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

[Vlastimil Babka <vbabka@suse.cz>]

On 8/19/20 10:09 AM, 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 

I don't think that commit is doing the test_and_set_skip() under lock to avoid
false sharing. I think it's done to simply make the test and set protected
against races without relying on e.g. a truly atomic test_and_set_bit(). It's
still noted that it's just a hint so it's not protected to others calling
set_pageblock_skip() from other contexts not under a lock.

> 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 
>