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Date: Mon, 4 Jul 2016 10:37:03 +0900 From: Minchan Kim <minchan@...nel.org> To: Mel Gorman <mgorman@...hsingularity.net> Cc: Andrew Morton <akpm@...ux-foundation.org>, Linux-MM <linux-mm@...ck.org>, Rik van Riel <riel@...riel.com>, Vlastimil Babka <vbabka@...e.cz>, Johannes Weiner <hannes@...xchg.org>, LKML <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 00/31] Move LRU page reclaim from zones to nodes v8 On Fri, Jul 01, 2016 at 09:01:08PM +0100, Mel Gorman wrote: > (Sorry for the resend, I accidentally sent the branch that still had the > Signed-off-by's from mmotm still applied which is incorrect.) > > Previous releases double accounted LRU stats on the zone and the node > because it was required by should_reclaim_retry. The last patch in the > series removes the double accounting. It's not integrated with the series > as reviewers may not like the solution. If not, it can be safely dropped > without a major impact to the results. > > Changelog since v7 > o Rebase onto current mmots > o Avoid double accounting of stats in node and zone > o Kswapd will avoid more reclaim if an eligible zone is available > o Remove some duplications of sc->reclaim_idx and classzone_idx > o Print per-node stats in zoneinfo > > Changelog since v6 > o Correct reclaim_idx when direct reclaiming for memcg > o Also account LRU pages per zone for compaction/reclaim > o Add page_pgdat helper with more efficient lookup > o Init pgdat LRU lock only once > o Slight optimisation to wake_all_kswapds > o Always wake kcompactd when kswapd is going to sleep > o Rebase to mmotm as of June 15th, 2016 > > Changelog since v5 > o Rebase and adjust to changes > > Changelog since v4 > o Rebase on top of v3 of page allocator optimisation series > > Changelog since v3 > o Rebase on top of the page allocator optimisation series > o Remove RFC tag > > This is the latest version of a series that moves LRUs from the zones to > the node that is based upon 4.7-rc4 with Andrew's tree applied. While this > is a current rebase, the test results were based on mmotm as of June 23rd. > Conceptually, this series is simple but there are a lot of details. Some > of the broad motivations for this are; > > 1. The residency of a page partially depends on what zone the page was > allocated from. This is partially combatted by the fair zone allocation > policy but that is a partial solution that introduces overhead in the > page allocator paths. > > 2. Currently, reclaim on node 0 behaves slightly different to node 1. For > example, direct reclaim scans in zonelist order and reclaims even if > the zone is over the high watermark regardless of the age of pages > in that LRU. Kswapd on the other hand starts reclaim on the highest > unbalanced zone. A difference in distribution of file/anon pages due > to when they were allocated results can result in a difference in > again. While the fair zone allocation policy mitigates some of the > problems here, the page reclaim results on a multi-zone node will > always be different to a single-zone node. > it was scheduled on as a result. > > 3. kswapd and the page allocator scan zones in the opposite order to > avoid interfering with each other but it's sensitive to timing. This > mitigates the page allocator using pages that were allocated very