linux-kernel - Re: [PATCH 0/4] promote zcache from staging

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Message-ID: <502BB125.7030607@linux.vnet.ibm.com>
Date:	Wed, 15 Aug 2012 09:24:37 -0500
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
CC:	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Nitin Gupta <ngupta@...are.org>,
	Minchan Kim <minchan@...nel.org>,
	Robert Jennings <rcj@...ux.vnet.ibm.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org,
	Kurt Hackel <kurt.hackel@...cle.com>
Subject: Re: [PATCH 0/4] promote zcache from staging

On 08/15/2012 04:38 AM, Konrad Rzeszutek Wilk wrote:
> On Fri, Aug 10, 2012 at 01:14:01PM -0500, Seth Jennings wrote:
>> On 08/09/2012 03:20 PM, Dan Magenheimer wrote
>>> I also wonder if you have anything else unusual in your
>>> test setup, such as a fast swap disk (mine is a partition
>>> on the same rotating disk as source and target of the kernel build,
>>> the default install for a RHEL6 system)?
>>
>> I'm using a normal SATA HDD with two partitions, one for
>> swap and the other an ext3 filesystem with the kernel source.
>>
>>> Or have you disabled cleancache?
>>
>> Yes, I _did_ disable cleancache.  I could see where having
>> cleancache enabled could explain the difference in results.
> 
> Why did you disable the cleancache? Having both (cleancache
> to compress fs data) and frontswap (to compress swap data) is the
> goal - while you turned one of its sources off.

I excluded cleancache to reduce interference/noise from the
benchmarking results. For this particular workload,
cleancache doesn't make a lot of sense since it will steal
pages that could otherwise be used for storing frontswap
pages to prevent swapin/swapout I/O.

In a test run with both enabled, I found that it didn't make
much difference under moderate to extreme memory pressure.
Both resulted in about 55% I/O reduction.  However, on light
memory pressure with 8 and 12 threads, it lowered the I/O
reduction ability of zcache to roughly 0 compared to ~20%
I/O reduction without cleancache.

In short, cleancache only had the power to harm in this
case, so I didn't enable it.

Seth

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