pidsmax

Pidsmax refers to the mechanism used in Linux control groups (cgroups) to cap the number of processes that can exist within a cgroup. It is a practical tool for resource management and containment, helping to prevent fork bombs and to enforce isolation in multi-tenant environments, containers, and service sandboxes.

In practice, pidsmax is implemented as a control file named pids.max that is part of the cgroup filesystem. The exact location varies by cgroup version: in older cgroup v1 setups the limit is exposed under the pids controller (for example, /sys/fs/cgroup/pids/pids.max), while in the unified cgroup v2 hierarchy the limit appears as a pids.max file within the directory representing a specific cgroup. Reading the file shows the current limit, and writing a value changes the limit for new process creations within that cgroup.

The limit counts processes associated with the cgroup, including child groups in a hierarchical configuration. When the number of processes reaches the configured maximum, new process creation requests for that cgroup are restricted by the kernel until processes exit and the count falls below the limit. A value that denotes no practical limit varies by kernel version and can be a large number or a special token such as max in some implementations.

Administration and usage often involve system management tools or container runtimes. Systemd can influence the limit via unit properties such as TasksMax, and container platforms can set pids.max to enforce bounds on contained workloads. Proper sizing of pidsmax requires balancing resource guarantees with the need for occasional bursts in workload.

See also: cgroups, pids controller, virtualized/container environments, systemd, Docker, Kubernetes.