linux-kernel - Re: [PATCHv4 0/7] zswap: compressed swap caching

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Date:	Mon, 04 Feb 2013 09:07:56 -0600
From:	Seth Jennings <sjenning@...ux.vnet.ibm.com>
To:	Simon Jeons <simon.jeons@...il.com>
CC:	Andrew Morton <akpm@...ux-foundation.org>,
	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>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, devel@...verdev.osuosl.org
Subject: Re: [PATCHv4 0/7] zswap: compressed swap caching

On 02/03/2013 07:03 PM, Simon Jeons wrote:
> On Fri, 2013-02-01 at 09:13 -0600, Seth Jennings wrote:
>> On 01/31/2013 07:39 PM, Simon Jeons wrote:
>>> Hi Seth,
>>> On Tue, 2013-01-29 at 15:40 -0600, Seth Jennings wrote:
>> <snip>
>>>> Performance, Kernel Building:
>>>>
>>>> Setup
>>>> ========
>>>> Gentoo w/ kernel v3.7-rc7
>>>> Quad-core i5-2500 @ 3.3GHz
>>>> 512MB DDR3 1600MHz (limited with mem=512m on boot)
>>>> Filesystem and swap on 80GB HDD (about 58MB/s with hdparm -t)
>>>> majflt are major page faults reported by the time command
>>>> pswpin/out is the delta of pswpin/out from /proc/vmstat before and after
>>>> then make -jN
>>>>
>>>> Summary
>>>> ========
>>>> * Zswap reduces I/O and improves performance at all swap pressure levels.
>>>>
>>>> * Under heavy swaping at 24 threads, zswap reduced I/O by 76%, saving
>>>>   over 1.5GB of I/O, and cut runtime in half.
>>>
>>> How to get your benchmark?
>>
>> It's just kernel building.  So "make" :)
>>
>> I intentionally choose this workload so people wouldn't have to jump
>> through hoops to replicate the results.
> 
> Since there already have zram which can handle anonymous pages
> compression, why need zswap? What's the difference of design between
> zram and zswap? 

zram is implemented is a virtual block device.  It interfaces with the
block device layer, not the swap code.  In fact, zram can be used as a
generic compressed RAM disk, not only for compressed swap.  One can
think of it as a RAM disk + compression.

So zram is the actual swap device while zswap is a caching layer above
the swap device.  zswap is not the swap device itself like zram.

Hope this clears up the difference :)

Seth

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