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From: Dave Hansen <dave.hansen@intel.com>
To: Mel Gorman <mgorman@techsingularity.net>
Cc: Linux-MM <linux-mm@kvack.org>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	Matthew Wilcox <willy@infradead.org>,
	Vlastimil Babka <vbabka@suse.cz>,
	Michal Hocko <mhocko@kernel.org>,
	Nicholas Piggin <npiggin@gmail.com>,
	LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH 3/6] mm/page_alloc: Adjust pcp->high after CPU hotplug events
Date: Mon, 24 May 2021 08:52:02 -0700	[thread overview]
Message-ID: <e9061a5c-bbef-e818-94f7-95e21a73a948@intel.com> (raw)
In-Reply-To: <20210524090726.GB30378@techsingularity.net>

On 5/24/21 2:07 AM, Mel Gorman wrote:
> On Fri, May 21, 2021 at 03:13:35PM -0700, Dave Hansen wrote:
>> On 5/21/21 3:28 AM, Mel Gorman wrote:
>>> The PCP high watermark is based on the number of online CPUs so the
>>> watermarks must be adjusted during CPU hotplug. At the time of
>>> hot-remove, the number of online CPUs is already adjusted but during
>>> hot-add, a delta needs to be applied to update PCP to the correct
>>> value. After this patch is applied, the high watermarks are adjusted
>>> correctly.
>>>
>>>   # grep high: /proc/zoneinfo  | tail -1
>>>               high:  649
>>>   # echo 0 > /sys/devices/system/cpu/cpu4/online
>>>   # grep high: /proc/zoneinfo  | tail -1
>>>               high:  664
>>>   # echo 1 > /sys/devices/system/cpu/cpu4/online
>>>   # grep high: /proc/zoneinfo  | tail -1
>>>               high:  649
>> This is actually a comment more about the previous patch, but it doesn't
>> really become apparent until the example above.
>>
>> In your example, you mentioned increased exit() performance by using
>> "vm.percpu_pagelist_fraction to increase the pcp->high value".  That's
>> presumably because of the increased batching effects and fewer lock
>> acquisitions.
>>
> Yes
> 
>> But, logically, doesn't that mean that, the more CPUs you have in a
>> node, the *higher* you want pcp->high to be?  If we took this to the
>> extreme and had an absurd number of CPUs in a node, we could end up with
>> a too-small pcp->high value.
>>
> I see your point but I don't think increasing pcp->high for larger
> numbers of CPUs is the right answer because then reclaim can be
> triggered simply because too many PCPs have pages.
> 
> To address your point requires much deeper surgery.
...
> There is value to doing something like this but it's beyond what this
> series is trying to do and doing the work without introducing regressions
> would be very difficult.

Agreed, such a solution is outside of the scope of what this set is
trying to do.

It would be nice to touch on this counter-intuitive property in the
changelog, and *maybe* add a WARN_ON_ONCE() if we hit an edge case.
Maybe WARN_ON_ONCE() if pcp->high gets below pcp->batch*SOMETHING.






  reply	other threads:[~2021-05-24 15:58 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-05-21 10:28 [RFC PATCH 0/6] Calculate pcp->high based on zone sizes and active CPUs Mel Gorman
2021-05-21 10:28 ` [PATCH 1/6] mm/page_alloc: Delete vm.percpu_pagelist_fraction Mel Gorman
2021-05-21 21:04   ` Dave Hansen
2021-05-21 10:28 ` [PATCH 2/6] mm/page_alloc: Disassociate the pcp->high from pcp->batch Mel Gorman
2021-05-21 21:52   ` Dave Hansen
2021-05-24  8:32     ` Mel Gorman
2021-05-21 10:28 ` [PATCH 3/6] mm/page_alloc: Adjust pcp->high after CPU hotplug events Mel Gorman
2021-05-21 22:13   ` Dave Hansen
2021-05-24  9:07     ` Mel Gorman
2021-05-24 15:52       ` Dave Hansen [this message]
2021-05-24 16:01         ` Mel Gorman
2021-05-21 10:28 ` [PATCH 4/6] mm/page_alloc: Scale the number of pages that are batch freed Mel Gorman
2021-05-21 22:36   ` Dave Hansen
2021-05-24  9:12     ` Mel Gorman
2021-05-21 10:28 ` [PATCH 5/6] mm/page_alloc: Limit the number of pages on PCP lists when reclaim is active Mel Gorman
2021-05-21 22:44   ` Dave Hansen
2021-05-24  9:22     ` 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
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 3/6] mm/page_alloc: Adjust pcp->high after CPU hotplug events Mel Gorman
2021-05-28 11:08   ` Vlastimil Babka

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