linux-kernel - Re: [PATCH v2] mm/zswap: change zswap to writethrough cache

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Message-ID: <CALZtONDyXdmzF_K7X+GNA0wY1yDn7k3t1FmQq4J=8=smX9XSoA@mail.gmail.com>
Date:	Thu, 21 Nov 2013 17:38:49 -0500
From:	Dan Streetman <ddstreet@...e.org>
To:	Bob Liu <bob.liu@...cle.com>
Cc:	Seth Jennings <sjennings@...iantweb.net>, linux-mm@...ck.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Minchan Kim <minchan@...nel.org>,
	Weijie Yang <weijie.yang@...sung.com>
Subject: Re: [PATCH v2] mm/zswap: change zswap to writethrough cache

On Wed, Nov 20, 2013 at 7:42 PM, Bob Liu <bob.liu@...cle.com> wrote:
> Hi Dan,
>
> On 11/21/2013 03:49 AM, Dan Streetman wrote:
>> Currently, zswap is writeback cache; stored pages are not sent
>> to swap disk, and when zswap wants to evict old pages it must
>> first write them back to swap cache/disk manually.  This avoids
>> swap out disk I/O up front, but only moves that disk I/O to
>> the writeback case (for pages that are evicted), and adds the
>> overhead of having to uncompress the evicted pages, and adds the
>> need for an additional free page (to store the uncompressed page)
>> at a time of likely high memory pressure.  Additionally, being
>> writeback adds complexity to zswap by having to perform the
>> writeback on page eviction.
>>
>
> Good work!
>
>> This changes zswap to writethrough cache by enabling
>> frontswap_writethrough() before registering, so that any
>> successful page store will also be written to swap disk.  All the
>> writeback code is removed since it is no longer needed, and the
>> only operation during a page eviction is now to remove the entry
>> from the tree and free it.
>>
>
> Could you do some testing using eg. SPECjbb? And compare the result with
> original zswap.

Sure, I have a small test program that I used for performance
comparisions, which I'll send and include some results, and I'll also
try to find a copy of SPECjbb to get results with.
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