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Date: Thu, 28 Feb 2013 10:13:28 -0800 (PST)
From: Dan Magenheimer <dan.magenheimer@oracle.com>
Subject: RE: [PATCHv6 4/8] zswap: add to mm/
References: <<1361397888-14863-1-git-send-email-sjenning@linux.vnet.ibm.com>>
 <<1361397888-14863-5-git-send-email-sjenning@linux.vnet.ibm.com>>
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To: Seth Jennings <sjenning@linux.vnet.ibm.com>, Andrew Morton <akpm@linux-foundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Nitin Gupta <ngupta@vflare.org>, Minchan Kim <minchan@kernel.org>, Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>, Dan Magenheimer <dan.magenheimer@oracle.com>, Robert Jennings <rcj@linux.vnet.ibm.com>, Jenifer Hopper <jhopper@us.ibm.com>, Mel Gorman <mgorman@suse.de>, Johannes Weiner <jweiner@redhat.com>, Rik van Riel <riel@redhat.com>, Larry Woodman <lwoodman@redhat.com>, Benjamin Herrenschmidt <benh@kernel.crashing.org>, Dave Hansen <dave@linux.vnet.ibm.com>, Joe Perches <joe@perches.com>, Joonsoo Kim <iamjoonsoo.kim@lge.com>, Cody P Schafer <cody@linux.vnet.ibm.com>, linux-mm@kvack.org, linux-kernel@vger.kernel.org, devel@driverdev.osuosl.org

> From: Seth Jennings [mailto:sjenning@linux.vnet.ibm.com]
> Subject: [PATCHv6 4/8] zswap: add to mm/
>=20
> +/*
> + * Maximum compression ratio, as as percentage, for an acceptable
> + * compressed page. Any pages that do not compress by at least
> + * this ratio will be rejected.
> +*/
> +static unsigned int zswap_max_compression_ratio =3D 80;
> +module_param_named(max_compression_ratio,
> +=09=09=09zswap_max_compression_ratio, uint, 0644);

Unless this is a complete coincidence, I believe that
the default value "80" is actually:

(100 * (1L >> ZS_MAX_ZSPAGE_ORDER)) /
        ((1L >> ZS_MAX_ZSPAGE_ORDER)) + 1)

(though the constant ZS_MAX_ZSPAGE_ORDER is not currently
defined outside of zsmalloc.c) because pages that compress
less efficiently than this always require a full pageframe
in zsmalloc.  True?

If this change were made, is there any real reason for this
to be a user-selectable parameter, i.e. given the compression
internals knowledge necessary to understand what value should
be selected, would any mortal sysadmin ever want to change it
or know what would be a reasonable value to change it to?

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