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

[Alex Shi]

在 2020/8/20 上午12:50, Alexander Duyck 写道:
> On Wed, Aug 19, 2020 at 1:11 AM Alex Shi <alex.shi@xxxxxxxxxxxxxxxxx> 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? :)
> 
> You keep bringing up false sharing but you don't fix the false sharing
> by doing this. You are still allowing the flags for multiple
> pageblocks per cacheline so you still have false sharing even after
> this.

yes, the cacheline false sharing is still there. But as you pointed, cmpxchg level
false sharing could be addressed much by the patchset.


> 
> What I believe you are attempting to address is the fact that multiple
> pageblocks share a single long value and that long is being used with
> a cmpxchg so you end up with multiple threads potentially all banging
> on the same value and watching it change. However the field currently
> consists of only 4 bits, 3 of them for migratetype and 1 for the skip
> bit. In the case of the 3 bit portion a cmpxchg makes sense and is
> usually protected by the zone lock so you would only have one thread
> accessing it in most cases with the possible exception of a section
> that spans multiple zones.
> 
> For the case such as the skip bit and MIGRATE_UNMOVABLE (0x0) where we
> would be clearing or setting the entire mask maybe it would make more
> sense to simply use an atomic_or or atomic_and depending on if you are
> setting or clearing the flag? It would allow you to avoid the spinning
> or having to read the word before performing the operation since you
> would just be directly applying an AND or OR via a mask value.

Right that the different level to fix this problem, but narrow the cmpxchg
comparsion is still needed and helpful.

Thanks
Alex
>