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SubjectMADV_HUGEPAGE vs. NUMA semantic (was: Re: [LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3% regression)
On Wed 05-12-18 16:58:02, Linus Torvalds wrote:
> I realize that we probably do want to just have explicit policies that
> do not exist right now, but what are (a) sane defaults, and (b) sane
> policies?

I would focus on the current default first (which is defrag=madvise).
This means that we only try the cheapest possible THP without
MADV_HUGEPAGE. If there is none we simply fallback. We do restrict to
the local node. I guess there is a general agreement that this is a sane

MADV_HUGEPAGE changes the picture because the caller expressed a need
for THP and is willing to go extra mile to get it. That involves
allocation latency and as of now also a potential remote access. We do
not have complete agreement on the later but the prevailing argument is
that any strong NUMA locality is just reinventing node-reclaim story
again or makes THP success rate down the toilet (to quote Mel). I agree
that we do not want to fallback to a remote node overeagerly. I believe
that something like the below would be sensible
1) THP on a local node with compaction not giving up too early
2) THP on a remote node in NOWAIT mode - so no direct
compaction/reclaim (trigger kswapd/kcompactd only for
3) fallback to the base page allocation

This would allow both full memory utilization and try to be as local as
possible. Whoever strongly prefers NUMA locality should be using
MPOL_NODE_RECLAIM (or similar) and that would skip 2 and make 1) and 2)
use more aggressive compaction and reclaim.

This will also fit into our existing NUMA api. MPOL_NODE_RECLAIM
wouldn't be restricted to THP obviously. It would act on base pages as
well and it would basically use the same implementation as we have for
the global node_reclaim and make it usable again.

Does this sound at least remotely sane?
Michal Hocko

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