Absolute time
Posted Dec 22, 2006 2:21 UTC (Fri) by
slamb (guest, #1070)
Parent article:
Kevent take 26
Is the absolute time CLOCK_REALTIME or CLOCK_MONOTONIC? If the
former...ugh...it's useless; I could see why Evgeniy didn't want it.
If the latter, it can provide more precise timing for those who worry about that sort of thing.
That group notably includes the designers of pthread_cond_timedwait(). A couple manpage
snippets:
If the Clock Selection option is supported, the condition variable
shall have a clock attribute which specifies the clock that shall be
used to measure the time specified by the abstime argument. When such
timeouts occur, pthread_cond_timedwait() shall nonetheless release and
re-acquire the mutex referenced by mutex. The pthread_cond_timedwait()
function is also a cancellation point.
...
An absolute time measure was chosen for specifying the timeout parameter for two
reasons. First, a relative time measure can be easily
implemented on top of a function that specifies absolute time, but
there is a race condition associated with specifying an absolute timeout on top of a function
that specifies relative timeouts. For example, assume that clock_gettime() returns the current
time and cond_relative_timed_wait()
uses relative timeouts:
clock_gettime(CLOCK_REALTIME, &now)
reltime = sleep_til_this_absolute_time -now;
cond_relative_timed_wait(c, m, &reltime);
If the thread is preempted between the first statement and the last
statement, the thread blocks for too long. Blocking, however, is irrelevant if an absolute
timeout is used. An absolute timeout also need not
be recomputed if it is used multiple times in a loop, such as that
enclosing a condition wait.
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