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From: Vlastimil Babka <vbabka@suse.cz>
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>
Subject: Re: [PATCH 6/6] mm/page_alloc: Introduce vm.percpu_pagelist_high_fraction
Date: Fri, 28 May 2021 13:59:37 +0200	[thread overview]
Message-ID: <018c4b99-81a5-bc12-03cd-662a938ef05a@suse.cz> (raw)
In-Reply-To: <20210525080119.5455-7-mgorman@techsingularity.net>

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.
> +
> +

  reply	other threads:[~2021-05-28 11:59 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-25  8:01 [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active CPUs Mel Gorman
2021-05-25  8:01 ` [PATCH 1/6] mm/page_alloc: Delete vm.percpu_pagelist_fraction Mel Gorman
2021-05-26 17:41   ` Vlastimil Babka
2021-05-25  8:01 ` [PATCH 2/6] mm/page_alloc: Disassociate the pcp->high from pcp->batch Mel Gorman
2021-05-26 18:14   ` Vlastimil Babka
2021-05-27 10:52     ` Mel Gorman
2021-05-28 10:27       ` Vlastimil Babka
2021-05-25  8:01 ` [PATCH 3/6] mm/page_alloc: Adjust pcp->high after CPU hotplug events Mel Gorman
2021-05-28 11:08   ` Vlastimil Babka
2021-05-25  8:01 ` [PATCH 4/6] mm/page_alloc: Scale the number of pages that are batch freed Mel Gorman
2021-05-28 11:19   ` Vlastimil Babka
2021-05-25  8:01 ` [PATCH 5/6] mm/page_alloc: Limit the number of pages on PCP lists when reclaim is active Mel Gorman
2021-05-28 11:43   ` Vlastimil Babka
2021-05-25  8:01 ` [PATCH 6/6] mm/page_alloc: Introduce vm.percpu_pagelist_high_fraction Mel Gorman
2021-05-28 11:59   ` Vlastimil Babka [this message]
2021-05-28 12:53     ` Mel Gorman
2021-05-28 14:38       ` Vlastimil Babka
2021-05-27 19:36 ` [PATCH 0/6 v2] Calculate pcp->high based on zone sizes and active CPUs Dave Hansen
2021-05-28  8:55   ` Mel Gorman
2021-05-28  9:03     ` David Hildenbrand
2021-05-28  9:08       ` David Hildenbrand
2021-05-28  9:49         ` Mel Gorman
2021-05-28  9:52           ` David Hildenbrand
2021-05-28 10:09             ` Mel Gorman
2021-05-28 10:21               ` David Hildenbrand
2021-05-28 12:12     ` Vlastimil Babka
2021-05-28 12:37       ` Mel Gorman
2021-05-28 14:39     ` Dave Hansen
2021-05-28 15:18       ` Mel Gorman
2021-05-28 16:17         ` Dave Hansen
2021-05-31 12:00           ` Feng Tang
  -- strict thread matches above, loose matches on Subject: below --
2021-05-21 10:28 [RFC PATCH 0/6] " Mel Gorman
2021-05-21 10:28 ` [PATCH 6/6] mm/page_alloc: Introduce vm.percpu_pagelist_high_fraction Mel Gorman
2021-05-21 22:57   ` Dave Hansen
2021-05-24  9:25     ` Mel Gorman

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