Re: [PATCH v2 2/2] mm/pageblock: remove false sharing in pageblock_flags

[Alex Shi]

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? ;)
> 
>