linux-kernel - Re: lmbench ctxsw regression with CFS

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Date:	Sat, 4 Aug 2007 08:50:37 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: lmbench ctxsw regression with CFS

* Nick Piggin <npiggin@...e.de> wrote:

> Oh good. Thanks for getting to the bottom of it. We have normally 
> disliked too much runtime tunables in the scheduler, so I assume these 
> are mostly going away or under a CONFIG option for 2.6.23? Or...?

yeah, they are all already under CONFIG_SCHED_DEBUG. (it's just that the 
add-on optimization is not upstream yet - the tunings are still being 
tested) Btw., with SCHED_DEBUG we now also have your domain-tree sysctl 
patch upstream, which has been in -mm for a near eternity.

> What CPU did you get these numbers on? Do the indirect calls hurt much 
> on those without an indirect predictor? (I'll try running some tests).

it was on an older Athlon64 X2. I never saw indirect calls really 
hurting on modern x86 CPUs - dont both CPU makers optimize them pretty 
efficiently? (as long as the target function is always the same - which 
it is here.)

> I must say that I don't really like the indirect calls a great deal, 
> and they could be eliminated just with a couple of branches and direct 
> calls.

yeah - i'll try that too. We can make the indirect call the uncommon 
case and a NULL pointer be the common case, combined with a 'default', 
direct function call. But i doubt it makes a big (or even measurable) 
difference.

	Ingo
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