linux-kernel - Re: [ck] Re: SD still better than CFS for 3d ?(was Re: 2.6.23-rc1)

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Date:	Tue, 31 Jul 2007 16:18:57 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Matthew Hawkins <darthmdh@...il.com>
Cc:	Kenneth Prugh <ken69267@...il.com>, ck@....kolivas.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Kasper Sandberg <lkml@...anurb.dk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [ck] Re: SD still better than CFS for 3d ?(was Re: 2.6.23-rc1)

* Matthew Hawkins <darthmdh@...il.com> wrote:

> On 7/31/07, Ingo Molnar <mingo@...e.hu> wrote:
> > * Kenneth Prugh <ken69267@...il.com> wrote:
> > > CFS generally seemed a lot smoother as the load increased, while 
> > > SD broke down to a highly unstable fps count that fluctuated 
> > > massively around the third loop. Seems like I will stick to CFS 
> > > for gaming now.
>
> My experience was quite similar.  I noticed after launching the second 
> loop that the FPS stuck down to 15 for about 20 seconds, then climbed 
> back up to 48.  After that it went rapidly downhill.  This is similar 
> to other benchmarks I've done of SD versus CFS in the past.  At a 
> "normal" load they're fairly similar but SD breaks down under 
> pressure.

ok, thanks for testing it!

> The only other thing of interest is that the -ck kernel had the WM 
> menus appear in about 3 seconds rather than 5-8 under the other two.

under what load is that - 10 loops? There's no disk or network IO going 
on during a WM menu appearance, correct?

This could be a time-slicing difference perhaps - if you have 
CONFIG_HZ=100 could you change it to 1000 (or if you have it at 1000, 
could you change it to 100) - does it show any sensitivity to that?

the other difference could be SCHED_FEAT_START_DEBIT, does that WM menu 
latency go down if you clear it from sched_features, i.e. to subtract 16 
from sched_features:

  echo 15 > /proc/sys/kernel/sched_features

to restore the default, do:

  echo 31 > /proc/sys/kernel/sched_features

(if you have CONFIG_SCHED_DEBUG=y). You might also want to try changing 
/proc/sys/kernel/sched_granularity_ns. Boundary conditions: make sure 
that if you change the sched_granularity value you also set 
/proc/sys/kernel/sched_runtime_limit_ns to 2*sched_granularity and set 
/proc/sys/kernel/sched_wakeup_granularity_ns to sched_granularity/2. 

Other interesting bits to experiment with in sched_features would be 
SCHED_FEAT_FAIR_SLEEPERS (mask '1' in the bitmask) and 
SCHED_FEAT_SKIP_INITIAL (mask '32' in the bitmask).

	Ingo
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