this function checks if class compaction will free any pages. rephrasing, do we have enough unused objects to form at least one ZS_EMPTY page and free it. it aborts compaction if class compaction will not result into any (further) savings. EXAMPLE (this debug output is not part of this patch set): -- class size -- number of allocated objects -- number of used objects, -- estimated number of pages that will be freed [..] [ 3303.108960] class-3072 objs:24652 inuse:24628 objs-per-page:4 pages-tofree:6 [ 3303.108965] class-3072 objs:24648 inuse:24628 objs-per-page:4 pages-tofree:5 [ 3303.108970] class-3072 objs:24644 inuse:24628 objs-per-page:4 pages-tofree:4 [ 3303.108973] class-3072 objs:24640 inuse:24628 objs-per-page:4 pages-tofree:3 [ 3303.108978] class-3072 objs:24636 inuse:24628 objs-per-page:4 pages-tofree:2 [ 3303.108982] class-3072 objs:24632 inuse:24628 objs-per-page:4 pages-tofree:1 [ 3303.108993] class-2720 objs:17970 inuse:17966 objs-per-page:3 pages-tofree:1 [ 3303.108997] class-2720 objs:17967 inuse:17966 objs-per-page:3 pages-tofree:0 [ 3303.108998] class-2720: Compaction is useless [ 3303.109000] class-2448 objs:7680 inuse:7674 objs-per-page:5 pages-tofree:1 [ 3303.109005] class-2336 objs:13510 inuse:13500 objs-per-page:7 pages-tofree:1 [ 3303.109010] class-2336 objs:13503 inuse:13500 objs-per-page:7 pages-tofree:0 [ 3303.109011] class-2336: Compaction is useless [ 3303.109013] class-1808 objs:1161 inuse:1154 objs-per-page:9 pages-tofree:0 [ 3303.109014] class-1808: Compaction is useless [ 3303.109016] class-1744 objs:2135 inuse:2131 objs-per-page:7 pages-tofree:0 [ 3303.109017] class-1744: Compaction is useless [ 3303.109019] class-1536 objs:1328 inuse:1323 objs-per-page:8 pages-tofree:0 [ 3303.109020] class-1536: Compaction is useless [ 3303.109022] class-1488 objs:8855 inuse:8847 objs-per-page:11 pages-tofree:0 [ 3303.109023] class-1488: Compaction is useless [ 3303.109025] class-1360 objs:14880 inuse:14878 objs-per-page:3 pages-tofree:0 [ 3303.109026] class-1360: Compaction is useless [ 3303.109028] class-1248 objs:3588 inuse:3577 objs-per-page:13 pages-tofree:0 [ 3303.109029] class-1248: Compaction is useless [ 3303.109031] class-1216 objs:3380 inuse:3372 objs-per-page:10 pages-tofree:0 [ 3303.109032] class-1216: Compaction is useless [ 3303.109033] class-1168 objs:3416 inuse:3401 objs-per-page:7 pages-tofree:2 [ 3303.109037] class-1168 objs:3409 inuse:3401 objs-per-page:7 pages-tofree:1 [ 3303.109042] class-1104 objs:605 inuse:599 objs-per-page:11 pages-tofree:0 [ 3303.109043] class-1104: Compaction is useless [..] every "Compaction is useless" indicates that we saved some CPU cycles. for example, class-1104 has 605 object allocated 599 objects used 11 objects per-page even if we have ALMOST_EMPTY page, we still don't have enough room to move all of its objects and free this page; so compaction will not make a lot of sense here, it's better to just leave it as is. Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@xxxxxxxxx> --- mm/zsmalloc.c | 25 +++++++++++++++++++++++++ 1 file changed, 25 insertions(+) diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c index 778b8db..9ef6f15 100644 --- a/mm/zsmalloc.c +++ b/mm/zsmalloc.c @@ -1673,6 +1673,28 @@ static struct page *isolate_source_page(struct size_class *class) return page; } +/* + * Make sure that we actually can compact this class, + * IOW if migration will empty at least one page. + * + * should be called under class->lock + */ +static bool zs_can_compact(struct size_class *class) +{ + /* + * calculate how many unused allocated objects we + * have and see if we can free any zspages. otherwise, + * compaction can just move objects back and forth w/o + * any memory gain. + */ + unsigned long ret = zs_stat_get(class, OBJ_ALLOCATED) - + zs_stat_get(class, OBJ_USED); + + ret /= get_maxobj_per_zspage(class->size, + class->pages_per_zspage); + return ret > 0; +} + static unsigned long __zs_compact(struct zs_pool *pool, struct size_class *class) { @@ -1686,6 +1708,9 @@ static unsigned long __zs_compact(struct zs_pool *pool, BUG_ON(!is_first_page(src_page)); + if (!zs_can_compact(class)) + break; + cc.index = 0; cc.s_page = src_page; -- 2.4.2.337.gfae46aa -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx"> email@xxxxxxxxx </a>