Re: [PATCH 6/6] mm/page_alloc: Introduce vm.percpu_pagelist_high_fraction
From: Vlastimil Babka <hidden>
Date: 2021-05-28 11:59:41
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On 5/25/21 10:01 AM, Mel Gorman wrote:
This introduces a new sysctl vm.percpu_pagelist_high_fraction. It is
similar to the old vm.percpu_pagelist_fraction. The old sysctl increased
both pcp->batch and pcp->high with the higher pcp->high potentially
reducing zone->lock contention. However, the higher pcp->batch value also
potentially increased allocation latency while the PCP was refilled.
This sysctl only adjusts pcp->high so that zone->lock contention is
potentially reduced but allocation latency during a PCP refill remains
the same.
# grep -E "high:|batch" /proc/zoneinfo | tail -2
high: 649
batch: 63
# sysctl vm.percpu_pagelist_high_fraction=8
# grep -E "high:|batch" /proc/zoneinfo | tail -2
high: 35071
batch: 63
# sysctl vm.percpu_pagelist_high_fraction=64
high: 4383
batch: 63
# sysctl vm.percpu_pagelist_high_fraction=0
high: 649
batch: 63
Signed-off-by: Mel Gorman <redacted>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>Acked-by: Vlastimil Babka <redacted> Documentation nit below:
quoted hunk ↗ jump to hunk
@@ -789,6 +790,25 @@ panic_on_oom=2+kdump gives you very strong tool to investigate why oom happens. You can get snapshot. +percpu_pagelist_high_fraction +============================= + +This is the fraction of pages in each zone that are allocated for each +per cpu page list. The min value for this is 8. It means that we do +not allow more than 1/8th of pages in each zone to be allocated in any +single per_cpu_pagelist.
This, while technically correct (as an upper limit) is somewhat misleading as the limit for a single per_cpu_pagelist also considers the number of local cpus.
This entry only changes the value of hot per +cpu pagelists. User can specify a number like 100 to allocate 1/100th +of each zone to each per cpu page list.
This is worse. Anyone trying to reproduce this example on a system with multiple cpus per node and checking the result will be puzzled. So I think the part about number of local cpus should be mentioned to avoid confusion.
+The batch value of each per cpu pagelist remains the same regardless of the +value of the high fraction so allocation latencies are unaffected. + +The initial value is zero. Kernel uses this value to set the high pcp->high +mark based on the low watermark for the zone and the number of local +online CPUs. If the user writes '0' to this sysctl, it will revert to +this default behavior. + +