Re: [PATCH v2 4/5] mm, kswapd: replace kswapd compaction with waking up kcompactd
From: Vlastimil Babka
Date: Tue Feb 09 2016 - 05:21:27 EST
On 02/08/2016 02:38 PM, Vlastimil Babka wrote:
> Similarly to direct reclaim/compaction, kswapd attempts to combine reclaim and
> compaction to attempt making memory allocation of given order available. The
> details differ from direct reclaim e.g. in having high watermark as a goal.
> The code involved in kswapd's reclaim/compaction decisions has evolved to be
> quite complex. Testing reveals that it doesn't actually work in at least one
> scenario, and closer inspection suggests that it could be greatly simplified
> without compromising on the goal (make high-order page available) or efficiency
> (don't reclaim too much). The simplification relieas of doing all compaction in
> kcompactd, which is simply woken up when high watermarks are reached by
> kswapd's reclaim.
>
> The scenario where kswapd compaction doesn't work was found with mmtests test
> stress-highalloc configured to attempt order-9 allocations without direct
> reclaim, just waking up kswapd. There was no compaction attempt from kswapd
> during the whole test. Some added instrumentation shows what happens:
>
> - balance_pgdat() sets end_zone to Normal, as it's not balanced
> - reclaim is attempted on DMA zone, which sets nr_attempted to 99, but it
> cannot reclaim anything, so sc.nr_reclaimed is 0
> - for zones DMA32 and Normal, kswapd_shrink_zone uses testorder=0, so it
> merely checks if high watermarks were reached for base pages. This is true,
> so no reclaim is attempted. For DMA, testorder=0 wasn't used, as
> compaction_suitable() returned COMPACT_SKIPPED
> - even though the pgdat_needs_compaction flag wasn't set to false, no
> compaction happens due to the condition sc.nr_reclaimed > nr_attempted
> being false (as 0 < 99)
> - priority-- due to nr_reclaimed being 0, repeat until priority reaches 0
> pgdat_balanced() is false as only the small zone DMA appears balanced
> (curiously in that check, watermark appears OK and compaction_suitable()
> returns COMPACT_PARTIAL, because a lower classzone_idx is used there)
>
> Now, even if it was decided that reclaim shouldn't be attempted on the DMA
> zone, the scenario would be the same, as (sc.nr_reclaimed=0 > nr_attempted=0)
> is also false. The condition really should use >= as the comment suggests.
> Then there is a mismatch in the check for setting pgdat_needs_compaction to
> false using low watermark, while the rest uses high watermark, and who knows
> what other subtlety. Hopefully this demonstrates that this is unsustainable.
>
> Luckily we can simplify this a lot. The reclaim/compaction decisions make
> sense for direct reclaim scenario, but in kswapd, our primary goal is to reach
> high watermark in order-0 pages. Afterwards we can attempt compaction just
> once. Unlike direct reclaim, we don't reclaim extra pages (over the high
> watermark), the current code already disallows it for good reasons.
>
> After this patch, we simply wake up kcompactd to process the pgdat, after we
> have either succeeded or failed to reach the high watermarks in kswapd, which
> goes to sleep. We pass kswapd's order and classzone_idx, so kcompactd can apply
> the same criteria to determine which zones are worth compacting. Note that we
> use the classzone_idx from wakeup_kswapd(), not balanced_classzone_idx which
> can include higher zones that kswapd tried to balance too, but didn't consider
> them in pgdat_balanced().
>
> Since kswapd now cannot create high-order pages itself, we need to adjust how
> it determines the zones to be balanced. The key element here is adding a
> "highorder" parameter to zone_balanced, which, when set to false, makes it
> consider only order-0 watermark instead of the desired higher order (this was
> done previously by kswapd_shrink_zone(), but not elsewhere). This false is
> passed for example in pgdat_balanced(). Importantly, wakeup_kswapd() uses true
> to make sure kswapd and thus kcompactd are woken up for a high-order allocation
> failure.
