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zsmalloc: introduce zs_can_compact() function

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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 in any (further) savings.

EXAMPLE (this debug output is not part of this patch set):

 - class size
 - number of allocated objects
 - number of used objects
 - max objects per zspage
 - pages per zspage
 - estimated number of pages that will be freed

[..]
class-512 objs:544 inuse:540 maxobj-per-zspage:8  pages-per-zspage:1 zspages-to-free:0
 ... class-512 compaction is useless. break
class-496 objs:660 inuse:570 maxobj-per-zspage:33 pages-per-zspage:4 zspages-to-free:2
class-496 objs:627 inuse:570 maxobj-per-zspage:33 pages-per-zspage:4 zspages-to-free:1
class-496 objs:594 inuse:570 maxobj-per-zspage:33 pages-per-zspage:4 zspages-to-free:0
 ... class-496 compaction is useless. break
class-448 objs:657 inuse:617 maxobj-per-zspage:9  pages-per-zspage:1 zspages-to-free:4
class-448 objs:648 inuse:617 maxobj-per-zspage:9  pages-per-zspage:1 zspages-to-free:3
class-448 objs:639 inuse:617 maxobj-per-zspage:9  pages-per-zspage:1 zspages-to-free:2
class-448 objs:630 inuse:617 maxobj-per-zspage:9  pages-per-zspage:1 zspages-to-free:1
class-448 objs:621 inuse:617 maxobj-per-zspage:9  pages-per-zspage:1 zspages-to-free:0
 ... class-448 compaction is useless. break
class-432 objs:728 inuse:685 maxobj-per-zspage:28 pages-per-zspage:3 zspages-to-free:1
class-432 objs:700 inuse:685 maxobj-per-zspage:28 pages-per-zspage:3 zspages-to-free:0
 ... class-432 compaction is useless. break
class-416 objs:819 inuse:705 maxobj-per-zspage:39 pages-per-zspage:4 zspages-to-free:2
class-416 objs:780 inuse:705 maxobj-per-zspage:39 pages-per-zspage:4 zspages-to-free:1
class-416 objs:741 inuse:705 maxobj-per-zspage:39 pages-per-zspage:4 zspages-to-free:0
 ... class-416 compaction is useless. break
class-400 objs:690 inuse:674 maxobj-per-zspage:10 pages-per-zspage:1 zspages-to-free:1
class-400 objs:680 inuse:674 maxobj-per-zspage:10 pages-per-zspage:1 zspages-to-free:0
 ... class-400 compaction is useless. break
class-384 objs:736 inuse:709 maxobj-per-zspage:32 pages-per-zspage:3 zspages-to-free:0
 ... class-384 compaction is useless. break
[..]

Every "compaction is useless" indicates that we saved CPU cycles.

class-512 has
	544	object allocated
	540	objects used
	8	objects per-page

Even if we have a ALMOST_EMPTY zspage, we still don't have enough room to
migrate all of its objects and free this zspage; so compaction will not
make a lot of sense, it's better to just leave it as is.

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
Acked-by: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
This commit is contained in:
Sergey Senozhatsky 2015-09-08 15:04:30 -07:00 committed by Linus Torvalds
parent 5724459419
commit 04f05909e0
1 changed files with 26 additions and 0 deletions
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26
mm/zsmalloc.c
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@ -1676,6 +1676,29 @@ static struct page *isolate_source_page(struct size_class *class)
return page;
}
/*
*
* Based on the number of unused allocated objects calculate
* and return the number of pages that we can free.
*
* Should be called under class->lock.
*/
static unsigned long zs_can_compact(struct size_class *class)
{
unsigned long obj_wasted;
if (!zs_stat_get(class, CLASS_ALMOST_EMPTY))
return 0;
obj_wasted = zs_stat_get(class, OBJ_ALLOCATED) -
zs_stat_get(class, OBJ_USED);
obj_wasted /= get_maxobj_per_zspage(class->size,
class->pages_per_zspage);
return obj_wasted * get_pages_per_zspage(class->size);
}
static unsigned long __zs_compact(struct zs_pool *pool,
struct size_class *class)
{
@ -1689,6 +1712,9 @@ 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;