Re: [LSF/MM TOPIC] NUMA remote THP vs NUMA local non-THP under MADV_HUGEPAGE

From: Mel Gorman
Date: Fri Feb 01 2019 - 09:17:40 EST

On Tue, Jan 29, 2019 at 06:40:58PM -0500, Andrea Arcangeli wrote:
> I posted some benchmark results showing that for tasks without strong
> NUMA locality the __GFP_THISNODE logic is not guaranteed to be optimal
> (and here of course I mean even if we ignore the large slowdown with
> swap storms at allocation time that might be caused by
> __GFP_THISNODE). The results also show NUMA remote THPs help
> intrasocket as well as intersocket.
> The following seems the interim conclusion which I happen to be in
> agreement with Michal and Mel:
> Hopefully this strict issue will be hot-fixed before April (like we
> had to hot-fix it in the enterprise kernels to avoid the 3 years old
> regression to break large workloads that can't fit it in a single NUMA
> node and I assume other enterprise distributions will follow suit),
> but whatever hot-fix will likely allow ample margin for discussions on
> what we can do better to optimize the decision between local non-THP
> and remote THP under MADV_HUGEPAGE.
> It is clear that the __GFP_THISNODE forced in the current code
> provides some minor advantage to apps using MADV_HUGEPAGE that can fit
> in a single NUMA node, but we should try to achieve it without major
> disadvantages to apps that can't fit in a single NUMA node.
> For example it was mentioned that we could allocate readily available
> already-free local 4k if local compaction fails and the watermarks
> still allows local 4k allocations without invoking reclaim, before
> invoking compaction on remote nodes. The same can be repeated at a
> second level with intra-socket non-THP memory before invoking
> compaction inter-socket. However we can't do things like that with the
> current page allocator workflow. It's possible some larger change is
> required than just sending a single gfp bitflag down to the page
> allocator that creates an implicit MPOL_LOCAL binding to make it
> behave like the obsoleted numa/zone reclaim behavior, but weirdly only
> applied to THP allocations.

I would also be interested in discussing this topic. My activity is
mostly compaction-related but I believe it will evolve into something
that returns more sane data to the page allocator. That should make it a
bit easier to detect when local compaction fails and make it easier to
improve the page allocator workflow without throwing another workload
under a bus.

Mel Gorman