11.2. Preemption

A process can voluntarily yield the CPU either because it has completed, or because it is waiting for an event (such as data from a disk, a key press, or for a network packet).

A process can also involuntarily yield the CPU. This is referred to as preemption, and occurs when a higher priority process wants to use the CPU. Preemption can have a particularly negative impact on performance, and constant preemption can lead to a state known as thrashing. This problem occurs when processes are constantly preempted and no process ever gets to run completely.

To check voluntary and involuntary preemption occurring on a single process, check the contents of the /proc/PID/status, where PID is the PID of the process. The following command checks the preemption of the process with PID 1000:

grep voluntary /proc/1000/status

~]# grep voluntary /proc/1000/status
voluntary_ctxt_switches: 194529
nonvoluntary_ctxt_switches: 195338

Changing the priority of a task can help reduce involuntary preemption.