HP-UX Reference (11i v1 05/09) - 3 Library Functions N-Z (vol 7)

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pthread_mutexattr_getspin_np(3T) pthread_mutexattr_getspin_np(3T)

(Pthread Library)

NAME

pthread_mutexattr_getspin_np(), pthread_mutexattr_setspin_np(), pthread_mutex_getyieldfreq_np(),

pthread_mutex_setyieldfreq_np() - get and set the mutex spin and yield frequency attributes

SYNOPSIS

#include <pthread.h>

int pthread_mutexattr_setspin_np(pthread_mutexattr_t *attr, int spin);

int pthread_mutexattr_getspin_np(const pthread_mutexattr_t *attr,

int *spin);

int pthread_mutex_setyieldfreq_np(int yield);

int pthread_mutex_getyieldfreq_np(int *yield);

PARAMETERS

attr Pointer to the mutex attributes object whose attributes are to be set/retrieved.

spin This parameter either speciﬁes the new value of the spin attribute (set function) or

points to the memory location where the spin attribute of attr is to be returned (get

function).

yield This parameter either speciﬁes the new value of the yield frequency process-wide

attribute (set function) or points to the memory location where the yield frequency

process-wide attribute is to be returned (get function).

DESCRIPTION

These attributes are used to tune the mutex locking behavior for optimized application performance on

multiprocessor systems.

For

pthread_mutexattr_setspin_np()

and pthread_mutexattr_getspin_np()

the

attributes object attr must have previously been initialized with the function

pthread_mutexattr_init()

before these functions are called.

ATTRIBUTE: spin

Mutexes can be initialized with a spin value which is used by

pthread_mutex_lock()

for busy-wait

iterations on the mutex lock. It is effective only on multiprocessor systems. It is ignored on uniprocessor

systems.

For bound threads, the procedure to block on a busy mutex is quite costly. On a multiprocessor system, the

thread holding the mutex may be a few instruction cycles away from releasing it. By performing a brief

busy-wait before actually blocking, the lock path can avoid a great deal of overhead in these situations.

For many applications, these situations are not rare. Consider a mutex used to protect a short sequence of

code (e.g., to increment a counter). The mutex is held only for a few instruction cycles (plus the function

call overhead for pthread_mutex_unlock().

The ability to set the spin attribute allows the application writer to adjust the busy-wait to suit the dura-

tion of the common-case critical section protected by each mutex.

The legal values for the spin attribute are:

positive integer

The pthread_mutex_lock() function will busy-wait on the mutex lock for the speciﬁed

number of iterations before blocking the thread, unless the lock is acquired sooner. Larger spin

values would be appropriate for mutexes associated with longer critical sections.

PTHREAD_MUTEX_SPINONLY_NP

Inhibits blocking on the mutex lock altogether. The pthread_mutex_lock() function will

busy-wait on the mutex lock until it is acquired.

PTHREAD_MUTEX_SPINDEFAULT_NP

Uses a built-in default value for the number of busy-wait iterations.

ATTRIBUTE: yield

Note that the yield attribute is not a per-mutex attribute, but is process-wide. That is, it affects the

behavior of all mutexes.

The yield attribute speciﬁes, for the pthread_mutex_lock() busy-wait, how frequently the processor

should be yielded (via sched_yield()), allowing other threads to execute.

HP-UX 11i Version 1: September 2005 − 1 − Hewlett-Packard Company Section 3−−733