linux-kernel - Re: [PATCH v4 0/8] Avoid cache trashing on clearing huge/gigantic page

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Date:	Fri, 14 Sep 2012 07:52:10 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	linux-mm@...ck.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Andi Kleen <ak@...ux.intel.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Alex Shi <alex.shu@...el.com>,
	Jan Beulich <jbeulich@...ell.com>,
	Robert Richter <robert.richter@....com>,
	Andy Lutomirski <luto@...capital.net>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	Mel Gorman <mgorman@...e.de>, linux-kernel@...r.kernel.org,
	linuxppc-dev@...ts.ozlabs.org, linux-mips@...ux-mips.org,
	linux-sh@...r.kernel.org, sparclinux@...r.kernel.org
Subject: Re: [PATCH v4 0/8] Avoid cache trashing on clearing huge/gigantic
 page


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Mon, 20 Aug 2012 16:52:29 +0300
> "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com> wrote:
> 
> > Clearing a 2MB huge page will typically blow away several levels of CPU
> > caches.  To avoid this only cache clear the 4K area around the fault
> > address and use a cache avoiding clears for the rest of the 2MB area.
> > 
> > This patchset implements cache avoiding version of clear_page only for
> > x86. If an architecture wants to provide cache avoiding version of
> > clear_page it should to define ARCH_HAS_USER_NOCACHE to 1 and implement
> > clear_page_nocache() and clear_user_highpage_nocache().
> 
> Patchset looks nice to me, but the changelogs are terribly 
> short of performance measurements.  For this sort of change I 
> do think it is important that pretty exhaustive testing be 
> performed, and that the results (or a readable summary of 
> them) be shown.  And that testing should be designed to probe 
> for slowdowns, not just the speedups!

That is my general impression as well.

Firstly, doing before/after "perf stat --repeat 3 ..." runs 
showing a statistically significant effect on a workload that is 
expected to win from this, and on a workload expected to be 
hurting from this would go a long way towards convincing me.

Secondly, if you can find some user-space simulation of the 
intended positive (and negative) effects then a 'perf bench' 
testcase designed to show weakness of any such approach, running 
the very kernel assembly code in user-space would also be rather 
useful.

See:

comet:~/tip> git grep x86 tools/perf/bench/ | grep inclu
tools/perf/bench/mem-memcpy-arch.h:#include "mem-memcpy-x86-64-asm-def.h"
tools/perf/bench/mem-memcpy-x86-64-asm.S:#include "../../../arch/x86/lib/memcpy_64.S"
tools/perf/bench/mem-memcpy.c:#include "mem-memcpy-x86-64-asm-def.h"
tools/perf/bench/mem-memset-arch.h:#include "mem-memset-x86-64-asm-def.h"
tools/perf/bench/mem-memset-x86-64-asm.S:#include "../../../arch/x86/lib/memset_64.S"
tools/perf/bench/mem-memset.c:#include "mem-memset-x86-64-asm-def.h"

that code uses the kernel-side assembly code and runs it in 
user-space.

Although obviously clearing pages on page faults needs some care 
to properly simulate in user-space.

Without repeatable hard numbers such code just gets into the 
kernel and bitrots there as new CPU generations come in - a few 
years down the line the original decisions often degrade to pure 
noise. We've been there, we've done that, we don't want to 
repeat it.

Thanks,

	Ingo
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