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? ;) > >
