linux-kernel - Re: Update cacheline size on X86

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Date:	Sat, 11 Oct 2008 22:42:27 +1100
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Dave Jones <davej@...hat.com>, x86@...nel.org,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: Update cacheline size on X86_GENERIC

On Saturday 11 October 2008 22:22, Andi Kleen wrote:
> On Sat, Oct 11, 2008 at 07:29:19PM +1100, Nick Piggin wrote:
> > I also think there are reasonable arguments the other way, and I
> > personally also think it might be better to leave it 128 (even
> > if it is unlikely, introducing a regression is not good).
>
> The issue is also that the regression will be likely large.

Yeah, that is what I'm worried about. If it was a simple case of
1% loss on P4 for 1% gain on Core2, it would be a good change.
But it might be huge losses on P4s.


> False sharing can really hurt when it hits as you know, because
> the penalties are so large.
>
> > > There are millions and millions of P4s around.
> > > And they're not that old, they're still shipping in fact.
> >
> > Still shipping in anything aside from 1s systems?
>
> Remember the first Core2 based 4S (Tigerton) Xeon was only introduced last
> year and that market is quite conservative. For 2S it's a bit longer, but
> it wouldn't surprise me there if new systems are still shipping.
>
> Also to be honest I doubt the theory that older systems
> are never upgraded to newer OS.

Yeah, fair enough.


> > That would be nice. It would be interesting to know what is causing
> > the slowdown.
>
> At least that test is extremly cache footprint sensitive. A lot of the
> cache misses are surprisingly in hd_struct, because it runs
> with hundred of disks and each needs hd_struct references in the fast path.
> The recent introduction of fine grained per partition statistics
> caused a large slowdown. But I don't think kernel workloads
> are normally that extremly cache sensitive.

That's interesting. struct device is pretty big. I wonder if fields
couldn't be rearranged to minimise the fastpath cacheline footprint?
I guess that's already been looked at?
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