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> 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. > +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. > + > +
