes
LimitNICE=       ulimit -e             Nice Level 
LimitRTPRIO=     ulimit -r             Realtime Priority  
LimitRTTIME=     No equivalent