linux-kernel - Re: [PATCH part2 v6 0/3] staging: zcache: Support zero-filled pages more efficiently

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Date:	Sun, 07 Apr 2013 17:12:47 +0800
From:	Ric Mason <ric.masonn@...il.com>
To:	Wanpeng Li <liwanp@...ux.vnet.ibm.com>
CC:	Dan Magenheimer <dan.magenheimer@...cle.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Seth Jennings <sjenning@...ux.vnet.ibm.com>,
	Konrad Rzeszutek Wilk <konrad@...nok.org>,
	Minchan Kim <minchan@...nel.org>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org,
	Geert Uytterhoeven <geert@...ux-m68k.org>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Bob Liu <bob.liu@...cle.com>
Subject: Re: [PATCH part2 v6 0/3] staging: zcache: Support zero-filled pages
 more efficiently

cc Bob
On 04/07/2013 05:03 PM, Wanpeng Li wrote:
> On Wed, Apr 03, 2013 at 06:16:20PM +0800, Wanpeng Li wrote:
>> Changelog:
>> v5 -> v6:
>>   * shove variables in debug.c and in debug.h just have an extern, spotted by Konrad
>>   * update patch description, spotted by Konrad
>> v4 -> v5:
>>   * fix compile error, reported by Fengguang, Geert
>>   * add check for !is_ephemeral(pool), spotted by Bob
>> v3 -> v4:
>>   * handle duplication in page_is_zero_filled, spotted by Bob
>>   * fix zcache writeback in dubugfs
>>   * fix pers_pageframes|_max isn't exported in debugfs
>>   * fix static variable defined in debug.h but used in multiple C files
>>   * rebase on Greg's staging-next
>> v2 -> v3:
>>   * increment/decrement zcache_[eph|pers]_zpages for zero-filled pages, spotted by Dan
>>   * replace "zero" or "zero page" by "zero_filled_page", spotted by Dan
>> v1 -> v2:
>>   * avoid changing tmem.[ch] entirely, spotted by Dan.
>>   * don't accumulate [eph|pers]pageframe and [eph|pers]zpages for
>>     zero-filled pages, spotted by Dan
>>   * cleanup TODO list
>>   * add Dan Acked-by.
>>
> Hi Dan,
>
> Some issues against Ramster:
>
> - Ramster who takes advantage of zcache also should support zero-filled
>    pages more efficiently, correct? It doesn't handle zero-filled pages well
>    currently.
> - Ramster DebugFS counters are exported in /sys/kernel/mm/, but zcache/frontswap/cleancache
>    all are exported in /sys/kernel/debug/, should we unify them?
> - If ramster also should move DebugFS counters to a single file like
>    zcache do?
>
> If you confirm these issues are make sense to fix, I will start coding. ;-)
>
> Regards,
> Wanpeng Li
>
>> Motivation:
>>
>> - Seth Jennings points out compress zero-filled pages with LZO(a lossless
>>   data compression algorithm) will waste memory and result in fragmentation.
>>   https://lkml.org/lkml/2012/8/14/347
>> - Dan Magenheimer add "Support zero-filled pages more efficiently" feature
>>   in zcache TODO list https://lkml.org/lkml/2013/2/13/503
>>
>> Design:
>>
>> - For store page, capture zero-filled pages(evicted clean page cache pages and
>>   swap pages), but don't compress them, set pampd which store zpage address to
>>   0x2(since 0x0 and 0x1 has already been ocuppied) to mark special zero-filled
>>   case and take advantage of tmem infrastructure to transform handle-tuple(pool
>>   id, object id, and an index) to a pampd. Twice compress zero-filled pages will
>>   contribute to one zcache_[eph|pers]_pageframes count accumulated.
>> - For load page, traverse tmem hierachical to transform handle-tuple to pampd
>>   and identify zero-filled case by pampd equal to 0x2 when filesystem reads
>>   file pages or a page needs to be swapped in, then refill the page to zero
>>   and return.
>>
>> Test:
>>
>> dd if=/dev/zero of=zerofile bs=1MB count=500
>> vmtouch -t zerofile
>> vmtouch -e zerofile
>>
>> formula:
>> - fragmentation level = (zcache_[eph|pers]_pageframes * PAGE_SIZE - zcache_[eph|pers]_zbytes)
>>   * 100 / (zcache_[eph|pers]_pageframes * PAGE_SIZE)
>> - memory zcache occupy = zcache_[eph|pers]_zbytes
>>
>> Result:
>>
>> without zero-filled awareness:
>> - fragmentation level: 98%
>> - memory zcache occupy: 238MB
>> with zero-filled awareness:
>> - fragmentation level: 0%
>> - memory zcache occupy: 0MB
>>
>> Wanpeng Li (3):
>>   staging: zcache: fix static variables defined in debug.h but used in
>>     mutiple C files
>>   staging: zcache: introduce zero-filled page stat count
>>   staging: zcache: clean TODO list
>>
>> drivers/staging/zcache/TODO          |    3 +-
>> drivers/staging/zcache/debug.c       |   35 +++++++++++++++
>> drivers/staging/zcache/debug.h       |   79 ++++++++++++++++++++-------------
>> drivers/staging/zcache/zcache-main.c |    4 ++
>> 4 files changed, 88 insertions(+), 33 deletions(-)
>>
>> -- 
>> 1.7.5.4
> --
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