>
> For testing, I used stress-highalloc configured to do order-9 allocations with
> GFP_NOWAIT|__GFP_HIGH|__GFP_COMP, so they relied just on kswapd/kcompactd
> reclaim/compaction (the interfering kernel builds in phases 1 and 2 work as
> usual):
>
> stress-highalloc
> 4.5-rc1 4.5-rc1
> 3-test 4-test
> Success 1 Min 1.00 ( 0.00%) 3.00 (-200.00%)
> Success 1 Mean 1.40 ( 0.00%) 4.00 (-185.71%)
> Success 1 Max 2.00 ( 0.00%) 6.00 (-200.00%)
> Success 2 Min 1.00 ( 0.00%) 3.00 (-200.00%)
> Success 2 Mean 1.80 ( 0.00%) 4.20 (-133.33%)
> Success 2 Max 3.00 ( 0.00%) 6.00 (-100.00%)
> Success 3 Min 34.00 ( 0.00%) 63.00 (-85.29%)
> Success 3 Mean 41.80 ( 0.00%) 64.60 (-54.55%)
> Success 3 Max 53.00 ( 0.00%) 67.00 (-26.42%)
>
> 4.5-rc1 4.5-rc1
> 3-test 4-test
> User 3166.67 3088.82
> System 1153.37 1142.01
> Elapsed 1768.53 1780.91
>
> 4.5-rc1 4.5-rc1
> 3-test 4-test
> Minor Faults 106940795 106582816
> Major Faults 829 813
> Swap Ins 482 311
> Swap Outs 6278 5598
> Allocation stalls 128 184
> DMA allocs 145 32
> DMA32 allocs 74646161 74843238
> Normal allocs 26090955 25886668
> Movable allocs 0 0
> Direct pages scanned 32938 31429
> Kswapd pages scanned 2183166 2185293
> Kswapd pages reclaimed 2152359 2134389
> Direct pages reclaimed 32735 31234
> Kswapd efficiency 98% 97%
> Kswapd velocity 1243.877 1228.666
> Direct efficiency 99% 99%
> Direct velocity 18.767 17.671
> Percentage direct scans 1% 1%
> Zone normal velocity 299.981 291.409
> Zone dma32 velocity 962.522 954.928
> Zone dma velocity 0.142 0.000
> Page writes by reclaim 6278.800 5598.600
> Page writes file 0 0
> Page writes anon 6278 5598
> Page reclaim immediate 93 96
> Sector Reads 4357114 4307161
> Sector Writes 11053628 11053091
> Page rescued immediate 0 0
> Slabs scanned 1592829 1555770
> Direct inode steals 1557 2025
> Kswapd inode steals 46056 45418
> Kswapd skipped wait 0 0
> THP fault alloc 579 614
> THP collapse alloc 304 324
> THP splits 0 0
> THP fault fallback 793 730
> THP collapse fail 11 14
> Compaction stalls 1013 959
> Compaction success 92 69
> Compaction failures 920 890
> Page migrate success 238457 662054
> Page migrate failure 23021 32846
> Compaction pages isolated 504695 1370326
> Compaction migrate scanned 661390 7025772
> Compaction free scanned 13476658 73302642
> Compaction cost 262 762
Also (after adjusting mmtests' ftrace monitor):
Time kswapd awake 2547781 2269241
Time kcompactd awake 0 119253
Time direct compacting 939937 557649
Time kswapd compacting 0 0
Time kcompactd compacting 0 119099
The decrease of overal time spent compacting doesn't match the increased
compaction stats. I suspect the tasks get rescheduled and the ftrace
monitor doesn't see that, so it's wall time, not CPU time. But I guess
that direct compactors care about overall latency anyway, whether busy
compacting or waiting for CPU doesn't matter...
It's also interesting how much time kswapd spent just going through all
the priorities and failing to even try compacting, over and over...
> After this patch we see improvements in allocation success rate (especially for
> phase 3) along with increased compaction activity. The compaction stalls
> (direct compaction) in the interfering kernel builds (probably THP's) also
> decreased somewhat to kcompactd activity, yet THP alloc successes improved a
^ thanks to
> bit.
