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mm/zswap.c: add allocation hysteresis if pool limit is hit

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zswap will always try to shrink pool when zswap is full.  If there is a
high pressure on zswap it will result in flipping pages in and out zswap
pool without any real benefit, and the overall system performance will
drop.  The previous discussion on this subject [1] ended up with a
suggestion to implement a sort of hysteresis to refuse taking pages into
zswap pool until it has sufficient space if the limit has been hit.
This is my take on this.

Hysteresis is controlled with a sysfs-configurable parameter (namely,
/sys/kernel/debug/zswap/accept_threhsold_percent).  It specifies the
threshold at which zswap would start accepting pages again after it
became full.  Setting this parameter to 100 disables the hysteresis and
sets the zswap behavior to pre-hysteresis state.

[1] https://lkml.org/lkml/2019/11/8/949

Link: http://lkml.kernel.org/r/20200108200118.15563-1-vitaly.wool@konsulko.com
Signed-off-by: Vitaly Wool <vitaly.wool@konsulko.com>
Cc: Dan Streetman <ddstreet@ieee.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Vitaly Wool
2020-01-30 22:15:04 -08:00
committed by Linus Torvalds
parent 3d680bdf60
commit 45190f01dd
2 changed files with 68 additions and 32 deletions
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13
Documentation/vm/zswap.rst
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@@ -130,6 +130,19 @@ checking for the same-value filled pages during store operation. However, the
existing pages which are marked as same-value filled pages remain stored
unchanged in zswap until they are either loaded or invalidated.
To prevent zswap from shrinking pool when zswap is full and there's a high
pressure on swap (this will result in flipping pages in and out zswap pool
without any real benefit but with a performance drop for the system), a
special parameter has been introduced to implement a sort of hysteresis to
refuse taking pages into zswap pool until it has sufficient space if the limit
has been hit. To set the threshold at which zswap would start accepting pages
again after it became full, use the sysfs ``accept_threhsold_percent``
attribute, e. g.::
echo 80 > /sys/module/zswap/parameters/accept_threhsold_percent
Setting this parameter to 100 will disable the hysteresis.
A debugfs interface is provided for various statistic about pool size, number
of pages stored, same-value filled pages and various counters for the reasons
pages are rejected.