Linux Process Management Concepts

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Internal

Process

The process with ID 0 is the swapper (or sched), the process responsible with paging. It is part of the kernel, rather than user-space.

The process with ID 1 is the init process, and it is responsible with starting and shutting down the system.

Threads

The number of threads for process can be read from the process' corresponding /proc/<pid>/status, as described here: "Threads:".

Maximum Number of Processes Allowed on the System

cat /proc/sys/kernel/pid_max

or

sysctl kernel.pid_max

For more details, see sysctl and kernel.pid_max.

To get the actual number of processes present in the system, see /proc/stat.

Maximum Number of Processes Available to a Single User

The current value can be read and set with ulimit -u.

For Java applications, this setting limits the number of threads a JVM can create, and it can cause the JVM to throw "java.lang.OutOfMemoryError: unable to create new native thread" exceptions when that limit is reached. For more details see
Java Threads and Linux Processes

The O/S level symptom of reaching the per-user process limit is:

-bash: fork: retry: Resource temporarily unavailable

while trying to execute an arbitrary process.