linux-kernel - Re: [PATCH v2 2/2] mm/pageblock: remove false sharing in pageblock

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Message-ID: <0dc33af2-4d2d-9531-03fa-d26654ab79fb@redhat.com>
Date:   Wed, 19 Aug 2020 10:13:20 +0200
From:   David Hildenbrand <david@...hat.com>
To:     Alex Shi <alex.shi@...ux.alibaba.com>,
        Anshuman Khandual <anshuman.khandual@....com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Hugh Dickins <hughd@...gle.com>,
        Alexander Duyck <alexander.h.duyck@...ux.intel.com>,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org
Subject: Re: [PATCH v2 2/2] mm/pageblock: remove false sharing in
 pageblock_flags

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 
> 
> 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? ;)


-- 
Thanks,

David / dhildenb