mm: page_alloc: High-order per-cpu page allocator
Subsystems (4)
PATCH 1/1 mm: page_alloc: High-order per-cpu page allocator
SLUB has been the default small kernel object allocator for quite some time but it is not universally used due to performance concerns and a reliance on high-order pages. The high-order concerns has two major components -- high-order pages are not always available and high-order page allocations potentially contend on the zone->lock. This patch addresses some concerns about the zone lock contention by extending the per-cpu page allocator to cache high-order pages. The patch makes the following modifications
o New per-cpu lists are added to cache the high-order pages. This increases the cache footprint of the per-cpu allocator and overall usage but for some workloads, this will be offset by reduced contention on zone->lock. The first MIGRATE_PCPTYPE entries in the list are per-migratetype. The remaining are high-order caches up to and including PAGE_ALLOC_COSTLY_ORDER
o pcp accounting during free is now confined to free_pcppages_bulk as it's impossible for the caller to know exactly how many pages were freed. Due to the high-order caches, the number of pages drained for a request is no longer precise.
o The high watermark for per-cpu pages is increased to reduce the probability that a single refill causes a drain on the next free.
The benefit depends on both the workload and the machine as ultimately the determining factor is whether cache line bounces on zone->lock or contention is a problem. The patch was tested on a variety of workloads and machines, some of which are reported here.
This is the result from netperf running UDP_STREAM on localhost. It was selected on the basis that it is slab-intensive and has been the subject of previous SLAB vs SLUB comparisons with the caveat that this is not testing between two physical hosts.
2-socket modern machine
4.9.0-rc5 4.9.0-rc5
vanilla hopcpu-v1r9
Hmean send-64 178.38 ( 0.00%) 256.74 ( 43.93%)
Hmean send-128 351.49 ( 0.00%) 507.52 ( 44.39%)
Hmean send-256 671.23 ( 0.00%) 1004.19 ( 49.60%)
Hmean send-1024 2663.60 ( 0.00%) 3910.42 ( 46.81%)
Hmean send-2048 5126.53 ( 0.00%) 7562.13 ( 47.51%)
Hmean send-3312 7949.99 ( 0.00%) 11565.98 ( 45.48%)
Hmean send-4096 9433.56 ( 0.00%) 12929.67 ( 37.06%)
Hmean send-8192 15940.64 ( 0.00%) 21587.63 ( 35.43%)
Hmean send-16384 26699.54 ( 0.00%) 32013.79 ( 19.90%)
Hmean recv-64 178.38 ( 0.00%) 256.72 ( 43.92%)
Hmean recv-128 351.49 ( 0.00%) 507.47 ( 44.38%)
Hmean recv-256 671.20 ( 0.00%) 1003.95 ( 49.57%)
Hmean recv-1024 2663.45 ( 0.00%) 3909.70 ( 46.79%)
Hmean recv-2048 5126.26 ( 0.00%) 7560.67 ( 47.49%)
Hmean recv-3312 7949.50 ( 0.00%) 11564.63 ( 45.48%)
Hmean recv-4096 9433.04 ( 0.00%) 12927.48 ( 37.04%)
Hmean recv-8192 15939.64 ( 0.00%) 21584.59 ( 35.41%)
Hmean recv-16384 26698.44 ( 0.00%) 32009.77 ( 19.89%)1-socket 6 year old machine
4.9.0-rc5 4.9.0-rc5
vanilla hopcpu-v1r9
Hmean send-64 87.47 ( 0.00%) 127.14 ( 45.36%)
Hmean send-128 174.36 ( 0.00%) 256.42 ( 47.06%)
Hmean send-256 347.52 ( 0.00%) 509.41 ( 46.59%)
Hmean send-1024 1363.03 ( 0.00%) 1991.54 ( 46.11%)
Hmean send-2048 2632.68 ( 0.00%) 3759.51 ( 42.80%)
Hmean send-3312 4123.19 ( 0.00%) 5873.28 ( 42.45%)
Hmean send-4096 5056.48 ( 0.00%) 7072.81 ( 39.88%)
Hmean send-8192 8784.22 ( 0.00%) 12143.92 ( 38.25%)
Hmean send-16384 15081.60 ( 0.00%) 19812.71 ( 31.37%)
Hmean recv-64 86.19 ( 0.00%) 126.59 ( 46.87%)
Hmean recv-128 173.93 ( 0.00%) 255.21 ( 46.73%)
Hmean recv-256 346.19 ( 0.00%) 506.72 ( 46.37%)
Hmean recv-1024 1358.28 ( 0.00%) 1980.03 ( 45.77%)
Hmean recv-2048 2623.45 ( 0.00%) 3729.35 ( 42.15%)
Hmean recv-3312 4108.63 ( 0.00%) 5831.47 ( 41.93%)
Hmean recv-4096 5037.25 ( 0.00%) 7021.59 ( 39.39%)
Hmean recv-8192 8762.32 ( 0.00%) 12072.44 ( 37.78%)
Hmean recv-16384 15042.36 ( 0.00%) 19690.14 ( 30.90%)This is somewhat dramatic but it's also not universal. For example, it was observed on an older HP machine using pcc-cpufreq that there was almost no difference but pcc-cpufreq is also a known performance hazard.
