[PATCH v2 0/4] zcache: Support zero-filled pages more efficiently

[Wanpeng Li]

Changelog:
 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.

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 (4):
  introduce zero-filled pages handler
  zero-filled pages awareness
  introduce zero-filled pages stat count
  clean TODO list

 drivers/staging/zcache/TODO          |    3 +-
 drivers/staging/zcache/zcache-main.c |  119 ++++++++++++++++++++++++++++++++--
 2 files changed, 114 insertions(+), 8 deletions(-)

-- 
1.7.7.6

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