zram: extend zero pages to same element pages
authorzhouxianrong <zhouxianrong@huawei.com>
Fri, 24 Feb 2017 22:59:27 +0000 (14:59 -0800)
committerLinus Torvalds <torvalds@linux-foundation.org>
Sat, 25 Feb 2017 01:46:56 +0000 (17:46 -0800)
commit8e19d540d107ee897eb9a874844060c94e2376c0
treea268e406f52137cf0f81150c8f27b0c9100c6bfb
parent517663edd6b5d2a822469885994f34e092e2cf9f
zram: extend zero pages to same element pages

The idea is that without doing more calculations we extend zero pages to
same element pages for zram.  zero page is special case of same element
page with zero element.

1. the test is done under android 7.0
2. startup too many applications circularly
3. sample the zero pages, same pages (none-zero element)
   and total pages in function page_zero_filled

the result is listed as below:

ZERO SAME TOTAL
36214 17842 598196

ZERO/TOTAL  SAME/TOTAL   (ZERO+SAME)/TOTAL ZERO/SAME
AVERAGE 0.060631909  0.024990816  0.085622726 2.663825038
STDEV 0.00674612  0.005887625  0.009707034 2.115881328
MAX 0.069698422  0.030046087  0.094975336 7.56043956
MIN 0.03959586  0.007332205  0.056055193 1.928985507

from the above data, the benefit is about 2.5% and up to 3% of total
swapout pages.

The defect of the patch is that when we recovery a page from non-zero
element the operations are low efficient for partial read.

This patch extends zero_page to same_page so if there is any user to
have monitored zero_pages, he will be surprised if the number is
increased but it's not harmful, I believe.

[minchan@kernel.org: do not free same element pages in zram_meta_free]
Link: http://lkml.kernel.org/r/20170207065741.GA2567@bbox
Link: http://lkml.kernel.org/r/1483692145-75357-1-git-send-email-zhouxianrong@huawei.com
Link: http://lkml.kernel.org/r/1486307804-27903-1-git-send-email-minchan@kernel.org
Signed-off-by: zhouxianrong <zhouxianrong@huawei.com>
Signed-off-by: Minchan Kim <minchan@kernel.org>
Cc: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Cc: Joonsoo Kim <iamjoonsoo.kim@lge.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Documentation/blockdev/zram.txt
drivers/block/zram/zram_drv.c
drivers/block/zram/zram_drv.h