recently > in the ideal case but it's sensitive to timing. When kswapd is allocating > from lower zones then it's great but during the rebalancing of the highest > zone, the page allocator and kswapd interfere with each other. It's worse > if the highest zone is small and difficult to balance. > > 4. slab shrinkers are node-based which makes it harder to identify the exact > relationship between slab reclaim and LRU reclaim. > > The reason we have zone-based reclaim is that we used to have > large highmem zones in common configurations and it was necessary > to quickly find ZONE_NORMAL pages for reclaim. Today, this is much > less of a concern as machines with lots of memory will (or should) use > 64-bit kernels. Combinations of 32-bit hardware and 64-bit hardware are > rare. Machines that do use highmem should have relatively low highmem:lowmem > ratios than we worried about in the past. Hello Mel, I agree the direction absolutely. However, I have a concern on highmem system as you already mentioned. Embedded products still use 2 ~ 3 ratio (highmem:lowmem). In such system, LRU churning by skipping other zone pages frequently might be significant for the performance. How big ratio between highmem:lowmem do you think a problem? > > Conceptually, moving to node LRUs should be easier to understand. The > page allocator plays fewer tricks to game reclaim and reclaim behaves > similarly on all nodes. > > The series has been tested on a 16 core UMA machine and a 2-socket 48 > core NUMA machine. The UMA results are presented in most cases as the NUMA > machine behaved similarly. I guess you would already test below with various highmem system(e.g., 2:1, 3:1, 4:1 and so on). If you have, could you mind sharing it? > > pagealloc > --------- > > This is a microbenchmark that shows the benefit of removing the fair zone > allocation policy. It was tested uip to order-4 but only orders 0 and 1 are > shown as the other orders were comparable. > > 4.7.0-rc4 4.7.0-rc4 > mmotm-20160623 nodelru-v8 > Min total-odr0-1 490.00 ( 0.00%) 463.00 ( 5.51%) > Min total-odr0-2 349.00 ( 0.00%) 325.00 ( 6.88%) > Min total-odr0-4 288.00 ( 0.00%) 272.00 ( 5.56%) > Min total-odr0-8 250.00 ( 0.00%) 235.00 ( 6.00%) > Min total-odr0-16 234.00 ( 0.00%) 222.00 ( 5.13%) > Min total-odr0-32 223.00 ( 0.00%) 205.00 ( 8.07%) > Min total-odr0-64 217.00 ( 0.00%) 202.00 ( 6.91%) > Min total-odr0-128 214.00 ( 0.00%) 207.00 ( 3.27%) > Min total-odr0-256 242.00 ( 0.00%) 242.00 ( 0.00%) > Min total-odr0-512 272.00 ( 0.00%) 265.00 ( 2.57%) > Min total-odr0-1024 290.00 ( 0.00%) 283.00 ( 2.41%) > Min total-odr0-2048 302.00 ( 0.00%) 296.00 ( 1.99%) > Min total-odr0-4096 311.00 ( 0.00%) 306.00 ( 1.61%) > Min total-odr0-8192 314.00 ( 0.00%) 309.00 ( 1.59%) > Min total-odr0-16384 315.00 ( 0.00%) 309.00 ( 1.90%) > Min total-odr1-1 741.00 ( 0.00%) 716.00 ( 3.37%) > Min total-odr1-2 565.00 ( 0.00%) 524.00 ( 7.26%) > Min total-odr1-4 457.00 ( 0.00%) 427.00 ( 6.56%) > Min total-odr1-8 408.00 ( 0.00%) 371.00 ( 9.07%) > Min total-odr1-16 383.00 ( 0.00%) 344.00 ( 10.18%) > Min total-odr1-32 378.00 ( 0.00%) 334.00 ( 11.64%) > Min total-odr1-64 383.00 ( 0.00%) 334.00 ( 12.79%) > Min total-odr1-128 376.00 ( 0.00%) 342.00 ( 9.04%) > Min total-odr1-256 381.00 ( 0.00%) 343.00 ( 9.97%) > Min total-odr1-512 388.00 ( 0.00%) 349.00 ( 10.05%) > Min