sysfscgrouppidspidsmax

sysfscgrouppidspidsmax is the name commonly used to refer to the sysfs control file that governs the maximum number of processes (PIDs) that can be created within a Linux cgroup. It is part of the cgroup subsystem and is exposed in the sysfs hierarchy to enforce per‑cgroup process limits.

Location and behavior

In the Linux cgroup architecture, the pids limit is exposed as a per‑cgroup file named pids.max. In the cgroup v2 unified hierarchy, this file appears in each cgroup’s directory under /sys/fs/cgroup, and in the older v1 world the limit is provided by the pids controller in each cgroup. The value written to this file determines how many processes may exist in that cgroup (including its descendants in the hierarchy, depending on the setup). A positive integer sets a finite limit, while the special value "max" typically represents no limit.

Usage and effects

Reading the file shows the current limit for that cgroup. Writing to the file requires appropriate privileges (usually root). Examples: to view, cat /sys/fs/cgroup/mygroup/pids.max; to enforce a limit of 2000 processes, echo 2000 > /sys/fs/cgroup/mygroup/pids.max; to remove the imposed limit (unlimited), echo max > /sys/fs/cgroup/mygroup/pids.max. The effective limit is the minimum of the limits assigned to this cgroup and any of its ancestors. When the limit is reached, attempts to create new processes in the cgroup are blocked or fail, helping to prevent resource exhaustion and to isolate workloads.

Systemd and administration

Systemd can configure per‑service process limits via properties such as PIDsMax, which map to the underlying pids.max mechanism. Administrators use these controls to enforce resource boundaries in multi‑tenant or tightly managed environments.

See also: Linux control groups (cgroups), pids controller, systemd, process accounting.