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SubjectRe: [RFC PATCH 0/5] compaction: changing initial position of scanners
Looks like wrapping got busted in the results part. Let's try again:
-----

Even after all the patches compaction received in last several versions, it
turns out that its effectivneess degrades considerably as the system ages
after reboot. For example, see how success rates of stress-highalloc from
mmtests degrades when we re-execute it several times, first time being after
fresh reboot:
3.19-rc4 3.19-rc4 3.19-rc4
4-nothp-1 4-nothp-2 4-nothp-3
Success 1 Min 25.00 ( 0.00%) 13.00 ( 48.00%) 9.00 ( 64.00%)
Success 1 Mean 36.20 ( 0.00%) 23.40 ( 35.36%) 16.40 ( 54.70%)
Success 1 Max 41.00 ( 0.00%) 34.00 ( 17.07%) 25.00 ( 39.02%)
Success 2 Min 25.00 ( 0.00%) 15.00 ( 40.00%) 10.00 ( 60.00%)
Success 2 Mean 37.20 ( 0.00%) 25.00 ( 32.80%) 17.20 ( 53.76%)
Success 2 Max 44.00 ( 0.00%) 36.00 ( 18.18%) 25.00 ( 43.18%)
Success 3 Min 84.00 ( 0.00%) 81.00 ( 3.57%) 78.00 ( 7.14%)
Success 3 Mean 85.80 ( 0.00%) 82.80 ( 3.50%) 80.40 ( 6.29%)
Success 3 Max 87.00 ( 0.00%) 84.00 ( 3.45%) 82.00 ( 5.75%)

Wouldn't it be much better, if it looked like this?

3.18 3.18 3.18
3.19-rc4 3.19-rc4 3.19-rc4
5-nothp-1 5-nothp-2 5-nothp-3
Success 1 Min 49.00 ( 0.00%) 42.00 ( 14.29%) 41.00 ( 16.33%)
Success 1 Mean 51.00 ( 0.00%) 45.00 ( 11.76%) 42.60 ( 16.47%)
Success 1 Max 55.00 ( 0.00%) 51.00 ( 7.27%) 46.00 ( 16.36%)
Success 2 Min 53.00 ( 0.00%) 47.00 ( 11.32%) 44.00 ( 16.98%)
Success 2 Mean 59.60 ( 0.00%) 50.80 ( 14.77%) 48.20 ( 19.13%)
Success 2 Max 64.00 ( 0.00%) 56.00 ( 12.50%) 52.00 ( 18.75%)
Success 3 Min 84.00 ( 0.00%) 82.00 ( 2.38%) 78.00 ( 7.14%)
Success 3 Mean 85.60 ( 0.00%) 82.80 ( 3.27%) 79.40 ( 7.24%)
Success 3 Max 86.00 ( 0.00%) 83.00 ( 3.49%) 80.00 ( 6.98%)

In my humble opinion, it would :) Much lower degradation, and a nice
improvement in the first iteration as a bonus.

So what sorcery is this? Nothing much, just a fundamental change of the
compaction scanners operation...

As everyone knows [1] the migration scanner starts at the first pageblock
of a zone, and goes towards the end, and the free scanner starts at the
last pageblock and goes towards the beginning. Somewhere in the middle of the
zone, the scanners meet:

zone_start zone_end
| |
-------------------------------------------------------------
MMMMMMMMMMMMM| => <= |FFFFFFFFFFFF
migrate_pfn free_pfn

In my tests, the scanners meet around the middle of the pageblock on the first
iteration, and around the 1/3 on subsequent iterations. Which means the
migration scanner doesn't see the larger part of the zone at all. For more
details why it's bad, see Patch 4 description.

To make sure we eventually scan the whole zone with the migration scanner, we
could e.g. reverse the directions after each run. But that would still be
biased, and with 1/3 of zone reachable from each side, we would still omit the
middle 1/3 of a zone.

Or we could stop terminating compaction when the scanners meet, and let them
continue to scan the whole zone. Mel told me it used to be the case long ago,
but that approach would result in migrating pages back and forth during single
compaction run, which wouldn't be cool.

So the approach taken by this patchset is to let scanners start at any
so-called pivot pfn within the zone, and keep their direction:

zone_start pivot zone_end
| | |
-------------------------------------------------------------
<= |FFFFFFFMMMMMM| =>
free_pfn migrate_pfn

Eventually, one of the scanners will reach the zone boundary and wrap around,
e.g. the in the case of the free scanner:

zone_start pivot zone_end
| | |
-------------------------------------------------------------
FFFFFFFFFFFFFFFFFFFFFFFFFFFFFMMMMMMMMMMMM| => <= |F
migrate_pfn free_pfn

Compaction will again terminate when the scanners meet.


