linux-kernel - Re: [PATCH 20/20] Get rid of the concept of hot/cold page freeing

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Message-Id: <20090223013723.1d8f11c1.akpm@linux-foundation.org>
Date:	Mon, 23 Feb 2009 01:37:23 -0800
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Mel Gorman <mel@....ul.ie>
Cc:	Linux Memory Management List <linux-mm@...ck.org>,
	Pekka Enberg <penberg@...helsinki.fi>,
	Rik van Riel <riel@...hat.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Johannes Weiner <hannes@...xchg.org>,
	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Lin Ming <ming.m.lin@...el.com>,
	Zhang Yanmin <yanmin_zhang@...ux.intel.com>
Subject: Re: [PATCH 20/20] Get rid of the concept of hot/cold page freeing

On Sun, 22 Feb 2009 23:17:29 +0000 Mel Gorman <mel@....ul.ie> wrote:

> Currently an effort is made to determine if a page is hot or cold when
> it is being freed so that cache hot pages can be allocated to callers if
> possible. However, the reasoning used whether to mark something hot or
> cold is a bit spurious. A profile run of kernbench showed that "cold"
> pages were never freed so it either doesn't happen generally or is so
> rare, it's barely measurable.
> 
> It's dubious as to whether pages are being correctly marked hot and cold
> anyway. Things like page cache and pages being truncated are are considered
> "hot" but there is no guarantee that these pages have been recently used
> and are cache hot. Pages being reclaimed from the LRU are considered
> cold which is logical because they cannot have been referenced recently
> but if the system is reclaiming pages, then we have entered allocator
> slowpaths and are not going to notice any potential performance boost
> because a "hot" page was freed.
> 
> This patch just deletes the concept of freeing hot or cold pages and
> just frees them all as hot.
> 

Well yes.  We waffled for months over whether to merge that code originally.

What tipped the balance was a dopey microbenchmark which I wrote which
sat in a loop extending (via write()) and then truncating the same file
by 32 kbytes (or thereabouts).  Its performance was increased by a lot
(2x or more, iirc) and no actual regressions were demonstrable, so we
merged it.

Could you check that please?  I'd suggest trying various values of 32k,
too.
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