HP-UX Reference (11i v1 00/12) - 3 Library Functions N-Z (vol 7)

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pthread_mutexattr_getspin_np(3T) pthread_mutexattr_getspin_np(3T)

(Pthread Library)

When the number of threads exceeds the number of processors in a system, a busy-wait on a mutex can

sometimes have an adverse effect. The busy-wait itself can prevent the thread holding the lock from com-

pleting the associated critical section of code. By yielding the processor on occasion, the thread attempting

the lock may allow the thread holding the lock to reach the point at which it can release the lock. How-

ever, it still avoids the costly path to block on the mutex.

The legal values for the yield attribute are:

positive integer

The busy-wait loop in pthread_mutex_lock() will yield the processor after each speciﬁed

number of iterations of the spin loop (where the total number of iterations is controlled by the

per-mutex spin attribute). PTHREAD_MUTEX_YIELDNEVER_NP Inhibits yielding in the

mutex lock altogether.

PTHREAD_MUTEX_YIELDFREQDEFAULT_NP

Uses a built-in default value for the frequency of yields in the busy-wait loop.

WARNINGS

The setting of the spin and yield attributes can, as well as improve application performance, easily lead to

degraded performance. The CPU consumption of the application may be increased. Settings which work

well for a small number of threads may do poorly for larger numbers of threads. The optimal settings will

vary depending upon hardware and operating system conﬁguration. Minor changes in the application itself

may require retuning of these attributes.

The programmer must analyze performance carefully, to obtain an understanding of mutex contention

within the application. Then, experiment with different attribute values, evaluating how mutex contention

is affected, response time, and CPU consumption.

RETURN VALUE

Upon successful completion, pthread_mutexattr_setspin_np()

pthread_mutexattr_getspin_np()

, pthread_mutex_setyieldfreq_np()

, and

pthread_mutex_getyieldfreq_np()

return zero. Otherwise, an error number is returned to indi-

cate the error (the

errno variable is not set).

ERRORS

For each of the following conditions, if the condition is detected, the

pthread_mutexattr_setspin_np()

, pthread_mutexattr_getspin_np()

pthread_mutex_setyieldfreq_np()

, and pthread_mutex_getyieldfreq_np()

functions

return the corresponding error number:

[EINVAL] The value speciﬁed by attr, spin,oryield is invalid.

AUTHOR

pthread_mutexattr_getspin_np(), pthread_mutexattr_setspin_np(),

pthread_mutex_getyieldfreq_np(), and pthread_mutex_setyieldfreq_np() were

developed by HP.