Re: [PATCH 1/1] mm: thp: Redefine default THP defrag behaviour disable it by default
From: Johannes Weiner
Date: Thu Feb 25 2016 - 14:45:42 EST
On Thu, Feb 25, 2016 at 05:12:19PM +0000, Mel Gorman wrote:
> This patch only makes sense on mmotm because it's heavily relying on an
> existing swapping-related fix and indirectly relying on the kcompactd
> patches. Even though the kernel says "4.4.0", the swapping and kcompactd
> patches have been cherry-picked from mmotm for the purposes of testing.
>
> THP defrag is enabled by default to direct reclaim/compact but not wake
> kswapd in the event of a THP allocation failure. The problem is that THP
> allocation requests potentially enter reclaim/compaction. This potentially
> incurs a severe stall that is not guaranteed to be offset by reduced TLB
> misses. While there has been considerable effort to reduce the impact
> of reclaim/compaction, it is still a high cost and workloads that should
> fit in memory fail to do so. Specifically, a simple anon/file streaming
> workload will enter direct reclaim on NUMA at least even though the working
> set size is 80% of RAM. It's been years and it's time to throw in the towel.
>
> First, this patch redefines what THP defrag means;
>
> o GFP_TRANSHUGE by default will neither reclaim/compact nor wake kswapd
> o For faults, defrag will not direct/reclaim but will wake kswapd
> o For khugepaged, defrag will enter direct/reclaim but not wake kswapd
>
> This means that a THP fault will no longer stall but may incur
> reclaim/compaction via kswapd reclaiming and kcompactd compacting. This
> is potentially destructive so the patch disables THP defrag by default.
> THP defrag for khugepaged remains enabled and will enter direct/reclaim
> but no wakeup kswapd or kcompactd.
>
> After this patch a THP allocation failure will quickly fallback and rely
> on khugepaged to recover the situation at some time in the future. In
> some cases, this will reduce THP usage but the benefit of THP is hard to
> measure and not a universal win where as a stall to reclaim/compaction is
> definitely measurable and can be painful.
>
> The first test for this is using "usemem" to read a large file and write
> a large anonymous mapping (to avoid the zero page) multiple times. The
> total size of the mappings is 80% of RAM and the benchmark simply measures
> how long it takes to complete. It uses multiple threads to see if that
> is a factor. On UMA, the performance is almost identical so is not reported
> but on NUMA, we see this
>
> usemem
> 4.4.0 4.4.0
> kcompactd-v1r1 nodefrag-v1r3
> Amean System-1 102.86 ( 0.00%) 46.81 ( 54.50%)
> Amean System-4 37.85 ( 0.00%) 34.02 ( 10.12%)
> Amean System-7 48.12 ( 0.00%) 46.89 ( 2.56%)
> Amean System-12 51.98 ( 0.00%) 56.96 ( -9.57%)
> Amean System-21 80.16 ( 0.00%) 79.05 ( 1.39%)
> Amean System-30 110.71 ( 0.00%) 107.17 ( 3.20%)
> Amean System-48 127.98 ( 0.00%) 124.83 ( 2.46%)
> Amean Elapsd-1 185.84 ( 0.00%) 105.51 ( 43.23%)
> Amean Elapsd-4 26.19 ( 0.00%) 25.58 ( 2.33%)
> Amean Elapsd-7 21.65 ( 0.00%) 21.62 ( 0.16%)
> Amean Elapsd-12 18.58 ( 0.00%) 17.94 ( 3.43%)
> Amean Elapsd-21 17.53 ( 0.00%) 16.60 ( 5.33%)
> Amean Elapsd-30 17.45 ( 0.00%) 17.13 ( 1.84%)
> Amean Elapsd-48 15.40 ( 0.00%) 15.27 ( 0.82%)
>
> For a single thread, the benchmark completes 43.23% faster with
> this patch applied with smaller benefits as the thread increases.
