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,
-- estimated number of pages that will be freed
[..]
class-3072 objs:24652 inuse:24628 maxobjs-per-page:4 pages-tofree:6
class-3072 objs:24648 inuse:24628 maxobjs-per-page:4 pages-tofree:5
class-3072 objs:24644 inuse:24628 maxobjs-per-page:4 pages-tofree:4
class-3072 objs:24640 inuse:24628 maxobjs-per-page:4 pages-tofree:3
class-3072 objs:24636 inuse:24628 maxobjs-per-page:4 pages-tofree:2
class-3072 objs:24632 inuse:24628 maxobjs-per-page:4 pages-tofree:1
class-2720 objs:17970 inuse:17966 maxobjs-per-page:3 pages-tofree:1
class-2720 objs:17967 inuse:17966 maxobjs-per-page:3 pages-tofree:0
class-2720: Compaction is useless
class-2448 objs:7680 inuse:7674 maxobjs-per-page:5 pages-tofree:1
class-2336 objs:13510 inuse:13500 maxobjs-per-page:7 pages-tofree:1
class-2336 objs:13503 inuse:13500 maxobjs-per-page:7 pages-tofree:0
class-2336: Compaction is useless
class-1808 objs:1161 inuse:1154 maxobjs-per-page:9 pages-tofree:0
class-1808: Compaction is useless
class-1744 objs:2135 inuse:2131 maxobjs-per-page:7 pages-tofree:0
class-1744: Compaction is useless
class-1536 objs:1328 inuse:1323 maxobjs-per-page:8 pages-tofree:0
class-1536: Compaction is useless
class-1488 objs:8855 inuse:8847 maxobjs-per-page:11 pages-tofree:0
class-1488: Compaction is useless
class-1360 objs:14880 inuse:14878 maxobjs-per-page:3 pages-tofree:0
class-1360: Compaction is useless
class-1248 objs:3588 inuse:3577 maxobjs-per-page:13 pages-tofree:0
class-1248: Compaction is useless
class-1216 objs:3380 inuse:3372 maxobjs-per-page:10 pages-tofree:0
class-1216: Compaction is useless
class-1168 objs:3416 inuse:3401 maxobjs-per-page:7 pages-tofree:2
class-1168 objs:3409 inuse:3401 maxobjs-per-page:7 pages-tofree:1
class-1104 objs:605 inuse:599 maxobjs-per-page:11 pages-tofree:0
class-1104: Compaction is useless
[..]
Every "Compaction is useless" indicates that we saved some CPU cycles.
class-1104 has
605 object allocated
599 objects used
11 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 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 0453347..5d8f1d4 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1682,6 +1682,28 @@ 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 unsigned long 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 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;
+}
+
static unsigned long __zs_compact(struct zs_pool *pool,
struct size_class *class)
{
@@ -1695,6 +1717,9 @@ 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;
--
2.4.2.387.gf86f31a
--
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