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Date: Sun, 3 Nov 2013 23:03:28 -0800
From: Christoph Hellwig <hch@infradead.org>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Davidlohr Bueso <davidlohr@hp.com>,
        Andrew Morton <akpm@linux-foundation.org>,
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        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, Dave Chinner <dchinner@redhat.com>
Subject: Re: [PATCH] mm: cache largest vma
Message-ID: <20131104070328.GA17995@infradead.org>
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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