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From: Nick Piggin <nickpiggin@yahoo.com.au>
To: Andi Kleen <andi@firstfloor.org>
Subject: Re: Update cacheline size on X86_GENERIC
Date: Fri, 10 Oct 2008 19:45:31 +1100
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Cc: Dave Jones <davej@redhat.com>, x86@kernel.org,
       Linux Kernel <linux-kernel@vger.kernel.org>
References: <20081009171453.GA15321@redhat.com> <200810101428.23662.nickpiggin@yahoo.com.au> <87tzbksxja.fsf@basil.nowhere.org>
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On Friday 10 October 2008 18:46, Andi Kleen wrote:
> Nick Piggin <nickpiggin@yahoo.com.au> writes:
> > On Friday 10 October 2008 04:14, Dave Jones wrote:
> >> I just noticed that configuring a kernel to use CONFIG_X86_GENERIC
> >> (as is typical for a distro kernel) configures it to use a 128 byte
> >> cacheline size. This made sense when that was commonplace (P4 era) but
> >> current
> >> Intel, AMD and VIA cpus use 64 byte cachelines.
> >
> > I think P4 technically did have 64 byte cachelines, but had some adjacent
> > line prefetching.
>
> The "coherency unit" on P4, which is what matters for SMP alignment
> purposes to avoid false sharing, is 128 bytes.
>
> > And AFAIK core2 CPUs can do similar prefetching (but
> > maybe it's smarter and doesn't cause so much bouncing?).
>
> On Core2 the coherency unit is 64bytes.

OK.


> > Anyway, GENERIC kernel should run well on all architectures, and while
> > going too big causes slightly increased structures sometimes, going too
> > small could result in horrible bouncing.
>
> Exactly.
>
> That is it costs one percent or so on TPC, but I think the fix
> for that is just to analyze where the problem is and size those
> data structures based on the runtime cache size. Some subsystems
> like slab do this already.

Costs 1% on TPC? Is that 128 byte aligning data structures on
Core2, or 64 byte aligning them on P4 that costs the performance?


> TPC is a bit of a extreme case because it is so extremly cache bound.

Still, it is a good canary.


> Overall the memory impact of the cache padding is getting less over
> time because more and more data is moving into the per CPU data areas.

Right.
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