On Thu, Jan 28, 2016 at 09:31:52PM +0300, Maxim Uvarov wrote:
> I have some thoughts and questions about timer implementation in
> linux-generic.
>
> Current implementation:
>
> sigev.sigev_notify = SIGEV_THREAD;
> sigev.sigev_notify_function = timer_notify;
> sigev.sigev_value.sival_ptr = tp;
> timer_create(CLOCK_MONOTONIC, &sigev, &tp->timerid);
> then:
> timer_settime(tp->timerid, 0, &ispec, NULL);
>
> where:
> timer_notify(sigval_t sigval)
> {
> uint64_t prev_tick = odp_atomic_fetch_inc_u64(&tp->cur_tick);
> /* Attempt to acquire the lock, check if the old value was clear */
> if (odp_spinlock_trylock(&tp->itimer_running)) {
> /* Scan timer array, looking for timers to expire */
> (void)odp_timer_pool_expire(tp, prev_tick);
> odp_spinlock_unlock(&tp->itimer_running);
> }
>
> }
>
> Now what I see from our test case.
> 1. We have bunch of workers.
> 2. Each worker starts timer.
> 3. Because it's SIGEV_THREAD on timer action new thread for notifier
> function started.
>
> Usually it works well. Until there is load on cpu. (something like
> busy loop app.) There a lot of threads
> just created by kernel. I.e. execution clone() call.
>
> Based that I have question I have questions which is not quite clear for me:
> 1. Why SIGEV_THREAD was used?
>
> 2. When each worker will run bunch of threads (timer handler), they
> will fight for cpu time for context
> switches between all that threads. Is there significant slowdown
> compare to one thread or signal usage?
>
> 3. What is priority of timer handler against to worker? Cpu affinity
> of handler thread? Should it
> be SHED_FIFO? I.e. do we need to specify that thread attrs?
>
> I think that creation thread each time only for increasing atomic
> counter is very expensive. So we can
> rewrite that code to use SIGEV_SIGNAL or start thread manually and
> SIGEV_THREAD_ID + semaphore.
>
> If we will think about core isolation, than probably we have to work
> with signals. Don't know if core isolation
> supports several threads on one core. Or even move all timer actions
> to separate core to not disturb worker
> cores.
>
> Thank you,
> Maxim.
+1
This is basically what was suggested here:
https://bugs.linaro.org/show_bug.cgi?id=1615#c18
--
Stuart.
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