total-odr1-1024 386.00 ( 0.00%) 356.00 ( 7.77%) > Min total-odr1-2048 389.00 ( 0.00%) 362.00 ( 6.94%) > Min total-odr1-4096 389.00 ( 0.00%) 362.00 ( 6.94%) > Min total-odr1-8192 389.00 ( 0.00%) 362.00 ( 6.94%) > > This shows a steady improvement throughout. The primary benefit is from > reduced system CPU usage which is obvious from the overall times; > > 4.7.0-rc4 4.7.0-rc4 > mmotm-20160623nodelru-v8 > User 191.39 191.61 > System 2651.24 2504.48 > Elapsed 2904.40 2757.01 > > The vmstats also showed that the fair zone allocation policy was definitely > removed as can be seen here; > > > 4.7.0-rc3 4.7.0-rc3 > mmotm-20160623 nodelru-v8 > DMA32 allocs 28794771816 0 > Normal allocs 48432582848 77227356392 > Movable allocs 0 0 > > tiobench on ext4 > ---------------- > > tiobench is a benchmark that artifically benefits if old pages remain resident > while new pages get reclaimed. The fair zone allocation policy mitigates this > problem so pages age fairly. While the benchmark has problems, it is important > that tiobench performance remains constant as it implies that page aging > problems that the fair zone allocation policy fixes are not re-introduced. > > 4.7.0-rc4 4.7.0-rc4 > mmotm-20160623 nodelru-v8 > Min PotentialReadSpeed 89.65 ( 0.00%) 90.34 ( 0.77%) > Min SeqRead-MB/sec-1 82.68 ( 0.00%) 83.13 ( 0.54%) > Min SeqRead-MB/sec-2 72.76 ( 0.00%) 72.15 ( -0.84%) > Min SeqRead-MB/sec-4 75.13 ( 0.00%) 74.23 ( -1.20%) > Min SeqRead-MB/sec-8 64.91 ( 0.00%) 65.25 ( 0.52%) > Min SeqRead-MB/sec-16 62.24 ( 0.00%) 62.76 ( 0.84%) > Min RandRead-MB/sec-1 0.88 ( 0.00%) 0.95 ( 7.95%) > Min RandRead-MB/sec-2 0.95 ( 0.00%) 0.94 ( -1.05%) > Min RandRead-MB/sec-4 1.43 ( 0.00%) 1.46 ( 2.10%) > Min RandRead-MB/sec-8 1.61 ( 0.00%) 1.58 ( -1.86%) > Min RandRead-MB/sec-16 1.80 ( 0.00%) 1.93 ( 7.22%) > Min SeqWrite-MB/sec-1 76.41 ( 0.00%) 78.84 ( 3.18%) > Min SeqWrite-MB/sec-2 74.11 ( 0.00%) 73.35 ( -1.03%) > Min SeqWrite-MB/sec-4 80.05 ( 0.00%) 78.69 ( -1.70%) > Min SeqWrite-MB/sec-8 72.88 ( 0.00%) 71.38 ( -2.06%) > Min SeqWrite-MB/sec-16 75.91 ( 0.00%) 75.81 ( -0.13%) > Min RandWrite-MB/sec-1 1.18 ( 0.00%) 1.12 ( -5.08%) > Min RandWrite-MB/sec-2 1.02 ( 0.00%) 1.02 ( 0.00%) > Min RandWrite-MB/sec-4 1.05 ( 0.00%) 0.99 ( -5.71%) > Min RandWrite-MB/sec-8 0.89 ( 0.00%) 0.92 ( 3.37%) > Min RandWrite-MB/sec-16 0.92 ( 0.00%) 0.89 ( -3.26%) > > This shows that the series has little or not impact on tiobench which is > desirable. It indicates that the fair zone allocation policy was removed > in a manner that didn't reintroduce one class of page aging bug. There > were only minor differences in overall reclaim activity > > 4.7.0-rc4 4.7.0-rc4 > mmotm-20160623nodelru-v8 > Minor Faults 645838 644036 > Major Faults 573 593 > Swap Ins 0 0 > Swap Outs 0 0 > Allocation stalls 24 0 > DMA allocs 0 0 > DMA32 allocs 46041453 44154171 > Normal allocs 78053072 79865782 > Movable allocs 0 0 > Direct pages scanned 10969 54504 > Kswapd pages scanned 93375144 93250583 > Kswapd pages reclaimed 93372243 93247714 > Direct pages reclaimed 10969 54504 > Kswapd efficiency 99% 99% > Kswapd velocity 13741.015 13711.950 > Direct efficiency 100% 100% > Direct velocity 1.614 8.014 > Percentage direct scans 0% 0% > Zone normal velocity 8641.875 13719.964 > Zone dma32 velocity 5100.754 0.000 > Zone dma velocity 0.000 0.000 > Page writes by reclaim 0.000 