These are quite different results but illustrate that the patch is dependent on the CPU. The results are similar for TCP_STREAM on the two-socket machine.
The observations on sockperf are different.
2-socket modern machine
sockperf-tcp-throughput
4.9.0-rc5 4.9.0-rc5
vanilla hopcpu-v2r4
Hmean 14 93.90 ( 0.00%) 92.79 ( -1.18%)
Hmean 100 1211.02 ( 0.00%) 1286.66 ( 6.25%)
Hmean 300 3081.59 ( 0.00%) 3347.84 ( 8.64%)
Hmean 500 4614.19 ( 0.00%) 4953.23 ( 7.35%)
Hmean 850 6521.74 ( 0.00%) 6951.72 ( 6.59%)
Stddev 14 0.89 ( 0.00%) 3.24 (-264.75%)
Stddev 100 5.95 ( 0.00%) 8.88 (-49.27%)
Stddev 300 11.16 ( 0.00%) 28.13 (-151.98%)
Stddev 500 36.32 ( 0.00%) 42.07 (-15.84%)
Stddev 850 29.61 ( 0.00%) 66.73 (-125.36%)sockperf-udp-throughput
4.9.0-rc5 4.9.0-rc5
vanilla hopcpu-v2r4
Hmean 14 16.82 ( 0.00%) 25.23 ( 50.03%)
Hmean 100 119.91 ( 0.00%) 180.63 ( 50.65%)
Hmean 300 358.11 ( 0.00%) 539.29 ( 50.59%)
Hmean 500 595.16 ( 0.00%) 892.15 ( 49.90%)
Hmean 850 989.44 ( 0.00%) 1496.01 ( 51.20%)
Stddev 14 0.05 ( 0.00%) 0.10 (-116.02%)
Stddev 100 0.53 ( 0.00%) 1.12 (-111.23%)
Stddev 300 1.43 ( 0.00%) 1.58 (-10.21%)
Stddev 500 3.93 ( 0.00%) 5.14 (-30.95%)
Stddev 850 4.02 ( 0.00%) 6.46 (-60.64%)Note that the improvements for TCP are nowhere near as dramatic as netperf, there is a slight loss for small packets and it's much more variable. While it's not presented here, it's known that running sockperf "under load" that packet latency is generally lower but not universally so. On the other hand, UDP improves performance but again, is much more variable.
This highlights that the patch is not necessarily a universal win and is going to depend heavily on both the workload and the CPU used.
