On 10-Mar-2001 Andi Kleen wrote:
> Davide Libenzi <[EMAIL PROTECTED]> writes:
>
>
>> Probably the rate at which is called sys_sched_yield() is not so high to let
>> the performance improvement to be measurable.
>
> LinuxThreads mutexes call sched_yield() when a lock is locked, so when you
> have a multithreaded program with some lock contention it'll be called a
> lot.
This is the linux thread spinlock acquire :
static void __pthread_acquire(int * spinlock)
{
int cnt = 0;
struct timespec tm;
while (testandset(spinlock)) {
if (cnt < MAX_SPIN_COUNT) {
sched_yield();
cnt++;
} else {
tm.tv_sec = 0;
tm.tv_nsec = SPIN_SLEEP_DURATION;
nanosleep(&tm, NULL);
cnt = 0;
}
}
}
Yes, it calls sched_yield() but this is not a std wait for mutex but for
spinlocks that are hold a very short time.
Real wait are implemented using signals.
More, with the new implementation of sys_sched_yield() the task release all its
time quantum so, even in a case where a task repeatedly calls sched_yield() the
call rate is not so high if there is at least one process to spin.
And if there isn't one task with goodness() > 0, nobody cares about
sched_yield() performance.
- Davide
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