0.000 > Page writes file 0 0 > Page writes anon 0 0 > Page reclaim immediate 37 54 > > kswapd activity was roughly comparable. There were differences in direct > reclaim activity but negligible in the context of the overall workload > (velocity of 8 pages per second with the patches applied, 1.6 pages per > second in the baseline kernel). Hmm, nodelru's allocation stall is zero above but how does direct page scanning/reclaimed happens? Above, DMA32 allocs in nodelru is almost same but zone dma32 velocity is zero. What does it means? > > pgbench read-only large configuration on ext4 > --------------------------------------------- > > pgbench is a database benchmark that can be sensitive to page reclaim > decisions. This also checks if removing the fair zone allocation policy > is safe > > pgbench Transactions > 4.7.0-rc4 4.7.0-rc4 > mmotm-20160623 nodelru-v8 > Hmean 1 188.26 ( 0.00%) 189.78 ( 0.81%) > Hmean 5 330.66 ( 0.00%) 328.69 ( -0.59%) > Hmean 12 370.32 ( 0.00%) 380.72 ( 2.81%) > Hmean 21 368.89 ( 0.00%) 369.00 ( 0.03%) > Hmean 30 382.14 ( 0.00%) 360.89 ( -5.56%) > Hmean 32 428.87 ( 0.00%) 432.96 ( 0.95%) > > Negligible differences again. As with tiobench, overall reclaim activity > was comparable. > > bonnie++ on ext4 > ---------------- > > No interesting performance difference, negligible differences on reclaim > stats. > > paralleldd on ext4 > ------------------ > > This workload uses varying numbers of dd instances to read large amounts of > data from disk. > > 4.7.0-rc3 4.7.0-rc3 > mmotm-20160615 nodelru-v7r17 > Amean Elapsd-1 181.57 ( 0.00%) 179.63 ( 1.07%) > Amean Elapsd-3 188.29 ( 0.00%) 183.68 ( 2.45%) > Amean Elapsd-5 188.02 ( 0.00%) 181.73 ( 3.35%) > Amean Elapsd-7 186.07 ( 0.00%) 184.11 ( 1.05%) > Amean Elapsd-12 188.16 ( 0.00%) 183.51 ( 2.47%) > Amean Elapsd-16 189.03 ( 0.00%) 181.27 ( 4.10%) > > 4.7.0-rc3 4.7.0-rc3 > mmotm-20160615nodelru-v7r17 > User 1439.23 1433.37 > System 8332.31 8216.01 > Elapsed 3619.80 3532.69 > > There is a slight gain in performance, some of which is from the reduced system > CPU usage. There areminor differences in reclaim activity but nothing significant > > 4.7.0-rc3 4.7.0-rc3 > mmotm-20160615nodelru-v7r17 > Minor Faults 362486 358215 > Major Faults 1143 1113 > Swap Ins 26 0 > Swap Outs 2920 482 > DMA allocs 0 0 > DMA32 allocs 31568814 28598887 > Normal allocs 46539922 49514444 > Movable allocs 0 0 > Allocation stalls 0 0 > Direct pages scanned 0 0 > Kswapd pages scanned 40886878 40849710 > Kswapd pages reclaimed 40869923 40835207 > Direct pages reclaimed 0 0 > Kswapd efficiency 99% 99% > Kswapd velocity 11295.342 11563.344 > Direct efficiency 100% 100% > Direct velocity 0.000 0.000 > Slabs scanned 131673 126099 > Direct inode steals 57 60 > Kswapd inode steals 762 18 > > It basically shows that kswapd was active at roughly the same rate in > both kernels. There was also comparable slab scanning activity and direct > reclaim was avoided in both cases. There appears to be a large difference > in numbers of inodes reclaimed but the workload has few active inodes and > is likely a timing artifact. It's interesting to note that the node-lru > did not swap in any pages but given the low swap activity, it's unlikely > to be significant. > > stutter > ------- > > stutter simulates a simple workload. One part uses a lot of anonymous > memory, a