hackbench was also tested with both socket and pipes and both processes and threads and the results are interesting in terms of how variability is imapcted
1-socket machine
hackbench-process-pipes
4.9.0-rc5 4.9.0-rc5
vanilla highmark-v1r12
Amean 1 12.9637 ( 0.00%) 13.1807 ( -1.67%)
Amean 3 13.4770 ( 0.00%) 13.6803 ( -1.51%)
Amean 5 18.5333 ( 0.00%) 18.7383 ( -1.11%)
Amean 7 24.5690 ( 0.00%) 23.0550 ( 6.16%)
Amean 12 39.7990 ( 0.00%) 36.7207 ( 7.73%)
Amean 16 56.0520 ( 0.00%) 48.2890 ( 13.85%)
Stddev 1 0.3847 ( 0.00%) 0.5853 (-52.15%)
Stddev 3 0.2652 ( 0.00%) 0.0295 ( 88.89%)
Stddev 5 0.5589 ( 0.00%) 0.2466 ( 55.87%)
Stddev 7 0.5310 ( 0.00%) 0.6680 (-25.79%)
Stddev 12 1.0780 ( 0.00%) 0.3230 ( 70.04%)
Stddev 16 2.1138 ( 0.00%) 0.6835 ( 67.66%)hackbench-process-sockets
Amean 1 4.8873 ( 0.00%) 4.7180 ( 3.46%) Amean 3 14.1157 ( 0.00%) 14.3643 ( -1.76%) Amean 5 22.5537 ( 0.00%) 23.1380 ( -2.59%) Amean 7 30.3743 ( 0.00%) 31.1520 ( -2.56%) Amean 12 49.1773 ( 0.00%) 50.3060 ( -2.30%) Amean 16 64.0873 ( 0.00%) 66.2633 ( -3.40%) Stddev 1 0.2360 ( 0.00%) 0.2201 ( 6.74%) Stddev 3 0.0539 ( 0.00%) 0.0780 (-44.72%) Stddev 5 0.1463 ( 0.00%) 0.1579 ( -7.90%) Stddev 7 0.1260 ( 0.00%) 0.3091 (-145.31%) Stddev 12 0.2169 ( 0.00%) 0.4822 (-122.36%) Stddev 16 0.0529 ( 0.00%) 0.4513 (-753.20%)
It's not a universal win for pipes but the differences are within the noise. What is interesting is that variability shows both gains and losses in stark contrast to the sockperf results. On the other hand, sockets generally show small losses albeit within the noise with more variability. Once again, the workload and CPU gets different results.
fsmark was tested with zero-sized files to continually allocate slab objects but didn't show any differences. This can be explained by the fact that the workload is only allocating and does not have mix of allocs/frees that would benefit from the caching. It was tested to ensure no major harm was done.
While it is recognised that this is a mixed bag of results, the patch helps a lot more workloads than it hurts and intuitively, avoiding the zone->lock in some cases is a good thing.
--- include/linux/mmzone.h | 20 +++++++++- mm/page_alloc.c | 105 +++++++++++++++++++++++++++++-------------------- 2 files changed, 82 insertions(+), 43 deletions(-)
include/linux/mmzone.h +19 -1
@@ -255,6 +255,24 @@ enum zone_watermarks { NR_WMARK }; +/* +* One per migratetype for order-0 pages and one per high-order up to +* and including PAGE_ALLOC_COSTLY_ORDER. This may allow unmovable +* allocations to contaminate reclaimable pageblocks if high-order +* pages are heavily used. +*/ +#define NR_PCP_LISTS (MIGRATE_PCPTYPES + PAGE_ALLOC_COSTLY_ORDER) + +static inline unsigned int pindex_to_order (unsigned int pindex) +{ +return pindex <MIGRATE_PCPTYPES ?0 : pindex -MIGRATE_PCPTYPES +1 ; +} + +static inline unsigned int order_to_pindex (int migratetype,unsigned int order) +{ +return (order ==0 ) ? migratetype :MIGRATE_PCPTYPES + order -1 ; +} +#define min_wmark_pages (z) (z->watermark[WMARK_MIN])#define low_wmark_pages (z) (z->watermark[WMARK_LOW])#define high_wmark_pages (z) (z->watermark[WMARK_HIGH])
@@ -265,7 +283,7 @@ struct per_cpu_pages {int batch;/* chunk size for buddy add/remove */ /* Lists of pages, one per migrate type stored on the pcp-lists */ -struct list_head lists[MIGRATE_PCPTYPES ]; +struct list_head lists[NR_PCP_LISTS ]; };struct per_cpu_pageset {
mm/page_alloc.c +63 -42
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