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Subject: Re: [PATCH v2 2/2] mm/pageblock: remove false sharing in
 pageblock_flags
To:     Anshuman Khandual <anshuman.khandual@arm.com>,
        Matthew Wilcox <willy@infradead.org>,
        David Hildenbrand <david@redhat.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>,
        Hugh Dickins <hughd@google.com>,
        Alexander Duyck <alexander.h.duyck@linux.intel.com>,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org
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From:   Alex Shi <alex.shi@linux.alibaba.com>
Message-ID: <a7376a9c-82e8-7dc1-b656-9c0516738801@linux.alibaba.com>
Date:   Wed, 19 Aug 2020 16:09:51 +0800
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在 2020/8/19 下午3:57, Anshuman Khandual 写道:
> 
> 
> On 08/19/2020 11:17 AM, Alex Shi wrote:
>> Current pageblock_flags is only 4 bits, so it has to share a char size
>> in cmpxchg when get set, the false sharing cause perf drop.
>>
>> If we incrase the bits up to 8, false sharing would gone in cmpxchg. and
>> the only cost is half char per pageblock, which is half char per 128MB
>> on x86, 4 chars in 1 GB.
> 
> Agreed that increase in memory utilization is negligible here but does
> this really improve performance ?
> 

It's no doubt in theory. and it would had a bad impact according to 
commit e380bebe4771548  mm, compaction: keep migration source private to a single 

but I do have some problem in running thpscale/mmtest. I'd like to see if anyone
could give a try.

BTW, I naturally hate the false sharing even it's in theory. Anyone who doesn't? :)

Thanks
Alex