HP-UX Reference (11i v2 04/09) - 3 Library Functions N-Z (vol 7)

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pthread_mutexattr_getspin_np(3T) pthread_mutexattr_getspin_np(3T)

(Pthread Library)

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.

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 attempt-

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

However, 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.

Mutex performance enhancement can also be achieved if the implementation does not have to strictly

adhere to the POSIX requirements for acquisition of mutexes by real-time threads. By calling the

pthread_mutexattr_disable_handoff_np()

function or the

pthread_mutex_disable_handoff_np()

function, these requirements will not be strictly

imposed, and the implementation can provide better mutex performance.

The

pthread_mutexattr_disable_handoff_np()

function enables performance improvement

for a speciﬁc mutex. This must be called after initializing the mutex attribute.

The

pthread_mutex_disable_handoff_np()

function enables performance improvement for all

mutexes in the process, and must be called by the initial thread before any other threads are created.

RETURN VALUE

pthread_mutexattr_setspin_np(), pthread_mutexattr_getspin_np()

pthread_mutex_setyieldfreq_np(), pthread_mutex_getyieldfreq_np()

pthread_mutexattr_disable_handoff_np()

, and

pthread_mutex_disable_handoff_np()

return the following values:

0 Successful completion.

<>0 Failure. The returned value is an error number deﬁned in the ERRORS section. (The errno vari-

able is not set).

ERRORS

If an error is detected,

pthread_mutexattr_setspin_np(),

pthread_mutexattr_getspin_np(), pthread_mutex_setyieldfreq_np(), and

pthread_mutex_getyieldfreq_np() return one of the following error numbers.

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

Section 3−−812 Hewlett-Packard Company − 2 − HP-UX 11i Version 2: September 2004