linux-kernel - Re: lmbench lat_mmap slowdown with CONFIG

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Date:	Wed, 21 Jan 2009 08:27:18 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jeremy Fitzhardinge <jeremy@...p.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>, hpa@...or.com,
	jeremy@...source.com, chrisw@...s-sol.org, zach@...are.com,
	rusty@...tcorp.com.au, Andrew Morton <akpm@...ux-foundation.org>,
	Xen-devel <xen-devel@...ts.xensource.com>
Subject: Re: lmbench lat_mmap slowdown with CONFIG_PARAVIRT

On Tue, Jan 20, 2009 at 09:56:53PM +0100, Ingo Molnar wrote:
> 
> * Jeremy Fitzhardinge <jeremy@...p.org> wrote:
> >> |       50234816 |     51303520  cache references            +2.12%
> >> |        5428258 |      5583728  cache misses                +2.86%
> >>   
> >
> > Is this I or D, or combined?
> 
> That's last-level-cache references+misses (L2 cache):
> 
>  Bit Position Event Name                UMask Event Select
>  CPUID.AH.EBX
>  3            LLC Reference             4FH   2EH
>  4            LLC Misses                41H   2EH

Oh, _llc_ references/misses? Ouch.

You have, what 32K L1I, 32K L1D, and 4MB L2? And even this microbenchmark
is seeing increased L2 misses by nearly 3%. Hmm, I wonder where that is
coming from? Instruction fetches?

It would be interesting to see how "the oltp" benchmark fares with
CONFIG_PARAVIRT turned on. That workload lives and dies by the cache :)
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