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

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Date:	Thu, 21 Nov 2013 11:50:47 +0800
From:	Weijie Yang <weijie.yang.kh@...il.com>
To:	Bob Liu <bob.liu@...cle.com>
Cc:	Dan Streetman <ddstreet@...e.org>,
	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

Hello Dan,

On Thu, Nov 21, 2013 at 8:42 AM, 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.
>>

Thanks for your work. I reviewed this patch, and it is good to me.

However, I am skeptical about it because:
1. it will add more IO than original zswap, how does it result in a
performance improvement ?
2. most embedded device use NAND, more IO will reduce its working life

Regards

> Could you do some testing using eg. SPECjbb? And compare the result with
> original zswap.
>
> Thanks,
> -Bob
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