> Similar, notice the large reduction in most cases in system CPU
> usage. The overall CPU time is
>
> 4.4.0 4.4.0
> kcompactd-v1r1 nodefrag-v1r3
> User 10357.65 10438.33
> System 3988.88 3543.94
> Elapsed 2203.01 1634.41
>
> Which is substantial. Now, the reclaim figures
>
> 4.4.0 4.4.0
> kcompactd-v1r1nodefrag-v1r3
> Minor Faults 128458477 278352931
> Major Faults 2174976 225
> Swap Ins 16904701 0
> Swap Outs 17359627 0
> Allocation stalls 43611 0
> DMA allocs 0 0
> DMA32 allocs 19832646 19448017
> Normal allocs 614488453 580941839
> Movable allocs 0 0
> Direct pages scanned 24163800 0
> Kswapd pages scanned 0 0
> Kswapd pages reclaimed 0 0
> Direct pages reclaimed 20691346 0
> Compaction stalls 42263 0
> Compaction success 938 0
> Compaction failures 41325 0
>
> This patch eliminates almost all swapping and direct reclaim activity. There
> is still overhead but it's from NUMA balancing which does not identify that
> it's pointless trying to do anything with this workload.
>
> I also tried the thpscale benchmark which forces a corner case where compaction
> can be used heavily and measures the latency of whether base or huge pages were
> used
>
> thpscale Fault Latencies
> 4.4.0 4.4.0
> kcompactd-v1r1 nodefrag-v1r3
> Amean fault-base-1 5288.84 ( 0.00%) 2817.12 ( 46.73%)
> Amean fault-base-3 6365.53 ( 0.00%) 3499.11 ( 45.03%)
> Amean fault-base-5 6526.19 ( 0.00%) 4363.06 ( 33.15%)
> Amean fault-base-7 7142.25 ( 0.00%) 4858.08 ( 31.98%)
> Amean fault-base-12 13827.64 ( 0.00%) 10292.11 ( 25.57%)
> Amean fault-base-18 18235.07 ( 0.00%) 13788.84 ( 24.38%)
> Amean fault-base-24 21597.80 ( 0.00%) 24388.03 (-12.92%)
> Amean fault-base-30 26754.15 ( 0.00%) 19700.55 ( 26.36%)
> Amean fault-base-32 26784.94 ( 0.00%) 19513.57 ( 27.15%)
> Amean fault-huge-1 4223.96 ( 0.00%) 2178.57 ( 48.42%)
> Amean fault-huge-3 2194.77 ( 0.00%) 2149.74 ( 2.05%)
> Amean fault-huge-5 2569.60 ( 0.00%) 2346.95 ( 8.66%)
> Amean fault-huge-7 3612.69 ( 0.00%) 2997.70 ( 17.02%)
> Amean fault-huge-12 3301.75 ( 0.00%) 6727.02 (-103.74%)
> Amean fault-huge-18 6696.47 ( 0.00%) 6685.72 ( 0.16%)
> Amean fault-huge-24 8000.72 ( 0.00%) 9311.43 (-16.38%)
> Amean fault-huge-30 13305.55 ( 0.00%) 9750.45 ( 26.72%)
> Amean fault-huge-32 9981.71 ( 0.00%) 10316.06 ( -3.35%)
>
> The average time to fault pages is substantially reduced in the
> majority of caseds but with the obvious caveat that fewer THPs
> are actually used in this adverse workload
>
> 4.4.0 4.4.0
> kcompactd-v1r1 nodefrag-v1r3
> Percentage huge-1 0.71 ( 0.00%) 14.04 (1865.22%)
> Percentage huge-3 10.77 ( 0.00%) 33.05 (206.85%)
> Percentage huge-5 60.39 ( 0.00%) 38.51 (-36.23%)
> Percentage huge-7 45.97 ( 0.00%) 34.57 (-24.79%)
> Percentage huge-12 68.12 ( 0.00%) 40.07 (-41.17%)
> Percentage huge-18 64.93 ( 0.00%) 47.82 (-26.35%)
> Percentage huge-24 62.69 ( 0.00%) 44.23 (-29.44%)
> Percentage huge-30 43.49 ( 0.00%) 55.38 ( 27.34%)
> Percentage huge-32 50.72 ( 0.00%) 51.90 ( 2.35%)
>
> 4.4.0 4.4.0
> kcompactd-v1r1nodefrag-v1r3
> Minor Faults 37429143 47564000
> Major Faults 1916 1558
> Swap Ins 1466 1079
> Swap Outs 2936863 149626
> Allocation stalls 62510 3
> DMA allocs 0 0
> DMA32 allocs 6566458 6401314
> Normal allocs 216361697 216538171
> Movable allocs 0 0
> Direct pages scanned 25977580 17998
> Kswapd pages scanned 0 3638931
> Kswapd pages reclaimed 0 207236
> Direct pages reclaimed 8833714 88
> Compaction stalls 103349 5
> Compaction success 270 4
> Compaction failures 103079 1
>
> Note again that while this does swap as it's an aggressive workload,
> the direct relcim activity and allocation stalls is substantially
> reduced. There is some kswapd activity but ftrace showed that the
> kswapd activity was due to normal wakeups from 4K pages being
> allocated. Compaction-related stalls and activity are almost
> eliminated.
