Re: [PATCH part2 v6 0/3] staging: zcache: Support zero-filled pagesmore efficiently

[Ric Mason]

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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