linux-kernel - Re: [Bug #11308] tbench regression on each kernel release from 2.6.22 -> 2.6.28

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Message-ID: <alpine.LFD.2.00.0811171218470.18283@nehalem.linux-foundation.org>
Date:	Mon, 17 Nov 2008 12:30:00 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	David Miller <davem@...emloft.net>
cc:	mingo@...e.hu, dada1@...mosbay.com, rjw@...k.pl,
	linux-kernel@...r.kernel.org, kernel-testers@...r.kernel.org,
	cl@...ux-foundation.org, efault@....de, a.p.zijlstra@...llo.nl,
	shemminger@...tta.com
Subject: Re: [Bug #11308] tbench regression on each kernel release from 2.6.22
 -&gt; 2.6.28

On Mon, 17 Nov 2008, David Miller wrote:
> 
> It's on my workstation which is a much simpler 2 processor
> UltraSPARC-IIIi (1.5Ghz) system.

Ok. It could easily be something like a cache footprint issue. And while I 
don't know my sparc cpu's very well, I think the Ultrasparc-IIIi is super- 
scalar but does no out-of-order and speculation, no? So I could easily see 
that the indirect branches in the scheduler hurt much more, and might 
explain why the x86 profile looks so different.

One thing that non-NMI profiles also tend to show is "clumping", which in 
turn tends to rather excessively pinpoint code sequences that release the 
irq flag - just because those points show up in profiles, rather than 
being a spread-out-mush. So it's possible that Ingo's profile did show the 
scheduler more, but it was in the form of much more spread out "noise" 
rather than the single spike you saw. 

		Linus
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