linux-kernel - Re: High scheduler wake up times

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Date:	Mon, 1 Feb 2010 13:37:35 -0600
From:	Shawn Bohrer <shawn.bohrer@...il.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>
Subject: Re: High scheduler wake up times

On Mon, Feb 01, 2010 at 09:51:30AM +0100, Peter Zijlstra wrote:
> Right, aside from that, CFS will only (potentially) delay your wakeup if
> there's someone else on the cpu at the moment of wakeup, and that's
> fully by design, you don't want to fix that, its bad for throughput.
> 
> If you want deterministic wakeup latencies use a RT scheduling class
> (and kernel).

I've confirmed that running my processes as SCHED_FIFO fixes the issue
and allows me to achieve ~999.99 iterations per second.

> As it stand it appears you have at least two bugs in your application,
> you rely on broken epoll behaviour and you have incorrect assumptions on
> what the regular scheduler class will guarantee you (which is in fact
> nothing other than that your application will at one point in the future
> receive some service, per posix).

Interestingly I can also achieve ~999.99 iterations per second by using an
infinite epoll timeout, and adding a 1 msec periodic timerfd handle to
the epoll set while still using SCHED_OTHER.

So it seems I have two solutions when using a new kernel so I'm
satisfied.  I'll see if I can clean up my patch to fix the broken epoll
behavior and send it in.

Thanks,
Shawn
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