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

To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Subject: Re: [PATCH v4 0/8] Avoid cache trashing on clearing huge/gigantic page
From: Andrew Morton <akpm@linux-foundation.org>
Date: Thu, 13 Sep 2012 16:05:06 -0700
Cc: linux-mm@kvack.org, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, x86@kernel.org, Andi Kleen <ak@linux.intel.com>, Tim Chen <tim.c.chen@linux.intel.com>, Alex Shi <alex.shu@intel.com>, Jan Beulich <jbeulich@novell.com>, Robert Richter <robert.richter@amd.com>, Andy Lutomirski <luto@amacapital.net>, Andrea Arcangeli <aarcange@redhat.com>, Johannes Weiner <hannes@cmpxchg.org>, Hugh Dickins <hughd@google.com>, KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>, Mel Gorman <mgorman@suse.de>, linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org, linux-mips@linux-mips.org, linux-sh@vger.kernel.org, sparclinux@vger.kernel.org
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On Mon, 20 Aug 2012 16:52:29 +0300
"Kirill A. Shutemov" <kirill.shutemov@linux.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!



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