From: Vlastimil Babka <vbabka@xxxxxxx>
Subject: mm, compaction: reorder fields in struct compact_control
Patch series "try to reduce fragmenting fallbacks", v3.
Last year, Johannes Weiner has reported a regression in page mobility
grouping [1] and while the exact cause was not found, I've come up with
some ways to improve it by reducing the number of allocations falling back
to different migratetype and causing permanent fragmentation.
The series was tested with mmtests stress-highalloc modified to do
GFP_KERNEL order-4 allocations, on 4.9 with "mm, vmscan: fix zone balance
check in prepare_kswapd_sleep" (without that, kcompactd indeed wasn't
woken up) on UMA machine with 4GB memory. There were 5 repeats of each
run, as the extfrag stats are quite volatile (note the stats below are
sums, not averages, as it was less perl hacking for me).
Success rate are the same, already high due to the low allocation order
used, so I'm not including them.
Compaction stats:
(the patches are stacked, and I haven't measured the non-functional-changes
patches separately)
patch 1 patch 2 patch 3 patch 4 patch 7 patch 8
Compaction stalls 22449 24680 24846 19765 22059 17480
Compaction success 12971 14836 14608 10475 11632 8757
Compaction failures 9477 9843 10238 9290 10426 8722
Page migrate success 3109022 3370438 3312164 1695105 1608435 2111379
Page migrate failure 911588 1149065 1028264 1112675 1077251 1026367
Compaction pages isolated 7242983 8015530 7782467 4629063 4402787 5377665
Compaction migrate scanned 980838938 987367943 957690188 917647238 947155598 1018922197
Compaction free scanned 557926893 598946443 602236894 594024490 541169699 763651731
Compaction cost 10243 10578 10304 8286 8398 9440
Compaction stats are mostly within noise until patch 4, which decreases
the number of compactions, and migrations. Part of that could be due to
more pageblocks marked as unmovable, and async compaction skipping those.
This changes a bit with patch 7, but not so much. Patch 8 increases free
scanner stats and migrations, which comes from the changed termination
criteria. Interestingly number of compactions decreases - probably the
fully compacted pageblock satisfies multiple subsequent allocations, so it
amortizes.
Next comes the extfrag tracepoint, where "fragmenting" means that an
allocation had to fallback to a pageblock of another migratetype which
wasn't fully free (which is almost all of the fallbacks). I have locally
added another tracepoint for "Page steal" into steal_suitable_fallback()
which triggers in situations where we are allowed to do
move_freepages_block(). If we decide to also do
set_pageblock_migratetype(), it's "Pages steal with pageblock" with break
down for which allocation migratetype we are stealing and from which
fallback migratetype. The last part "due to counting" comes from patch 4
and counts the events where the counting of movable pages allowed us to
change pageblock's migratetype, while the number of free pages alone
wouldn't be enough to cross the threshold.
patch 1 patch 2 patch 3 patch 4 patch 7 patch 8
Page alloc extfrag event 10155066 8522968 10164959 15622080 13727068 13140319
Extfrag fragmenting 10149231 8517025 10159040 15616925 13721391 13134792
Extfrag fragmenting for unmovable 159504 168500 184177 97835 70625 56948
Extfrag fragmenting unmovable placed with movable 153613 163549 172693 91740 64099 50917
Extfrag fragmenting unmovable placed with reclaim. 5891 4951 11484 6095 6526 6031
Extfrag fragmenting for reclaimable 4738 4829 6345 4822 5640 5378
Extfrag fragmenting reclaimable placed with movable 1836 1902 1851 1579 1739 1760
Extfrag fragmenting reclaimable placed with unmov. 2902 2927 4494 3243 3901 3618
Extfrag fragmenting for movable 9984989 8343696 9968518 15514268 13645126 13072466
Pages steal 179954 192291 210880 123254 94545 81486
Pages steal with pageblock 22153 18943 20154 33562 29969 33444
Pages steal with pageblock for unmovable 14350 12858 13256 20660 19003 20852
Pages steal with pageblock for unmovable from mov. 12812 11402 11683 19072 17467 19298
Pages steal with pageblock for unmovable from recl. 1538 1456 1573 1588 1536 1554
Pages steal with pageblock for movable 7114 5489 5965 11787 10012 11493
Pages steal with pageblock for movable from unmov. 6885 5291 5541 11179 9525 10885
Pages steal with pageblock for movable from recl. 229 198 424 608 487 608
Pages steal with pageblock for reclaimable 689 596 933 1115 954 1099
Pages steal with pageblock for reclaimable from unmov. 273 219 537 658 547 667
Pages steal with pageblock for reclaimable from mov. 416 377 396 457 407 432
Pages steal with pageblock due to counting 11834 10075 7530
... for unmovable 8993 7381 4616
... for movable 2792 2653 2851
... for reclaimable 49 41 63
What we can see is that "Extfrag fragmenting for unmovable" and "...
