On Thu, 15 May 2008, Daniel Eischen wrote:
On Thu, 15 May 2008, Andriy Gapon wrote:
Or even more realistic: there should be a feeder thread that puts things on
the queue, it would never be able to enqueue new items until the queue
becomes empty if worker thread's code looks like the following:
while(1)
{
pthread_mutex_lock(&work_mutex);
while(queue.is_empty())
pthread_cond_wait(...);
//dequeue item
...
pthread_mutex_unlock(&work mutex);
//perform some short and non-blocking processing of the item
...
}
Because the worker thread (while the queue is not empty) would never enter
cond_wait and would always re-lock the mutex shortly after unlocking it.
Well in theory, the kernel scheduler will let both threads run fairly
with regards to their cpu usage, so this should even out the enqueueing
and dequeueing threads.
You could also optimize the above a little bit by dequeueing everything
in the queue instead of one at a time.
I suppose you could also enforce your own scheduling with
something like the following:
pthread_cond_t writer_cv;
pthread_cond_t reader_cv;
pthread_mutex_t q_mutex;
...
thingy_q_t thingy_q;
int writers_waiting = 0;
int readers_waiting = 0;
...
void
enqueue(thingy_t *thingy)
{
pthread_mutex_lock(q_mutex);
/* Insert into thingy q */
...
if (readers_waiting > 0) {
pthread_cond_broadcast(&reader_cv, &q_mutex);
readers_waiting = 0;
}
while (thingy_q.size > ENQUEUE_THRESHOLD_HIGH) {
writers_waiting++;
pthread_cond_wait(&writer_cv, &q_mutex);
}
pthread_mutex_unlock(&q_mutex);
}
thingy_t *
dequeue(void)
{
thingy_t *thingy;
pthread_mutex_lock(&q_mutex);
while (thingy_q.size == 0) {
readers_waiting++;
pthread_cond_wait(&reader_cv, &q_mutex);
}
/* Dequeue thingy */
...
if ((writers_waiting > 0)
&& thingy_q.size < ENQUEUE_THRESHOLD_LOW)) {
/* Wakeup the writers. */
pthread_cond_broadcast(&writer_cv, &q_mutex);
writers_waiting = 0;
}
pthread_mutex_unlock(&q_mutex);
return (thingy);
}
The above is completely untested and probably contains some
bugs ;-)
You probably shouldn't need anything like that if the kernel
scheduler is scheduling your threads fairly.
--
DE
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