linux-kernel - RE: [PATCHv6 4/8] zswap: add to mm/

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Date:	Thu, 28 Feb 2013 10:13:28 -0800 (PST)
From:	Dan Magenheimer <dan.magenheimer@...cle.com>
To:	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Minchan Kim <minchan@...nel.org>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>,
	Jenifer Hopper <jhopper@...ibm.com>,
	Mel Gorman <mgorman@...e.de>,
	Johannes Weiner <jweiner@...hat.com>,
	Rik van Riel <riel@...hat.com>,
	Larry Woodman <lwoodman@...hat.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Dave Hansen <dave@...ux.vnet.ibm.com>,
	Joe Perches <joe@...ches.com>,
	Joonsoo Kim <iamjoonsoo.kim@....com>,
	Cody P Schafer <cody@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: RE: [PATCHv6 4/8] zswap: add to mm/

> From: Seth Jennings [mailto:sjenning@...ux.vnet.ibm.com]
> Subject: [PATCHv6 4/8] zswap: add to mm/
> 
> +/*
> + * 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 = 80;
> +module_param_named(max_compression_ratio,
> +			zswap_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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