linux-kernel - Re: [PATCH v3] mm/zswap: change zswap to writethrough cache

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Message-ID: <CALZtONByDcL+yd3rMfFmrW_a2dV3ZV8XKiWcxba-oWTWeGNVVQ@mail.gmail.com>
Date:	Sat, 23 Nov 2013 15:35:35 -0500
From:	Dan Streetman <ddstreet@...e.org>
To:	Weijie Yang <weijie.yang.kh@...il.com>
Cc:	Seth Jennings <sjennings@...iantweb.net>, linux-mm@...ck.org,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Bob Liu <bob.liu@...cle.com>, Minchan Kim <minchan@...nel.org>,
	Weijie Yang <weijie.yang@...sung.com>
Subject: Re: [PATCH v3] mm/zswap: change zswap to writethrough cache

On Fri, Nov 22, 2013 at 9:37 PM, Weijie Yang <weijie.yang.kh@...il.com> wrote:
> Hello Dan,
>
> On Sat, Nov 23, 2013 at 6:10 AM, Dan Streetman <ddstreet@...e.org> 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.
>>
>> 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.
>
> I agree with Seth, It is not good to embedded device.
> May be we can find its place in others like server.
> I guess it is good to medium workload when swap io is not frequent.
>
> My suggestion is would you please make it configurable so that user
> can choice to use writethrough or writeback mode?
>

Having to support both significantly increases complexity and I think
would make further improvements more difficult.  My opinion is the
writeback code should be removed.  Is there anyone else who thinks
both should be available by a param?
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