Limits

Unix systems put various sensible limits on the use of different resources. Open files, number of processes, locked files, cpu usage, etc can be limited.

• soft limits are the enforced limitation during the session, can be self-configured
• hard limits are the ceiling for the soft limit, set by admin
• processes inherit the limit of their parent process, so daemonized processes will likely inherit from pid 1

/proc/*/limits

shows the current limits set for a running process. These limits are set based on a number of possible configuration locations.

ulimit

Utility for setting limits at a user-level for system-wide resource usage, which is useful for shared systems. These are limits for the sum of all processes owned by a user.

/etc/security/limits.conf

Part of linux's pam_limits, the persistent ulimits configuration file. Sets for logged in users, not for system processes

• config file for the pam_module, pluggable authentication module
• acts as adapter for multiple low level auth protocols (facade design pattern?)
• /etc/security/limits.d/* are other configurations loaded after limits.conf

pam_limits are enforced when a session is opened. They do not affect daemon processes

/etc/sysctl.conf

Contains system-wide configurations, kernel-level configurations

• less graceful to hit limits here

source for file descriptors limit