[RFC][PATCH 07/10] zsmalloc: introduce auto-compact support

Sergey Senozhatsky
2015-05-29T15:09:44Z

perform class compaction in zs_free(), if zs_free() has created
a ZS_ALMOST_EMPTY page. this is the most trivial `policy'.

probably it would make zs_can_compact() to return an estimated number
of pages that potentially will be free and trigger auto-compaction
only when it's above some limit (e.g. at least 4 zs pages); or put it
under config option.

this also tweaks __zs_compact() -- we can't do reschedule
anymore, waiting for new pages in the current class. so we
compact as much as we can and return immediately if compaction
is not possible anymore.

auto-compaction is not a replacement of manual compaction.

compiled linux kernel with auto-compaction:

cat /sys/block/zram0/mm_stat
2339885056 1601034235 1624076288 0 1624076288 19961 1106

performing additional manual compaction:

echo 1 > /sys/block/zram0/compact
cat /sys/block/zram0/mm_stat
2339885056 1601034235 1624051712 0 1624076288 19961 1114

manual compaction was able to migrate additional 8 objects. so
auto-compaction is 'good enough'.

TEST

this test copies a 1.3G linux kernel tar to mounted zram disk,
and extracts it.

w/auto-compaction:

cat /sys/block/zram0/mm_stat
1171456 26006 86016 0 86016 32781 0

time tar xf linux-3.10.tar.gz -C linux

real 0m16.970s
user 0m15.247s
sys 0m8.477s

du -sh linux
2.0G linux

cat /sys/block/zram0/mm_stat
3547353088 2993384270 3011088384 0 3011088384 24310 108

=====================================================================

w/o auto compaction:

cat /sys/block/zram0/mm_stat
1171456 26000 81920 0 81920 32781 0

time tar xf linux-3.10.tar.gz -C linux

real 0m16.983s
user 0m15.267s
sys 0m8.417s

du -sh linux
2.0G linux

cat /sys/block/zram0/mm_stat
3548917760 2993566924 3011317760 0 3011317760 23928 0

=====================================================================

iozone shows that auto-compacted code runs faster in several
tests, which is hardly trustworthy. anyway.

iozone -t 3 -R -r 16K -s 60M -I +Z

test base auto-compact (compacted 66123 objs)
Initial write 1603682.25 1645112.38
Rewrite 2502243.31 2256570.31
Read 7040860.00 7130575.00
Re-read 7036490.75 7066744.25
Reverse Read 6617115.25 6155395.50
Stride read 6705085.50 6350030.38
Random read 6668497.75 6350129.38
Mixed workload 5494030.38 5091669.62
Random write 2526834.44 2500977.81
Pwrite 1656874.00 1663796.94
Pread 3322818.91 3359683.44
Fwrite 4090124.25 4099773.88
Fread 10358916.25 10324409.75

Signed-off-by: Sergey Senozhatsky <sergey.senozhatsky@gmail.com>
---
mm/zsmalloc.c | 25 +++++++++++++------------
1 file changed, 13 insertions(+), 12 deletions(-)

diff --git a/mm/zsmalloc.c b/mm/zsmalloc.c
index c2a640a..70bf481 100644
--- a/mm/zsmalloc.c
+++ b/mm/zsmalloc.c
@@ -1515,34 +1515,28 @@ static void __zs_compact(struct zs_pool *pool, struct size_class *class)

while ((dst_page = isolate_target_page(class))) {
cc.d_page = dst_page;
- /*
- * If there is no more space in dst_page, resched
- * and see if anyone had allocated another zspage.
- */
+
if (!migrate_zspage(pool, class, &cc))
- break;
+ goto out;

putback_zspage(pool, class, dst_page);
}

- /* Stop if we couldn't find slot */
- if (dst_page == NULL)
+ if (!dst_page)
break;
-
putback_zspage(pool, class, dst_page);
putback_zspage(pool, class, src_page);
- spin_unlock(&class->lock);
- cond_resched();
- spin_lock(&class->lock);
}

+out:
+ if (dst_page)
+ putback_zspage(pool, class, dst_page);
if (src_page)
putback_zspage(pool, class, src_page);

spin_unlock(&class->lock);
}

-
unsigned long zs_get_total_pages(struct zs_pool *pool)
{
return atomic_long_read(&pool->pages_allocated);
@@ -1741,6 +1735,13 @@ void zs_free(struct zs_pool *pool, unsigned long handle)
unpin_tag(handle);

free_handle(pool, handle);
+
+ /*
+ * actual fullness might have changed, __zs_compact() checks
+ * if compaction makes sense
+ */
+ if (fullness == ZS_ALMOST_EMPTY)
+ __zs_compact(pool, class);
}
EXPORT_SYMBOL_GPL(zs_free);

--
2.4.2.337.gfae46aa