Semaphores slow???

From: Rogier Wolff (
Date: Wed Dec 27 2000 - 09:15:35 EST


We have a typical semaphore application that has a producer and a

Without the semaphores we are limited by the rest of the stuff to
10000 times around the loop per second. That's good.

When we put the "push the semaphore" call in there, the rate drops to
around 8000 per second. I'm not happy about that, but ok. When we add
the "wait for bufferspace" semaphore wait in there, the rate drops to
4000 per second. This is way too low.

Does anybody have an idea why linux semaphores are so slow?

        sem_init (&write_sem, 1, 0);
        sem_init (&write_buffer_sem, 1, 0);


        while (1) {
                sem_wait (&write_buffer_sem);

                sem_post (&write_sem);

        for (i=0;i<nbufs;i++) {
                Create_buffer (i);
                sem_post (&write_buffer_sem);

        while (1) {
                sem_wait (&write_sem);

                sem_post (&write_buffer_sem);

(pshared == 0 doesn't work: returns error. The manpage claims that
thsi would happen for pshared != 0... )


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