Re: [PATCH v4 0/8] Avoid cache trashing on clearing huge/giganticpage

From: Ingo Molnar
Date: Fri Sep 14 2012 - 01:52:13 EST

* Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx> wrote:

> On Mon, 20 Aug 2012 16:52:29 +0300
> "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> 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


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

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.


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