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To:     Mel Gorman <mgorman@techsingularity.net>,
        Andrew Morton <akpm@linux-foundation.org>
Cc:     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>
From:   Vlastimil Babka <vbabka@suse.cz>
Subject: Re: [PATCH 6/6] mm/page_alloc: Introduce
 vm.percpu_pagelist_high_fraction
Message-ID: <018c4b99-81a5-bc12-03cd-662a938ef05a@suse.cz>
Date:   Fri, 28 May 2021 13:59:37 +0200
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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>

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.
> +
> +