As you can imagine, the required code changes made the termination detection
and the scanners themselves quite hairy. There are lots of corner cases and
the code is often hard to wrap one's head around [puns intended]. The scanner
functions isolate_migratepages() and isolate_freepages() were recently cleaned
up a lot, and this makes them messy again, as they can no longer rely on the
fact that they will meet the other scanner and not the zone boundary.

But the improvements seem to make these complications worth, and I hope
somebody can suggest more elegant solutions to the various parts of the code.
So here it is as a RFC. Patches 1-3 are cleanups that could be applied in any
case. Patch 4 implements the main changes, but leaves the pivot to be the
first zone's pfn, so that free scanner wraps immediately and there's no
actual change. Patch 5 updates the pivot in a conservative way, as explained
in the changelog.

Even with the conservative approach, stress-highalloc results are improved,
mainly on the 2+ iterations after fresh reboot. Here are again the results
before the patches, including compaction stats:

3.19-rc4 3.19-rc4 3.19-rc4
4-nothp-1 4-nothp-2 4-nothp-3
Success 1 Min 25.00 ( 0.00%) 13.00 ( 48.00%) 9.00 ( 64.00%)
Success 1 Mean 36.20 ( 0.00%) 23.40 ( 35.36%) 16.40 ( 54.70%)
Success 1 Max 41.00 ( 0.00%) 34.00 ( 17.07%) 25.00 ( 39.02%)
Success 2 Min 25.00 ( 0.00%) 15.00 ( 40.00%) 10.00 ( 60.00%)
Success 2 Mean 37.20 ( 0.00%) 25.00 ( 32.80%) 17.20 ( 53.76%)
Success 2 Max 44.00 ( 0.00%) 36.00 ( 18.18%) 25.00 ( 43.18%)
Success 3 Min 84.00 ( 0.00%) 81.00 ( 3.57%) 78.00 ( 7.14%)
Success 3 Mean 85.80 ( 0.00%) 82.80 ( 3.50%) 80.40 ( 6.29%)
Success 3 Max 87.00 ( 0.00%) 84.00 ( 3.45%) 82.00 ( 5.75%)

3.19-rc4 3.19-rc4 3.19-rc4
4-nothp-1 4-nothp-2 4-nothp-3
User 6781.16 6931.44 6905.44
System 1073.97 1071.20 1067.92
Elapsed 2349.71 2290.40 2255.59

3.19-rc4 3.19-rc4 3.19-rc4
4-nothp-1 4-nothp-2 4-nothp-3
Minor Faults 198270153 197020929 197146656
Major Faults 498 470 527
Swap Ins 60 34 105
Swap Outs 2780 1011 425
Allocation stalls 8325 4615 4769
DMA allocs 161 177 141
DMA32 allocs 124477754 123412713 123385291
Normal allocs 59366406 59040066 58909035
Movable allocs 0 0 0
Direct pages scanned 413858 280997 267341
Kswapd pages scanned 2204723 2184556 2147546
Kswapd pages reclaimed 2199430 2181305 2144395
Direct pages reclaimed 413062 280323 266557
Kswapd efficiency 99% 99% 99%
Kswapd velocity 914.497 977.868 943.877
Direct efficiency 99% 99% 99%
Direct velocity 171.664 125.782 117.500
Percentage direct scans 15% 11% 11%
Zone normal velocity 359.993 356.888 339.577
Zone dma32 velocity 726.152 746.745 721.784
Zone dma velocity 0.016 0.017 0.016
Page writes by reclaim 2893.000 1119.800 507.600
Page writes file 112 108 81
Page writes anon 2780 1011 425
Page reclaim immediate 1134 1197 1412
Sector Reads 4747006 4606605 4524025
Sector Writes 12759301 12562316 12629243
Page rescued immediate 0 0 0
Slabs scanned 1807821 1528751 1498929
Direct inode steals 24428 12649 13346
Kswapd inode steals 33213 29804 30194
Kswapd skipped wait 0 0 0
THP fault alloc 217 207 222
THP collapse alloc 500 539 373
THP splits 13 14 13
THP fault fallback 50 9 16
THP collapse fail 16 17 22
Compaction stalls 3136 2310 2072
Compaction success 1123 897 828
Compaction failures 2012 1413 1244
Page migrate success 4319697 3682666 3133699
Page migrate failure 19012 9964 7488
Compaction pages isolated 8974417 7634911 6528981
Compaction migrate scanned 182447073 122423031 127292737
Compaction free scanned 389193883 291257587 269516820
Compaction cost 5931 4824 4267

As the allocation success rates degrade, so do the compaction success rates,
and so does the scanner activity.

