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Date: Wed, 21 Jul 2010 10:37:20 -0700 (PDT)
From: Dan Magenheimer <dan.magenheimer@oracle.com>
Subject: RE: [PATCH 0/8] zcache: page cache compression support
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Cc: Pekka Enberg <penberg@cs.helsinki.fi>, Hugh Dickins <hugh.dickins@tiscali.co.uk>, Andrew Morton <akpm@linux-foundation.org>, Greg KH <greg@kroah.com>, Rik van Riel <riel@redhat.com>, Avi Kivity <avi@redhat.com>, Christoph Hellwig <hch@infradead.org>, Minchan Kim <minchan.kim@gmail.com>, Konrad Wilk <konrad.wilk@oracle.com>, linux-mm <linux-mm@kvack.org>, linux-kernel <linux-kernel@vger.kernel.org>
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> > Maybe the best solution is to make the threshold a sysfs
> > settable?  Or maybe BOTH the single-page threshold and
> > the average threshold as two different sysfs settables?
> > E.g. throw away a put page if either it compresses poorly
> > or adding it to the pool would push the average over.
>=20
> Considering overall compression average instead of bothering about
> individual page compressibility seems like a good point. Still, I think
> storing completely incompressible pages isn't desirable.
>=20
> So, I agree with the idea of separate sysfs tunables for average and
> single-page
> compression thresholds with defaults conservatively set to 50% and
> PAGE_SIZE/2
> respectively. I will include these in "v2" patches.

Unless the single-page compression threshold is higher than the
average, the average is useless.  IMHO I'd suggest at least
5*PAGE_SIZE/8 as the single-page threshold, possibly higher.

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