> We can also configure stress-highalloc to perform both direct
> reclaim/compaction and wakeup kswapd/kcompactd, by using
> GFP_KERNEL|__GFP_HIGH|__GFP_COMP:
>
> stress-highalloc
> 4.5-rc1 4.5-rc1
> 3-test2 4-test2
> Success 1 Min 4.00 ( 0.00%) 6.00 (-50.00%)
> Success 1 Mean 8.00 ( 0.00%) 8.40 ( -5.00%)
> Success 1 Max 12.00 ( 0.00%) 13.00 ( -8.33%)
> Success 2 Min 4.00 ( 0.00%) 6.00 (-50.00%)
> Success 2 Mean 8.20 ( 0.00%) 8.60 ( -4.88%)
> Success 2 Max 13.00 ( 0.00%) 12.00 ( 7.69%)
> Success 3 Min 75.00 ( 0.00%) 75.00 ( 0.00%)
> Success 3 Mean 75.60 ( 0.00%) 75.60 ( 0.00%)
> Success 3 Max 77.00 ( 0.00%) 76.00 ( 1.30%)
>
> 4.5-rc1 4.5-rc1
> 3-test2 4-test2
> User 3344.73 3258.62
> System 1194.24 1177.92
> Elapsed 1838.04 1837.02
>
> 4.5-rc1 4.5-rc1
> 3-test2 4-test2
> Minor Faults 111269736 109392253
> Major Faults 806 755
> Swap Ins 671 155
> Swap Outs 5390 5790
> Allocation stalls 4610 4562
> DMA allocs 250 34
> DMA32 allocs 78091501 76901680
> Normal allocs 27004414 26587089
> Movable allocs 0 0
> Direct pages scanned 125146 108854
> Kswapd pages scanned 2119757 2131589
> Kswapd pages reclaimed 2073183 2090937
> Direct pages reclaimed 124909 108699
> Kswapd efficiency 97% 98%
> Kswapd velocity 1161.027 1160.870
> Direct efficiency 99% 99%
> Direct velocity 68.545 59.283
> Percentage direct scans 5% 4%
> Zone normal velocity 296.678 294.389
> Zone dma32 velocity 932.841 925.764
> Zone dma velocity 0.053 0.000
> Page writes by reclaim 5392.000 5790.600
> Page writes file 1 0
> Page writes anon 5390 5790
> Page reclaim immediate 104 218
> Sector Reads 4350232 4376989
> Sector Writes 11126496 11102113
> Page rescued immediate 0 0
> Slabs scanned 1705294 1692486
> Direct inode steals 8700 16266
> Kswapd inode steals 36352 28364
> Kswapd skipped wait 0 0
> THP fault alloc 599 567
> THP collapse alloc 323 326
> THP splits 0 0
> THP fault fallback 806 805
> THP collapse fail 17 18
> Compaction stalls 2457 2070
> Compaction success 906 527
> Compaction failures 1551 1543
> Page migrate success 2031423 2423657
> Page migrate failure 32845 28790
> Compaction pages isolated 4129761 4916017
> Compaction migrate scanned 11996712 19370264
> Compaction free scanned 214970969 360662356
> Compaction cost 2271 2745
Time kswapd awake 2532984 2326824
Time kcompactd awake 0 257916
Time direct compacting 864839 735130
Time kswapd compacting 0 0
Time kcompactd compacting 0 257585
>
> Here, this patch doesn't change the success rate as direct compaction already
> tries what it can. There's however significant reduction in direct compaction
> stalls, made entirely of the successful stalls. This means the offload to
> kcompactd is working as expected, and direct compaction is reduced either due
> to detecting contention, or compaction deferred by kcompactd. In the previous
> version of this patchset there was some apparent reduction of success rate,
> but the changes in this version (such as using sync compaction only), new
> baseline kernel, and/or averaging results from 5 executions (my bet), made this
> go away.
>
> Signed-off-by: Vlastimil Babka <vbabka@xxxxxxx>
> ---