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Subject: Re: [PATCH v2 2/2] mm/pageblock: remove false sharing in
 pageblock_flags
To:     David Hildenbrand <david@redhat.com>,
        Anshuman Khandual <anshuman.khandual@arm.com>,
        Matthew Wilcox <willy@infradead.org>
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: <420c8091-f0c7-f7e6-be95-85d81f877aae@linux.alibaba.com>
Date:   Wed, 19 Aug 2020 16:36:36 +0800
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在 2020/8/19 下午4:13, David Hildenbrand 写道:
> On 19.08.20 10:09, Alex Shi wrote:
>>
>>
>> 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 

The above commit is a good disproof, the false sharing caused performance issue,
and need using a lock protect cmpxchg, the instruction was designed for lockless
using. The false sharing, it was a trouble, and would trouble again sometime.

Thanks
Alex

>>
>> 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? :)
> 
> I hate wasting memory, even if it's just a little bit, anyone who
> doesn't? ;)
> 
>