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

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Message-Id: <20081117.125826.193693115.davem@davemloft.net>
Date:	Mon, 17 Nov 2008 12:58:26 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	torvalds@...ux-foundation.org
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

From: Linus Torvalds <torvalds@...ux-foundation.org>
Date: Mon, 17 Nov 2008 12:30:00 -0800 (PST)

> 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?

I does only very simple speculation, but you're description is accurate.

> 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.

Right.

> 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. 

Sure.
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