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

David Miller
2008-11-17T20:58:26Z

From: Linus Torvalds <torvalds-de/tnXTf+JLsfHDXvbKv3WD2FQJk+8+b@public.gmane.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.