placed with movable" drops with almost each patch, which is good as we are
polluting less movable pageblocks with unmovable pages.
The most significant change is patch 4 with movable page counting. On the
other hand it increases "Extfrag fragmenting for movable" by 50%. "Pages
steal" drops though, so these movable allocation fallbacks find only small
free pages and are not allowed to steal whole pageblocks back. "Pages
steal with pageblock" raises, because the patch increases the chances of
pageblock migratetype changes to happen. This affects all migratetypes.
The summary is that patch 4 is not a clear win wrt these stats, but I
believe that the tradeoff it makes is a good one. There's less pollution
of movable pageblocks by unmovable allocations. There's less stealing
between pageblock, and those that remain have higher chance of changing
migratetype also the pageblock itself, so it should more faithfully
reflect the migratetype of the pages within the pageblock. The increase
of movable allocations falling back to unmovable pageblock might look
dramatic, but those allocations can be migrated by compaction when needed,
and other patches in the series (7-9) improve that aspect.
Patches 7 and 8 continue the trend of reduced unmovable fallbacks and also
reduce the impact on movable fallbacks from patch 4.
[1] https://www.spinics.net/lists/linux-mm/msg114237.html
This patch (of 8):
While currently there are (mostly by accident) no holes in struct
compact_control (on x86_64), but we are going to add more bool flags, so
place them all together to the end of the structure. While at it, just
order all fields from largest to smallest.
Link: http://lkml.kernel.org/r/20170307131545.28577-2-vbabka@xxxxxxx
Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
Acked-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: Joonsoo Kim <iamjoonsoo.kim@xxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
mm/internal.h | 10 +++++-----
1 file changed, 5 insertions(+), 5 deletions(-)
diff -puN mm/internal.h~mm-compaction-reorder-fields-in-struct-compact_control mm/internal.h
--- a/mm/internal.h~mm-compaction-reorder-fields-in-struct-compact_control
+++ a/mm/internal.h
@@ -183,6 +183,7 @@ extern int user_min_free_kbytes;
struct compact_control {
struct list_head freepages; /* List of free pages to migrate to */
struct list_head migratepages; /* List of pages being migrated */
+ struct zone *zone;
unsigned long nr_freepages; /* Number of isolated free pages */
unsigned long nr_migratepages; /* Number of pages to migrate */
unsigned long total_migrate_scanned;
@@ -190,16 +191,15 @@ struct compact_control {
unsigned long free_pfn; /* isolate_freepages search base */
unsigned long migrate_pfn; /* isolate_migratepages search base */
unsigned long last_migrated_pfn;/* Not yet flushed page being freed */
+ const gfp_t gfp_mask; /* gfp mask of a direct compactor */
+ int order; /* order a direct compactor needs */
+ const unsigned int alloc_flags; /* alloc flags of a direct compactor */
+ const int classzone_idx; /* zone index of a direct compactor */
enum migrate_mode mode; /* Async or sync migration mode */
bool ignore_skip_hint; /* Scan blocks even if marked skip */
bool ignore_block_suitable; /* Scan blocks considered unsuitable */
bool direct_compaction; /* False from kcompactd or /proc/... */
bool whole_zone; /* Whole zone should/has been scanned */
- int order; /* order a direct compactor needs */
- const gfp_t gfp_mask; /* gfp mask of a direct compactor */
- const unsigned int alloc_flags; /* alloc flags of a direct compactor */
- const int classzone_idx; /* zone index of a direct compactor */
- struct zone *zone;
bool contended; /* Signal lock or sched contention */
};
_
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