Now after the series:

3.19-rc4 3.19-rc4 3.19-rc4
5-nothp-1 5-nothp-2 5-nothp-3
Success 1 Min 49.00 ( 0.00%) 42.00 ( 14.29%) 41.00 ( 16.33%)
Success 1 Mean 51.00 ( 0.00%) 45.00 ( 11.76%) 42.60 ( 16.47%)
Success 1 Max 55.00 ( 0.00%) 51.00 ( 7.27%) 46.00 ( 16.36%)
Success 2 Min 53.00 ( 0.00%) 47.00 ( 11.32%) 44.00 ( 16.98%)
Success 2 Mean 59.60 ( 0.00%) 50.80 ( 14.77%) 48.20 ( 19.13%)
Success 2 Max 64.00 ( 0.00%) 56.00 ( 12.50%) 52.00 ( 18.75%)
Success 3 Min 84.00 ( 0.00%) 82.00 ( 2.38%) 78.00 ( 7.14%)
Success 3 Mean 85.60 ( 0.00%) 82.80 ( 3.27%) 79.40 ( 7.24%)
Success 3 Max 86.00 ( 0.00%) 83.00 ( 3.49%) 80.00 ( 6.98%)

3.19-rc4 3.19-rc4 3.19-rc4
5-nothp-1 5-nothp-2 5-nothp-3
User 6675.75 6742.26 6707.92
System 1069.78 1070.77 1070.25
Elapsed 2450.31 2363.98 2442.21

3.19-rc4 3.19-rc4 3.19-rc4
5-nothp-1 5-nothp-2 5-nothp-3
Minor Faults 197652452 197164571 197946824
Major Faults 882 743 934
Swap Ins 113 96 144
Swap Outs 2144 1340 1799
Allocation stalls 10304 9060 10261
DMA allocs 142 227 181
DMA32 allocs 124497911 123947671 124010151
Normal allocs 59233600 59160910 59669421
Movable allocs 0 0 0
Direct pages scanned 591368 481119 525158
Kswapd pages scanned 2217381 2164036 2170388
Kswapd pages reclaimed 2212285 2160162 2166794
Direct pages reclaimed 590064 480297 524013
Kswapd efficiency 99% 99% 99%
Kswapd velocity 921.119 937.864 936.558
Direct efficiency 99% 99% 99%
Direct velocity 245.659 208.511 226.615
Percentage direct scans 21% 18% 19%
Zone normal velocity 378.957 376.819 383.604
Zone dma32 velocity 787.805 769.530 779.552
Zone dma velocity 0.015 0.025 0.016
Page writes by reclaim 2284.600 1432.400 1972.400
Page writes file 140 92 173
Page writes anon 2144 1340 1799
Page reclaim immediate 1689 1315 1440
Sector Reads 4920699 4830343 4830263
Sector Writes 12643658 12588410 12713518
Page rescued immediate 0 0 0
Slabs scanned 2084358 1922421 1962867
Direct inode steals 35881 37170 45512
Kswapd inode steals 35973 35741 30339
Kswapd skipped wait 0 0 0
THP fault alloc 191 224 170
THP collapse alloc 554 533 516
THP splits 15 15 11
THP fault fallback 45 0 76
THP collapse fail 16 18 18
Compaction stalls 4069 3746 3951
Compaction success 1507 1388 1366
Compaction failures 2562 2357 2585
Page migrate success 4533617 4912832 5278583
Page migrate failure 20127 15273 17862
Compaction pages isolated 9495626 10235697 10993942
Compaction migrate scanned 93585708 99204656 92052138
Compaction free scanned 510786018 503529022 541298357
Compaction cost 5541 5988 6332

Less degradation in success rates and no degradation in activity.
Let's look again at compactions stats without the series:

Compaction stalls 3136 2310 2072
Compaction success 1123 897 828
Compaction failures 2012 1413 1244
Page migrate success 4319697 3682666 3133699
Page migrate failure 19012 9964 7488
Compaction pages isolated 8974417 7634911 6528981
Compaction migrate scanned 182447073 122423031 127292737
Compaction free scanned 389193883 291257587 269516820
Compaction cost 5931 4824 4267

Interestingly, the number of migrate scanned pages was higher before the
series, even twice in the first iteration. That suggests that the scanner
was indeed scanning in a limited number of pageblocks over and over, despite
their compaction potential "depleted". After the patches, it achieves better
results with less scanned blocks, as it has a better chance of encountering
non-depleted blocks. The free scanner activity has increased after the series,
which just means that higher success rates lead to less deferred compaction.
There is also some increase in page migrations, but it is likely also a
consequence of better success and not due to useless back and forth migrations
due to pivot changes.

Preliminary testing with THP-like allocations has shown similar improvements,
which is somewhat surprising, because THP allocations do not use sync
and thus do not defer compaction; but changing the pivot is currently tied
to restarting from deferred compaction. Possibly this is due to the other
allocation activity than the stress-highalloc test itself. I will post these
results when full measurements are done.

[1] http://lwn.net/Articles/368869/


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