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SubjectRe: MADV_HUGEPAGE vs. NUMA semantic (was: Re: [LKP] [mm] ac5b2c1891: vm-scalability.throughput -61.3% regression)
On Thu, 6 Dec 2018, Michal Hocko wrote:

> 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
> defrag=defer+madvise)
> 3) fallback to the base page allocation
>

I disagree that MADV_HUGEPAGE should take on any new semantic that
overrides the preference of node local memory for a hugepage, which is the
nearly four year behavior. The order of MADV_HUGEPAGE preferences listed
above would cause current users to regress who rely on local small page
fallback rather than remote hugepages because the access latency is much
better. I think the preference of remote hugepages over local small pages
needs to be expressed differently to prevent regression.

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