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Subject: Re: [PATCH 6/6] mm/page_alloc: Introduce
 vm.percpu_pagelist_high_fraction
To:     Mel Gorman <mgorman@techsingularity.net>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Hillf Danton <hdanton@sina.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        Michal Hocko <mhocko@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Linux-MM <linux-mm@kvack.org>
References: <20210525080119.5455-1-mgorman@techsingularity.net>
 <20210525080119.5455-7-mgorman@techsingularity.net>
 <018c4b99-81a5-bc12-03cd-662a938ef05a@suse.cz>
 <20210528125334.GP30378@techsingularity.net>
From:   Vlastimil Babka <vbabka@suse.cz>
Message-ID: <3a6670d3-63eb-f97a-62a0-ec752d933a13@suse.cz>
Date:   Fri, 28 May 2021 16:38:04 +0200
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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
>