linux-kernel - Re: [RFC patch 1/2] sched: dynamically adapt granularity with nr

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Date:	Mon, 13 Sep 2010 16:56:28 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Mike Galbraith <efault@....de>,
	LKML <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Tony Lindgren <tony@...mide.com>
Subject: Re: [RFC patch 1/2] sched: dynamically adapt granularity with
	nr_running

* Mathieu Desnoyers (mathieu.desnoyers@...icios.com) wrote:
> * Ingo Molnar (mingo@...e.hu) wrote:
> > 
> > * Mike Galbraith <efault@....de> wrote:
> > 
> > > On Sun, 2010-09-12 at 14:16 -0400, Mathieu Desnoyers wrote:
> > 
> > > > Or am I missing your point ?
> > > 
> > > Yes and no.  I'm pondering the parent, but by the same token, the 
> > > vfork child shouldn't be penalized either.
> > > 
> > > Does your latency go down drastically if you turn START_DEBIT off? 
> > > Seems like it should.  Perhaps START_DEBIT should not start a task 
> > > further right than rightmost.  I've done that before.
> > > 
> > > maximum latency: 19221.5 µs
> > > average latency: 5159.0 µs
> > > missed timer events: 0
> > > 
> > > maximum latency: 43901.0 µs
> > > average latency: 8430.1 µs
> > > missed timer events: 0
> > > 
> > > Turning it off here cut latency roughly in half (i've piddled vfork 
> > > though, but not completely).  Limiting child placement to no further 
> > > right than rightmost should help quite a bit.
> > 
> > Very interesting observation. Mathieu, mind testing Mike's suggestion 
> > with wakeup-latency.c?
> 
> Sure. this is with the smaller min_granularity:
> 
> With START_DEBIT:
> 
> maximum latency: 21111.1 µs
> average latency: 4188.2 µs
> missed timer events: 0
> 
> Without:
> 
> maximum latency: 6670.2 µs
> average latency: 1586.0 µs
> missed timer events: 0
> 
> So yes, as expected, it makes a huge difference. This is because SIGEV_THREAD
> creates a new thread each time the timer fires, and newly created tasks are put
> at the end of the runqueue with START_DEBIT.
> 
> However, removing START_DEBIT makes my Xorg feel less responsive (again, just my
> own impression). We might need a more suitable way to deal with forks than just
> putting the newly forked task at the end of the spread, but just putting it at
> the beginning of the spread does not seem to do well neither.
> 
> One idea: we could temporarily tweak the nice value of both the parent and the
> child after a fork to a lower nice value, but only apply this for their first
> slice after the fork. The goal behind this is that their respective vruntime
> will increment faster in the first slice after the fork, so a fork bomb
> (worse-case) will end up running with a very very low nice level. With this
> measure in place, START_DEBIT might not be needed. Thoughts ?

A small note: Steven made me realize that when I say "low nice level" here, I
actually mean "high nice values". Less is more when we talk about nice levels.

Thanks,

Mathieu

> 
> Thanks,
> 
> Mathieu
> 
> > 
> > Thanks,
> > 
> > 	Ingo
> 
> -- 
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant
> EfficiOS Inc.
> http://www.efficios.com

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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