On Thu, 19 Jun 2014 12:01:10 EDT erik quanstrom <quans...@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.
Could there've been a context switch?
