Weird. I assume cycles is using rdtsc or rdtscp. Perhaps some of it is due
to a combination of contention and rdtsc(p) being serializing instructions?
On Jun 19, 2014 12:04 PM, "erik quanstrom" <quanstro@quanstro.net> wrote:

> i'm seeing some mighty interesting timing on my intel ivy bridge.
> i found a bug in the file server aoe implementation (can't happen
> if you're using the uniprocessor x86 version) that happens because
> the Srb is freed before wakeup completes.  to solve this there is
> some code that sets the state (this is from ken's ancient scheduler,
> by way of sape)
>
>         wakeup(&srb);
>         srb->state = Free;
>
> code that receives it is like this
>
>         sleep(&srb, srbdone, srb);
>         cycles(&t0);
>         for(n = 0; srb->state != Free; n++){
>                 if(srb->wmach == m->machno)
>                         sched();
>                 else
>                         monmwait(&srb->state, Alloc);
>         }
>         cycles(&t1);
>         free(srb);
>
> the astounding thing is that t1-t0 is often ~ 60,000 cycles.
> it only hits a small fraction of the time, and the average is
> much lower.  but that just blows the mind.  60000 cycles!
>
> (other versions with sched were much worse.)
>
> as far as i can tell, there are no funny bits in the scheduler that
> can cause this, and no wierd scheduling is going on.
>
> i'm baffled.
>
> - erik
>
>