NAME¶

PR_SET_TIMERSLACK - set the "current" timer slack value for the calling thread

LIBRARY¶

Standard C library (libc, -lc)

SYNOPSIS¶

#include <linux/prctl.h>  /* Definition of PR_* constants */
#include <sys/prctl.h>

int prctl(PR_SET_TIMERSLACK, unsigned long slack);

DESCRIPTION¶

Each thread has two associated timer slack values: a "default" value, and a "current" value. This operation sets the "current" timer slack value for the calling thread. slack is an unsigned long value in the range [1L, ULONG_MAX]. If the nanosecond value supplied in slack is greater than zero, then the "current" value is set to this value. If slack is 0L, the "current" timer slack is reset to the thread's "default" timer slack value.

The "current" timer slack is used by the kernel to group timer expirations for the calling thread that are close to one another; as a consequence, timer expirations for the thread may be up to the specified number of nanoseconds late (but will never expire early). Grouping timer expirations can help reduce system power consumption by minimizing CPU wake-ups.

The timer expirations affected by timer slack are those set by select(2), pselect(2), poll(2), ppoll(2), epoll_wait(2), epoll_pwait(2), clock_nanosleep(2), nanosleep(2), and futex(2) (and thus the library functions implemented via futexes, including pthread_cond_timedwait(3), pthread_mutex_timedlock(3), pthread_rwlock_timedrdlock(3), pthread_rwlock_timedwrlock(3), and sem_timedwait(3)).

Timer slack is not applied to threads that are scheduled under a real-time scheduling policy (see sched_setscheduler(2)).

When a new thread is created, the two timer slack values are made the same as the "current" value of the creating thread. Thereafter, a thread can adjust its "current" timer slack value via PR_SET_TIMERSLACK. The "default" value can't be changed. The timer slack values of init (PID 1), the ancestor of all processes, are 50,000 nanoseconds (50 microseconds). The timer slack value is inherited by a child created via fork(2), and is preserved across execve(2).

RETURN VALUE¶

On success, 0 is returned. On error, -1 is returned, and errno is set to indicate the error.

FILES¶

/proc/pid/timerslack_ns

Since Linux 4.6, the "current" timer slack value of any process can be examined and changed via this file.

STANDARDS¶

Linux.

HISTORY¶

Linux 2.6.28.

SEE ALSO¶

prctl(2), PR_GET_TIMERSLACK(2const), proc_pid_timerslack_ns(5)