On 5/28/21 2:53 PM, Mel Gorman wrote: > 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? Ack, thanks > 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 >
