Mailing List Archive

[PATCH 0/4] zcache: Support zero-filled pages more efficiently
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
add pageframes count once compress zero-filled pages twice

drivers/staging/zcache/tmem.c | 4 +-
drivers/staging/zcache/tmem.h | 5 +
drivers/staging/zcache/zcache-main.c | 141 +++++++++++++++++++++++++++++++---
3 files changed, 139 insertions(+), 11 deletions(-)

--
1.7.7.6

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