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

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Message-Id: <20120913160506.d394392a.akpm@linux-foundation.org>
Date:	Thu, 13 Sep 2012 16:05:06 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:	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

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!


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