linux-kernel - Re: [PATCH] mm: cache largest vma

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Date:	Sun, 3 Nov 2013 23:03:28 -0800
From:	Christoph Hellwig <hch@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Davidlohr Bueso <davidlohr@...com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	Ingo Molnar <mingo@...nel.org>, Mel Gorman <mgorman@...e.de>,
	Rik van Riel <riel@...hat.com>,
	Guan Xuetao <gxt@...c.pku.edu.cn>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	linux-mm <linux-mm@...ck.org>, Dave Chinner <dchinner@...hat.com>
Subject: Re: [PATCH] mm: cache largest vma

On Sun, Nov 03, 2013 at 10:51:27AM -0800, Linus Torvalds wrote:
> Ugh. This patch makes me angry. It looks way too ad-hoc.
> 
> I can well imagine that our current one-entry cache is crap and could
> be improved, but this looks too random. Different code for the
> CONFIG_MMU case? Same name, but for non-MMU it's a single entry, for
> MMU it's an array? And the whole "largest" just looks odd. Plus why do
> you set LAST_USED if you also set LARGEST?
> 
> Did you try just a two- or four-entry pseudo-LRU instead, with a
> per-thread index for "last hit"? Or even possibly a small fixed-size
> hash table (say "idx = (add >> 10) & 3" or something)?

Btw, Dave Chiner has recently implemented a simple look aside cache for
the buffer cache, which also uses a rbtree.  Might beworth into making
that into a generic library and use it here:

	http://thread.gmane.org/gmane.comp.file-systems.xfs.general/56220
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