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If zone->lock is heavily contended at the same time, latency spikes may occur even for casual page allocation/freeing. Although reducing the batch number cannot make zone->lock contended lighter, it can reduce the latency spikes effectively. To demonstrate this, I wrote a Python script: import mmap size = 6 * 1024**3 while True: mm = mmap.mmap(-1, size) mm[:] = b'\xff' * size mm.close() Run this script 10 times in parallel and measure the allocation latency by measuring the duration of rmqueue_bulk() with the BCC tools funclatency[0]: funclatency -T -i 600 rmqueue_bulk Here are the results for both AMD and Intel CPUs. AMD EPYC 7W83 64-Core Processor, single NUMA node, KVM virtual server ===================================================================== - Default value of 5 nsecs : count distribution 0 -> 1 : 0 | | 2 -> 3 : 0 | | 4 -> 7 : 0 | | 8 -> 15 : 0 | | 16 -> 31 : 0 | | 32 -> 63 : 0 | | 64 -> 127 : 0 | | 128 -> 255 : 0 | | 256 -> 511 : 0 | | 512 -> 1023 : 12 | | 1024 -> 2047 : 9116 | | 2048 -> 4095 : 2004 | | 4096 -> 8191 : 2497 | | 8192 -> 16383 : 2127 | | 16384 -> 32767 : 2483 | | 32768 -> 65535 : 10102 | | 65536 -> 131071 : 212730 |******************* | 131072 -> 262143 : 314692 |***************************** | 262144 -> 524287 : 430058 |****************************************| 524288 -> 1048575 : 224032 |******************** | 1048576 -> 2097151 : 73567 |****** | 2097152 -> 4194303 : 17079 |* | 4194304 -> 8388607 : 3900 | | 8388608 -> 16777215 : 750 | | 16777216 -> 33554431 : 88 | | 33554432 -> 67108863 : 2 | | avg = 449775 nsecs, total: 587066511229 nsecs, count: 1305242 The avg alloc latency can be 449us, and the max latency can be higher than 30ms. - Value set to 0 nsecs : count distribution 0 -> 1 : 0 | | 2 -> 3 : 0 | | 4 -> 7 : 0 | | 8 -> 15 : 0 | | 16 -> 31 : 0 | | 32 -> 63 : 0 | | 64 -> 127 : 0 | | 128 -> 255 : 0 | | 256 -> 511 : 0 | | 512 -> 1023 : 92 | | 1024 -> 2047 : 8594 | | 2048 -> 4095 : 2042818 |****** | 4096 -> 8191 : 8737624 |************************** | 8192 -> 16383 : 13147872 |****************************************| 16384 -> 32767 : 8799951 |************************** | 32768 -> 65535 : 2879715 |******** | 65536 -> 131071 : 659600 |** | 131072 -> 262143 : 204004 | | 262144 -> 524287 : 78246 | | 524288 -> 1048575 : 30800 | | 1048576 -> 2097151 : 12251 | | 2097152 -> 4194303 : 2950 | | 4194304 -> 8388607 : 78 | | avg = 19359 nsecs, total: 708638369918 nsecs, count: 36604636 The avg was reduced significantly to 19us, and the max latency is reduced to less than 8ms. - Conclusion On this AMD CPU, reducing vm.pcp_batch_scale_max significantly helps reduce latency. Latency-sensitive applications will benefit from this tuning. However, I don't have access to other types of AMD CPUs, so I was unable to test it on different AMD models. Intel(R) Xeon(R) Platinum 8260 CPU @ 2.40GHz, two NUMA nodes ============================================================ - Default value of 5 nsecs : count distribution 0 -> 1 : 0 | | 2 -> 3 : 0 | | 4 -> 7 : 0 | | 8 -> 15 : 0 | | 16 -> 31 : 0 | | 32 -> 63 : 0 | | 64 -> 127 : 0 | | 128 -> 255 : 0 | | 256 -> 511 : 0 | | 512 -> 1023 : 2419 | | 1024 -> 2047 : 34499 |* | 2048 -> 4095 : 4272 | | 4096 -> 8191 : 9035 | | 8192 -> 16383 : 4374 | | 16384 -> 32767 : 2963 | | 32768 -> 65535 : 6407 | | 65536 -> 131071 : 884806 |****************************************| 131072 -> 262143 : 145931 |****** | 262144 -> 524287 : 13406 | | 524288 -> 1048575 : 1874 | | 1048576 -> 2097151 : 249 | | 2097152 -> 4194303 : 28 | | avg = 96173 nsecs, total: 106778157925 nsecs, count: 1110263 - Conclusion This Intel CPU works fine with the default setting. Intel(R) Xeon(R) Platinum 8260 CPU @ 2.40GHz, single NUMA node ============================================================== Using the cpuset cgroup, we can restrict the test script to run on NUMA node 0 only. - Default value of 5 nsecs : count distribution 0 -> 1 : 0 | | 2 -> 3 : 0 | | 4 -> 7 : 0 | | 8 -> 15 : 0 | | 16 -> 31 : 0 | | 32 -> 63 : 0 | | 64 -> 127 : 0 | | 128 -> 255 : 0 | | 256 -> 511 : 46 | | 512 -> 1023 : 695 | | 1024 -> 2047 : 19950 |* | 2048 -> 4095 : 1788 | | 4096 -> 8191 : 3392 | | 8192 -> 16383 : 2569 | | 16384 -> 32767 : 2619 | | 32768 -> 65535 : 3809 | | 65536 -> 131071 : 616182 |****************************************| 131072 -> 262143 : 295587 |******************* | 262144 -> 524287 : 75357 |**** | 524288 -> 1048575 : 15471 |* | 1048576 -> 2097151 : 2939 | | 2097152 -> 4194303 : 243 | | 4194304 -> 8388607 : 3 | | avg = 144410 nsecs, total: 150281196195 nsecs, count: 1040651 The zone->lock contention becomes severe when there is only a single NUMA node. The average latency is approximately 144us, with the maximum latency exceeding 4ms. - Value set to 0 nsecs : count distribution 0 -> 1 : 0 | | 2 -> 3 : 0 | | 4 -> 7 : 0 | | 8 -> 15 : 0 | | 16 -> 31 : 0 | | 32 -> 63 : 0 | | 64 -> 127 : 0 | | 128 -> 255 : 0 | | 256 -> 511 : 24 | | 512 -> 1023 : 2686 | | 1024 -> 2047 : 10246 | | 2048 -> 4095 : 4061529 |********* | 4096 -> 8191 : 16894971 |****************************************| 8192 -> 16383 : 6279310 |************** | 16384 -> 32767 : 1658240 |*** | 32768 -> 65535 : 445760 |* | 65536 -> 131071 : 110817 | | 131072 -> 262143 : 20279 | | 262144 -> 524287 : 4176 | | 524288 -> 1048575 : 436 | | 1048576 -> 2097151 : 8 | | 2097152 -> 4194303 : 2 | | avg = 8401 nsecs, total: 247739809022 nsecs, count: 29488508 After setting it to 0, the avg latency is reduced to around 8us, and the max latency is less than 4ms. - Conclusion On this Intel CPU, this tuning doesn't help much. Latency-sensitive applications work well with the default setting. It is worth noting that all the above data were tested using the upstream kernel. Why introduce a systl knob? =========================== From the above data, it's clear that different CPU types have varying allocation latencies concerning zone->lock contention. Typically, people don't release individual kernel packages for each type of x86_64 CPU. Furthermore, for latency-insensitive applications, we can keep the default setting for better throughput. In our production environment, we set this value to 0 for applications running on Kubernetes servers while keeping it at the default value of 5 for other applications like big data. It's not common to release individual kernel packages for each application. Future work =========== To ultimately mitigate the zone->lock contention issue, several suggestions have been proposed. One approach involves dividing large zones into multi smaller zones, as suggested by Matthew[1], while another entails splitting the zone->lock using a mechanism similar to memory arenas and shifting away from relying solely on zone_id to identify the range of free lists a particular page belongs to, as suggested by Mel[2]. However, implementing these solutions is likely to necessitate a more extended development effort. Link: https://github.com/iovisor/bcc/blob/master/tools/funclatency.py [0] Link: https://lore.kernel.org/linux-mm/ZnTrZ9mcAIRodnjx@casper.infradead.org/ [1] Link: https://lore.kernel.org/linux-mm/20240705130943.htsyhhhzbcptnkcu@techsingularity.net/ [2] Signed-off-by: Yafang Shao Cc: "Huang, Ying" Cc: Mel Gorman Cc: Matthew Wilcox Cc: David Rientjes --- Documentation/admin-guide/sysctl/vm.rst | 17 +++++++++++++++++ mm/Kconfig | 11 ----------- mm/page_alloc.c | 23 +++++++++++++++++------ 3 files changed, 34 insertions(+), 17 deletions(-) diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst index f48eaa98d22d..4971289dfb79 100644 --- a/Documentation/admin-guide/sysctl/vm.rst +++ b/Documentation/admin-guide/sysctl/vm.rst @@ -66,6 +66,7 @@ Currently, these files are in /proc/sys/vm: - page-cluster - page_lock_unfairness - panic_on_oom +- pcp_batch_scale_max - percpu_pagelist_high_fraction - stat_interval - stat_refresh @@ -883,6 +884,22 @@ panic_on_oom=2+kdump gives you very strong tool to investigate why oom happens. You can get snapshot. +pcp_batch_scale_max +=================== + +In page allocator, PCP (Per-CPU pageset) is refilled and drained in +batches. The batch number is scaled