linux-kernel - Re: [PATCH v4 1/4] mm/pageblock: mitigation cmpxchg false sharing in pageblock flags

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Message-ID: <20200903072447.GB3179@techsingularity.net>
Date:   Thu, 3 Sep 2020 08:24:47 +0100
From:   Mel Gorman <mgorman@...hsingularity.net>
To:     Alex Shi <alex.shi@...ux.alibaba.com>
Cc:     Anshuman Khandual <anshuman.khandual@....com>,
        David Hildenbrand <david@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Alexander Duyck <alexander.duyck@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/4] mm/pageblock: mitigation cmpxchg false sharing in
 pageblock flags

On Thu, Sep 03, 2020 at 03:01:20PM +0800, Alex Shi wrote:
> pageblock_flags is used as long, since every pageblock_flags is just 4
> bits, 'long' size will include 8(32bit machine) or 16 pageblocks' flags,
> that flag setting has to sync in cmpxchg with 7 or 15 other pageblock
> flags. It would cause long waiting for sync.
> 
> If we could change the pageblock_flags variable as char, we could use
> char size cmpxchg, which just sync up with 2 pageblock flags. it could
> relief the false sharing in cmpxchg.
> 
> Signed-off-by: Alex Shi <alex.shi@...ux.alibaba.com>

Page block types were not known to change at high frequency that would
cause a measurable performance drop. If anything, the performance hit
from pageblocks is the lookup paths which is a lot more frequent.

What was the workload you were running that altered pageblocks at a high
enough frequency for collisions to occur when updating adjacent
pageblocks?

-- 
Mel Gorman
SUSE Labs