>
> I also tried the stutter benchmark. For this, I do not have figures for
> NUMA but it's something that does impact UMA so I'll report what is available
>
> stutter
> 4.4.0 4.4.0
> kcompactd-v1r1 nodefrag-v1r3
> Min mmap 7.3571 ( 0.00%) 7.3438 ( 0.18%)
> 1st-qrtle mmap 7.5278 ( 0.00%) 17.9200 (-138.05%)
> 2nd-qrtle mmap 7.6818 ( 0.00%) 21.6055 (-181.25%)
> 3rd-qrtle mmap 11.0889 ( 0.00%) 21.8881 (-97.39%)
> Max-90% mmap 27.8978 ( 0.00%) 22.1632 ( 20.56%)
> Max-93% mmap 28.3202 ( 0.00%) 22.3044 ( 21.24%)
> Max-95% mmap 28.5600 ( 0.00%) 22.4580 ( 21.37%)
> Max-99% mmap 29.6032 ( 0.00%) 25.5216 ( 13.79%)
> Max mmap 4109.7289 ( 0.00%) 4813.9832 (-17.14%)
> Mean mmap 12.4474 ( 0.00%) 19.3027 (-55.07%)
>
> This benchmark is trying to fault an anonymous mapping while there is
> a heavy IO load -- a scenario that desktop users used to complain about
> frequently. This shows a mix because the ideal case of mapping with THP
> is not hit as often. However, note that 99% of the mappings complete
> 13.79% faster. The CPU usage here is particularly interesting
>
> 4.4.0 4.4.0
> kcompactd-v1r1nodefrag-v1r3
> User 67.50 0.99
> System 1327.88 91.30
> Elapsed 2079.00 2128.98
>
> And once again we look at the reclaim figures
>
> 4.4.0 4.4.0
> kcompactd-v1r1nodefrag-v1r3
> Minor Faults 335241922 1314582827
> Major Faults 715 819
> Swap Ins 0 0
> Swap Outs 0 0
> Allocation stalls 532723 0
> DMA allocs 0 0
> DMA32 allocs 1822364341 1177950222
> Normal allocs 1815640808 1517844854
> Movable allocs 0 0
> Direct pages scanned 21892772 0
> Kswapd pages scanned 20015890 41879484
> Kswapd pages reclaimed 19961986 41822072
> Direct pages reclaimed 21892741 0
> Compaction stalls 1065755 0
> Compaction success 514 0
> Compaction failures 1065241 0
>
> Allocation stalls and all direct reclaim activity is eliminated as well
> as compaction-related stalls.
>
> THP gives impressive gains in some cases but only if they are quickly
> available. We're not going to reach the point where they are completely
> free so lets take the costs out of the fast paths finally and defer the
> cost to kswapd, kcompactd and khugepaged where it belongs.
>
> Signed-off-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
Acked-by: Johannes Weiner <hannes@xxxxxxxxxxx>
The cornercases Rik pointed out aside, if the mapping isn't long-lived
enough that it can wait for khugepaged, what are the odds that the
defrag work will be offset by the TLB savings? However, for mappings
where it would pay off, having to do the same defrag work but doing it
at a later time is actually a net loss. Should we consider keeping
direct reclaim and compaction as a configurable option as least?
Regardless, this looks like much saner defaults than what we have.