Re: semaphore redesign

[Rich Felker <dalias@libc.org>]

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On Sat, Feb 28, 2015 at 02:21:22AM +0300, Alexander Monakov wrote:
> Hello,
> 
> As new cancellation has landed (except that __timedwait fails to propagate
> ECANCELED in addition to ETIMEDOUT and EINTR), I'm happy to post semaphore
> redesign.  We discussed this implementation with Rich on IRC once, and
> presently I'm not aware of any issues (finally!), but still testing and
> performance comparison need to be done.

Thanks! I'm attaching a performance testing program I used in the
past. Its time measurement is not very precise, but it should show
large-scale differences. It measures the number of messages that can
be send back and forth between two threads in 2 seconds by cancelling
the two threads after sleep(2).

Some other things we should test for performance:

- Do something like the attached but with multiple threads contending
  on semaphores rather than uncontended message passing.

- Time/cycles per call for sem_post and sem_trywait and uncontended
  sem_wait -- this looks like it should be an obvious win though.

- Perhaps something to hammer posts and trywait -- it's not clear that
  this would model any real-world behavior, but it could show
  something interesting anyway.

I doubt we'll see any measurable differences in usage cases where the
futex syscall is expected; it should dominate there.

Rich

[-- Attachment #2: sem_bench.c --]
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#include <stdio.h>
#include <unistd.h>
#include <pthread.h>
#include <semaphore.h>

static sem_t sem[2];
static long count[2];

static void *func(void *arg)
{
	long my_id = (long) arg;

	for (;;) {
		sem_wait(&sem[my_id]);
		count[my_id]++;
		sem_post(&sem[1-my_id]);
	}

	return NULL;
}

int main(void)
{
	void *ret;
	pthread_t t1, t2;

	sem_init(&sem[0], 0, 1);
	sem_init(&sem[1], 0, 0);

	pthread_create(&t1, NULL, func, (void*) 0);
	pthread_create(&t2, NULL, func, (void*) 1);

	sleep(2);

	pthread_cancel(t1);
	pthread_cancel(t2);

	pthread_join(t1, &ret);
	pthread_join(t2, &ret);

	printf("count=%ld\n", count[0] + count[1]);

	return 0;
}