On Fri, May 28, 2021 at 01:59:37PM +0200, Vlastimil Babka wrote: > 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 <mgorman@xxxxxxxxxxxxxxxxxxx> > > Acked-by: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> > > Acked-by: Vlastimil Babka <vbabka@xxxxxxx> > Thanks. > Documentation nit below: > > > @@ -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. > Is this any better? diff --git a/Documentation/admin-guide/sysctl/vm.rst b/Documentation/admin-guide/sysctl/vm.rst index e85c2f21d209..2da25735a629 100644 --- a/Documentation/admin-guide/sysctl/vm.rst +++ b/Documentation/admin-guide/sysctl/vm.rst @@ -793,15 +793,16 @@ 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 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. - -The batch value of each per cpu pagelist remains the same regardless of the -value of the high fraction so allocation latencies are unaffected. +This is the fraction of pages in each zone that are can be stored to +per-cpu page lists. It is an upper boundary that is divided depending +on the number of online CPUs. The min value for this is 8 which means +that we do not allow more than 1/8th of pages in each zone to be stored +on per-cpu page lists. This entry only changes the value of hot per-cpu +page lists. A user can specify a number like 100 to allocate 1/100th of +each zone between per-cpu lists. + +The batch value of each per-cpu page list 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 -- Mel Gorman SUSE Labs
