linux-kernel - Re: lmbench lat_mmap slowdown with CONFIG

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Date:	Thu, 22 Jan 2009 14:28:31 -0800
From:	Zachary Amsden <zach@...are.com>
To:	Jeremy Fitzhardinge <jeremy@...p.org>
Cc:	Nick Piggin <npiggin@...e.de>, Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"jeremy@...source.com" <jeremy@...source.com>,
	"chrisw@...s-sol.org" <chrisw@...s-sol.org>,
	"rusty@...tcorp.com.au" <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 Wed, 2009-01-21 at 14:23 -0800, Jeremy Fitzhardinge wrote:
> Nick Piggin wrote:
> > 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?
> >   
> 
> I assume so.  There should be no extra data accesses with 
> CONFIG_PARAVIRT (hm, there's probably some extra stack/spill traffic, 
> but I surely hope that's not falling out of cache).

These fragments, from native_pgd_val, certainly don't help:

c0120f60:       55                      push   %ebp
c0120f61:       89 e5                   mov    %esp,%ebp
c0120f63:       5d                      pop    %ebp
c0120f64:       c3                      ret
c0120f65:       8d 74 26 00             lea    0x0(%esi,%eiz,1),%esi
c0120f69:       8d bc 27 00 00 00 00    lea    0x0(%edi,%eiz,1),%edi

That is really disgusting.  We absolutely should be patching away the
function calls here in the native case.. not sure we do that today.

Zach

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