second measures mmap latency and a third copies a large file. > The primary metric is checking for mmap latency. > > stutter > 4.7.0-rc4 4.7.0-rc4 > mmotm-20160623 nodelru-v8 > Min mmap 16.6283 ( 0.00%) 16.1394 ( 2.94%) > 1st-qrtle mmap 54.7570 ( 0.00%) 55.2975 ( -0.99%) > 2nd-qrtle mmap 57.3163 ( 0.00%) 57.5230 ( -0.36%) > 3rd-qrtle mmap 58.9976 ( 0.00%) 58.0537 ( 1.60%) > Max-90% mmap 59.7433 ( 0.00%) 58.3910 ( 2.26%) > Max-93% mmap 60.1298 ( 0.00%) 58.4801 ( 2.74%) > Max-95% mmap 73.4112 ( 0.00%) 58.5537 ( 20.24%) > Max-99% mmap 92.8542 ( 0.00%) 58.9673 ( 36.49%) > Max mmap 1440.6569 ( 0.00%) 137.6875 ( 90.44%) > Mean mmap 59.3493 ( 0.00%) 55.5153 ( 6.46%) > Best99%Mean mmap 57.2121 ( 0.00%) 55.4194 ( 3.13%) > Best95%Mean mmap 55.9113 ( 0.00%) 55.2813 ( 1.13%) > Best90%Mean mmap 55.6199 ( 0.00%) 55.1044 ( 0.93%) > Best50%Mean mmap 53.2183 ( 0.00%) 52.8330 ( 0.72%) > Best10%Mean mmap 45.9842 ( 0.00%) 42.3740 ( 7.85%) > Best5%Mean mmap 43.2256 ( 0.00%) 38.8660 ( 10.09%) > Best1%Mean mmap 32.9388 ( 0.00%) 27.7577 ( 15.73%) > > This shows a number of improvements with the worst-case outlier greatly > improved. > > Some of the vmstats are interesting > > 4.7.0-rc4 4.7.0-rc4 > mmotm-20160623nodelru-v8 > Swap Ins 163 239 > Swap Outs 0 0 > Allocation stalls 2603 0 > DMA allocs 0 0 > DMA32 allocs 618719206 1303037965 > Normal allocs 891235743 229914091 > Movable allocs 0 0 > Direct pages scanned 216787 3173 > Kswapd pages scanned 50719775 41732250 > Kswapd pages reclaimed 41541765 41731168 > Direct pages reclaimed 209159 3173 > Kswapd efficiency 81% 99% > Kswapd velocity 16859.554 14231.043 > Direct efficiency 96% 100% > Direct velocity 72.061 1.082 > Percentage direct scans 0% 0% > Zone normal velocity 8431.777 14232.125 > Zone dma32 velocity 8499.838 0.000 > Zone dma velocity 0.000 0.000 > Page writes by reclaim 6215049.000 0.000 > Page writes file 6215049 0 > Page writes anon 0 0 > Page reclaim immediate 70673 143 > Sector Reads 81940800 81489388 > Sector Writes 100158984 99161860 > Page rescued immediate 0 0 > Slabs scanned 1366954 21196 > > While this is not guaranteed in all cases, this particular test showed > a large reduction in direct reclaim activity. It's also worth noting > that no page writes were issued from reclaim context. > > This series is not without its hazards. There are at least three areas > that I'm concerned with even though I could not reproduce any problems in > that area. > > 1. Reclaim/compaction is going to be affected because the amount of reclaim is > no longer targetted at a specific zone. Compaction works on a per-zone basis > so there is no guarantee that reclaiming a few THP's worth page pages will > have a positive impact on compaction success rates. > > 2. The Slab/LRU reclaim ratio is affected because the frequency the shrinkers > are called is now different. This may or may not be a problem but if it > is, it'll be because shrinkers are not called enough and some balancing > is required. > > 3. The anon/file reclaim ratio may be affected. Pages about to be dirtied are > distributed between zones and the fair zone allocation policy used to do > something very similar for anon. The distribution is now different but not > necessarily in any way that matters but it's still worth bearing in mind.
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