From: "Devon H. O'Dell" <devon.odell@gmail.com>
To: Fans of the OS Plan 9 from Bell Labs <9fans@9fans.net>
Subject: Re: [9fans] cache lines, and 60000 cycles of doom
Date: Fri, 20 Jun 2014 01:03:05 -0400 [thread overview]
Message-ID: <CAFgOgC9oeGtBp3mA2QRLfsgeziTxYjhwMBvFGuQjoYiCAHCDBQ@mail.gmail.com> (raw)
In-Reply-To: <399b6c5c08b7aeab53f52ce0414714bf@brasstown.quanstro.net>
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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
>
>
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next prev parent reply other threads:[~2014-06-20 5:03 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-06-19 16:01 erik quanstrom
2014-06-20 5:03 ` Devon H. O'Dell [this message]
2014-06-20 11:50 ` erik quanstrom
2014-06-20 12:47 ` Devon H. O'Dell
2014-06-20 13:07 ` erik quanstrom
2014-06-20 5:09 ` Bakul Shah
2014-06-20 11:45 ` erik quanstrom
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