Re: [PATCH v2 00/10] try to reduce fragmenting fallbacks

Vlastimil Babka
2017-02-20T12:30:43Z

On 02/13/2017 12:07 PM, Mel Gorman wrote:
> On Fri, Feb 10, 2017 at 06:23:33PM +0100, Vlastimil Babka wrote:
>
> By and large, I like the series, particularly patches 7 and 8. I cannot
> make up my mind about the RFC patches 9 and 10 yet. Conceptually they
> seem sound but they are much more far reaching than the rest of the
> series.
>
> It would be nice if patches 1-8 could be treated in isolation with data
> on the number of extfrag events triggered, time spent in compaction and
> the success rate. Patches 9 and 10 are tricy enough that they would need
> data per patch where as patches 1-8 should be ok with data gathered for
> the whole series.

Ok let's try again with a fresh subthread after fixing automation and
postprocessing...

I've got the results 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 alllocation order used.

patch 1 patch 2 patch 3 patch 4 patch 7 patch 8 patch 9 patch 10
Compaction stalls 22449 24680 24846 19765 22059 17480 29499 58284
Compaction success 12971 14836 14608 10475 11632 8757 16697 12544
Compaction failures 9477 9843 10238 9290 10426 8722 12801 45739
Page migrate success 3109022 3370438 3312164 1695105 1608435 2111379 2445824 3288822
Page migrate failure 911588 1149065 1028264 1112675 1077251 1026367 1014035 398158
Compaction pages isolated 7242983 8015530 7782467 4629063 4402787 5377665 6062703 7180216
Compaction migrate scanned 980838938 987367943 957690188 917647238 947155598 1018922197 1041367620 209082744
Compaction free scanned 557926893 598946443 602236894 594024490 541169699 763651731 827822984 396678647
Compaction cost 10243 10578 10304 8286 8398 9440 9957 5019

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.
Patch 9 increases compaction attempts as we force them before fallbacks. Success
vs failure rate increases, so it might be worth it.
Patch 10 looks quite bad for compaction - lots of attempt and failures, but
scanner stats went down. I probably need to check if the new migratetype is
considered as suitable for compaction optimally.

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 patch 9 patch 10
Page alloc extfrag event 10155066 8522968 10164959 15622080 13727068 13140319 6584820 2030419
Extfrag fragmenting 10149231 8517025 10159040 15616925 13721391 13134792 6579315 2024038
Extfrag fragmenting for unmovable 159504 168500 184177 97835 70625 56948 50413 166200
Extfrag fragmenting unmovable placed with movable 153613 163549 172693 91740 64099 50917 44845 20256
Extfrag fragmenting unmovable placed with reclaim. 5891 4951 11484 6095 6526 6031 5568 26540
Extfrag fragmenting for reclaimable 4738 4829 6345 4822 5640 5378 4213 6599
Extfrag fragmenting reclaimable placed with movable 1836 1902 1851 1579 1739 1760 1918 965
Extfrag fragmenting reclaimable placed with unmov. 2902 2927 4494 3243 3901 3618 2295 3867
Extfrag fragmenting for movable 9984989 8343696 9968518 15514268 13645126 13072466 6524689 1851239
Pages steal 179954 192291 210880 123254 94545 81486 72717 2024038
Pages steal with pageblock 22153 18943 20154 33562 29969 33444 32871 1572912
Pages steal with pageblock for unmovable 14350 12858 13256 20660 19003 20852 20265 21010
Pages steal with pageblock for unmovable from mov. 12812 11402 11683 19072 17467 19298 18791 5271
Pages steal with pageblock for unmovable from recl. 1538 1456 1573 1588 1536 1554 1474 1421
Pages steal with pageblock for movable 7114 5489 5965 11787 10012 11493 11586 1550723
Pages steal with pageblock for movable from unmov. 6885 5291 5541 11179 9525 10885 10874 29787
Pages steal with pageblock for movable from recl. 229 198 424 608 487 608 712 1190
Pages steal with pageblock for reclaimable 689 596 933 1115 954 1099 1020 1179
Pages steal with pageblock for reclaimable from unmov. 273 219 537 658 547 667 584 629
Pages steal with pageblock for reclaimable from mov. 416 377 396 457 407 432 436 324
Pages steal with pageblock due to counting 11834 10075 7530 6927 1381357
... for unmovable 8993 7381 4616 3863 344
... for movable 2792 2653 2851 2972 1380981
... for reclaimable 49 41 63 92 32

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.
Same for patch 9, which also reduces the movable fallbacks to half. It's not
completely clear to me why. Perhaps the more aggressive compaction of unmovable
blocks results in unmovable allocations (such as those GFP_KERNEL ones from the
workload) fitting within less blocks, and thus reclaim has higher changes of
freeing the LRU pages within movable blocks, and new movable allocations don't
have to fallback that much.
Patch 10 kills all the improvements to "Pages steal with pageblock" so I'll have
to investigate.

To sum up, patches 1-8 look OK to me. Patch 9 looks also very promising, but
there's danger of increased allocation latencies due to the forced compaction.
Patch 10 has either implementation bugs or there's some unforeseen consequence
of its design.