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. > > https://lkml.kernel.org/r/20181210044916.GC24097@xxxxxxxxxx > https://lkml.kernel.org/r/20181212104418.GE1130@xxxxxxxxxx > > The following seems the interim conclusion which I happen to be in > agreement with Michal and Mel: > > https://lkml.kernel.org/r/20181212095051.GO1286@xxxxxxxxxxxxxx > https://lkml.kernel.org/r/20181212170016.GG1130@xxxxxxxxxx > > 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 SUSE Labs
