linux-kernel - Re: lmbench ctxsw regression with CFS

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Date:	Mon, 13 Aug 2007 14:30:31 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	Ingo Molnar <mingo@...e.hu>,
	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

On Mon, Aug 06 2007, Nick Piggin wrote:
> > > 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 think a lot of CPUs do. I think ia64 does not. It predicts
> based on the contents of a branch target register which has to
> be loaded I presume before instructoin fetch reaches the branch.
> I don't know if this would hurt or not.

Testing on ia64 showed that the indirect calls in the io scheduler hurt
quite a bit, so I'd be surprised if the impact here wasn't an issue
there.

-- 
Jens Axboe

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