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SubjectRe: [LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3% regression
On Mon, 3 Dec 2018, Andrea Arcangeli wrote:

> It's trivial to reproduce the badness by running a memhog process that
> allocates more than the RAM of 1 NUMA node, under defrag=always
> setting (or by changing memhog to use MADV_HUGEPAGE) and it'll create
> swap storms despite 75% of the RAM is completely free in a 4 node NUMA
> (or 50% of RAM free in a 2 node NUMA) etc..
>
> How can it be ok to push the system into gigabytes of swap by default
> without any special capability despite 50% - 75% or more of the RAM is
> free? That's the downside of the __GFP_THISNODE optimizaton.
>

The swap storm is the issue that is being addressed. If your remote
memory is as low as local memory, the patch to clear __GFP_THISNODE has
done nothing to fix it: you still get swap storms and memory compaction
can still fail if the per-zone freeing scanner cannot utilize the
reclaimed memory. Recall that this patch to clear __GFP_THISNODE was
measured by me to have a 40% increase in allocation latency for fragmented
remote memory on Haswell. It makes the problem much, much worse.

> __GFP_THISNODE helps increasing NUMA locality if your app can fit in a
> single node which is the common David's workload. But if his workload
> would more often than not fit in a single node, he would also run into
> an unacceptable slowdown because of the __GFP_THISNODE.
>

Which is why I have suggested that we do not do direct reclaim, as the
page allocator implementation expects all thp page fault allocations to
have __GFP_NORETRY set, because no amount of reclaim can be shown to be
useful to the memory compaction freeing scanner if it is iterated over by
the migration scanner.

> I think there's lots of room for improvement for the future, but in my
> view that __GFP_THISNODE as it was implemented was an incomplete hack,
> that opened the door for bad VM corner cases that should not happen.
>

__GFP_THISNODE is intended specifically because of the remote access
latency increase that is encountered if you fault remote hugepages over
local pages of the native page size.

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