automatically to improve page +allocation/free throughput. But too large scale factor may hurt +latency. This option sets the upper limit of scale factor to limit +the maximum latency. + +The range for this parameter spans from 0 to 6, with a default value of 5. +The value assigned to 'N' signifies that during each refilling or draining +process, a maximum of (batch << N) pages will be involved, where "batch" +represents the default batch size automatically computed by the kernel for +each zone. + + percpu_pagelist_high_fraction ============================= diff --git a/mm/Kconfig b/mm/Kconfig index 7b716ac80272..14f64b4f744a 100644 --- a/mm/Kconfig +++ b/mm/Kconfig @@ -690,17 +690,6 @@ config HUGETLB_PAGE_SIZE_VARIABLE config CONTIG_ALLOC def_bool (MEMORY_ISOLATION && COMPACTION) || CMA -config PCP_BATCH_SCALE_MAX - int "Maximum scale factor of PCP (Per-CPU pageset) batch allocate/free" - default 5 - range 0 6 - help - In page allocator, PCP (Per-CPU pageset) is refilled and drained in - batches. The batch number is scaled automatically to improve page - allocation/free throughput. But too large scale factor may hurt - latency. This option sets the upper limit of scale factor to limit - the maximum latency. - config PHYS_ADDR_T_64BIT def_bool 64BIT diff --git a/mm/page_alloc.c b/mm/page_alloc.c index 5a842cc13314..bf0c94a0b659 100644 --- a/mm/page_alloc.c +++ b/mm/page_alloc.c @@ -273,6 +273,8 @@ int min_free_kbytes = 1024; int user_min_free_kbytes = -1; static int watermark_boost_factor __read_mostly = 15000; static int watermark_scale_factor = 10; +static int pcp_batch_scale_max = 5; +static int sysctl_6 = 6; /* movable_zone is the "real" zone pages in ZONE_MOVABLE are taken from */ int movable_zone; @@ -2391,7 +2393,7 @@ static void drain_pages_zone(unsigned int cpu, struct zone *zone) count = pcp->count; if (count) { int to_drain = min(count, - pcp->batch << CONFIG_PCP_BATCH_SCALE_MAX); + pcp->batch << pcp_batch_scale_max); free_pcppages_bulk(zone, to_drain, pcp, 0); count -= to_drain; @@ -2519,7 +2521,7 @@ static int nr_pcp_free(struct per_cpu_pages *pcp, int batch, int high, bool free /* Free as much as possible if batch freeing high-order pages. */ if (unlikely(free_high)) - return min(pcp->count, batch << CONFIG_PCP_BATCH_SCALE_MAX); + return min(pcp->count, batch << pcp_batch_scale_max); /* Check for PCP disabled or boot pageset */ if (unlikely(high < batch)) @@ -2551,7 +2553,7 @@ static int nr_pcp_high(struct per_cpu_pages *pcp, struct zone *zone, return 0; if (unlikely(free_high)) { - pcp->high = max(high - (batch << CONFIG_PCP_BATCH_SCALE_MAX), + pcp->high = max(high - (batch << pcp_batch_scale_max), high_min); return 0; } @@ -2621,9 +2623,9 @@ static void free_unref_page_commit(struct zone *zone, struct per_cpu_pages *pcp, } else if (pcp->flags & PCPF_PREV_FREE_HIGH_ORDER) { pcp->flags &= ~PCPF_PREV_FREE_HIGH_ORDER; } - if (pcp->free_count < (batch << CONFIG_PCP_BATCH_SCALE_MAX)) + if (pcp->free_count < (batch << pcp_batch_scale_max)) pcp->free_count = min(pcp->free_count + (1 << order), - batch << CONFIG_PCP_BATCH_SCALE_MAX); + batch << pcp_batch_scale_max); high = nr_pcp_high(pcp, zone, batch, free_high); if (pcp->count >= high) { free_pcppages_bulk(zone, nr_pcp_free(pcp, batch, high, free_high), @@ -2964,7 +2966,7 @@ static int nr_pcp_alloc(struct per_cpu_pages *pcp, struct zone *zone, int order) * subsequent allocation of order-0 pages without any freeing. */ if (batch <= max_nr_alloc && - pcp->alloc_factor < CONFIG_PCP_BATCH_SCALE_MAX) + pcp->alloc_factor < pcp_batch_scale_max) pcp->alloc_factor++; batch = min(batch, max_nr_alloc); } @@ -6341,6 +6343,15 @@ static struct ctl_table page_alloc_sysctl_table[] = { .proc_handler = percpu_pagelist_high_fraction_sysctl_handler, .extra1 = SYSCTL_ZERO, }, + { + .procname = "pcp_batch_scale_max", + .data = &pcp_batch_scale_max, + .maxlen = sizeof(pcp_batch_scale_max), + .mode = 0644, + .proc_handler = proc_dointvec_minmax, + .extra1 = SYSCTL_ZERO, + .extra2 = &sysctl_6, + }, { .procname = "lowmem_reserve_ratio", .data = &sysctl_lowmem_reserve_ratio,