[RFC][PATCH 10/10] zsmalloc: lower ZS_ALMOST_FULL waterline

Sergey Senozhatsky
2015-05-29T15:09:02Z

get_fullness_group() considers 3/4 full pages as almost empty.
that, unfortunately, marks as ALMOST_EMPTY pages that we would
probably like to keep in ALMOST_FULL list.

ALMOST_EMPTY:
[..]
inuse: 3 max_objexts: 4
inuse: 5 max_objexts: 7
inuse: 5 max_objexts: 7
inuse: 2 max_objexts: 3
[..]

so, for "inuse: 5 max_objexts: 7" ALMOST_EMPTY page, for example,
it'll take 2 obj_malloc to make the page FULL and 5 obj_free to
make it EMPTY. compaction selects ALMOST_EMPTY pages as source
pages, which can result in extra object moves.

iow, in terms of compaction, it makes more sense to fill this
page, rather than drain it.

decrease ALMOST_FULL waterline to 2/3 of max capacity; which is,
of course, still imperfect.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
mm/zsmalloc.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index 0524c4a..a8a3eae 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -196,7 +196,7 @@ static int zs_size_classes;
*
* (see: fix_fullness_group())
*/
-static const int fullness_threshold_frac = 4;
+static const int fullness_threshold_frac = 3;

struct size_class {
spinlock_t lock;
@@ -612,7 +612,7 @@ static enum fullness_group get_fullness_group(struct page *page)
fg = ZS_EMPTY;
else if (inuse == max_objects)
fg = ZS_FULL;
- else if (inuse <= 3 * max_objects / fullness_threshold_frac)
+ else if (inuse <= 2 * max_objects / fullness_threshold_frac)
fg = ZS_ALMOST_EMPTY;
else
fg = ZS_ALMOST_FULL;
--
2.4.2.337.gfae46aa