linux-kernel - Re: [PATCH] mm: fix pcp count beyond pcp high in pcplist allocation

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Date:   Thu, 3 Nov 2022 20:46:46 +0800
From:   Chen Wandun <chenwandun@...wei.com>
To:     Mel Gorman <mgorman@...hsingularity.net>
CC:     <akpm@...ux-foundation.org>, <vbabka@...e.cz>,
        <linux-mm@...ck.org>, <linux-kernel@...r.kernel.org>,
        <wangkefeng.wang@...wei.com>
Subject: Re: [PATCH] mm: fix pcp count beyond pcp high in pcplist allocation



在 2022/11/1 18:40, Mel Gorman 写道:
> On Mon, Oct 31, 2022 at 11:37:35AM +0800, Chen Wandun wrote:
>>>>>     As is, the patch could result in a batch request of 0 and
>>>>    I foget this, the patch need some improve, thanks.
>>>>
>>>>> fall through to allocating from the zone list anyway defeating the
>>>>> purpose of the PCP allocator and probably regressing performance in some
>>>>> csaes.
>>>> Same as I understand???how about set high/batch for each order in pcplist???
>>> Using anything would than (X >> order) consumes storage. Even if storage
>>> was to be used, selecting a value per-order would be impossible because
>>> the correct value would depend on frequency of requests for each order.
>>> That can only be determined at runtime and the cost of determining the
>>> value would likely exceed the benefit.
>> Can we set a experience value for pcp batch for each order during init
>> stage?
> I'm not sure what you mean by "experience value" but maybe you meant
> experimental value?
yes, experimental value,  sorry for that.
>
>> If so we can make accurately control for pcp size. Nowdays, the size of each
>> order in pcp list is full of randomness. I dont konw which scheme is better
>> for performance.
>>
> It is something that could be experimented with but the main question is
> -- what should those per-order values be? One option would be to enforce
> pcp->high for all high-order values except THP if THP is enabled. That would
> limit some of the issues with pcp->high being exceeded as even if two THPs
> are refilled, one of them is allocated immediately. I wasn't convinced it was
> necessary when implementing high-order PCP support but it could be evaluated.
Thank you for your suggestion, I will do some tests.