Re: [patch 0/2 for-4.20] mm, thp: fix remote access and allocation regressions
From: David Rientjes
Date: Tue Dec 04 2018 - 17:25:58 EST
On Tue, 4 Dec 2018, Michal Hocko wrote:
> > This fixes a 13.9% of remote memory access regression and 40% remote
> > memory allocation regression on Haswell when the local node is fragmented
> > for hugepage sized pages and memory is being faulted with either the thp
> > defrag setting of "always" or has been madvised with MADV_HUGEPAGE.
> >
> > The usecase that initially identified this issue were binaries that mremap
> > their .text segment to be backed by transparent hugepages on startup.
> > They do mmap(), madvise(MADV_HUGEPAGE), memcpy(), and mremap().
>
> Do you have something you can share with so that other people can play
> and try to reproduce?
>
This is a single MADV_HUGEPAGE usecase, there is nothing special about it.
It would be the same as if you did mmap(), madvise(MADV_HUGEPAGE), and
faulted the memory with a fragmented local node and then measured the
remote access latency to the remote hugepage that occurs without setting
__GFP_THISNODE. You can also measure the remote allocation latency by
fragmenting the entire system and then faulting.
(Remapping the text segment only involves parsing /proc/self/exe, mmap,
madvise, memcpy, and mremap.)
> > This requires a full revert and partial revert of commits merged during
> > the 4.20 rc cycle. The full revert, of ac5b2c18911f ("mm: thp: relax
> > __GFP_THISNODE for MADV_HUGEPAGE mappings"), was anticipated to fix large
> > amounts of swap activity on the local zone when faulting hugepages by
> > falling back to remote memory. This remote allocation causes the access
> > regression and, if fragmented, the allocation regression.
>
> Have you tried to measure any of the workloads Mel and Andrea have
> pointed out during the previous review discussion? In other words what
> is the impact on the THP success rate and allocation latencies for other
> usecases?
It isn't a property of the workload, it's a property of the how fragmented
both local and remote memory is. In Andrea's case, I believe he has
stated that memory compaction has failed locally and the resulting reclaim
activity ends up looping and causing it the thrash the local node whereas
75% of remote memory is free and not fragmented. So we have local
fragmentation and reclaim is very expensive to enable compaction to
succeed, if it ever does succeed[*], and mostly free remote memory.
If remote memory is also fragmented, Andrea's case will run into a much
more severe swap storm as a result of not setting __GFP_THISNODE. The
premise of the entire change is that his remote memory is mostly free so
fallback results in a quick allocation. For balanced nodes, that's not
going to be the case. The fix to prevent the heavy reclaim activity is to
set __GFP_NORETRY as the page allocator suspects, which patch 2 here does.
That's an interesting memory state to
[*] Reclaim here would only be beneficial if we fail the order-0
watermark check in __compaction_suitable() *and* the reclaimed
memory can be accessed during isolate_freepages().