Re: [PATCH 6/6] mm/page_alloc: Introduce vm.percpu_pagelist_high_fraction

Vlastimil Babka
2021-05-28T14:38:09Z

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@techsingularity.net>
>> > Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
>>
>> Acked-by: Vlastimil Babka <vbabka@suse.cz>
>>
>
> 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
>