On Sun, Aug 19, 2018 at 11:22:02PM -0400, Andrea Arcangeli wrote: > Hello, > > we detected a regression compared to older kernels, only happening > with defrag=always or by using MADV_HUGEPAGE (and QEMU uses it). > > I haven't bisected but I suppose this started since commit > 5265047ac30191ea24b16503165000c225f54feb combined with previous > commits that introduced the logic to not try to invoke reclaim for THP > allocations in the remote nodes. > > Once I looked into it the problem was pretty obvious and there are two > possible simple fixes, one is not to invoke reclaim and stick to > compaction in the local node only (still __GFP_THISNODE model). > > This approach keeps the logic the same and prioritizes for NUMA > locality over THP generation. > > Then I'll send the an alternative that drops the __GFP_THISNODE logic > if_DIRECT_RECLAIM is set. That however changes the behavior for > MADV_HUGEPAGE and prioritizes THP generation over NUMA locality. > > A possible incremental improvement for this __GFP_COMPACT_ONLY > solution would be to remove __GFP_THISNODE (and in turn > __GFP_COMPACT_ONLY) after checking the watermarks if there's no free > PAGE_SIZEd memory in the local node. However checking the watermarks > in mempolicy.c is not ideal so it would be a more messy change and > it'd still need to use __GFP_COMPACT_ONLY as implemented here for when > there's no PAGE_SIZEd free memory in the local node. That further > improvement wouldn't be necessary if there's agreement to prioritize > THP generation over NUMA locality (the alternative solution I'll send > in a separate post). I personally prefer to prioritize NUMA locality over THP (__GFP_COMPACT_ONLY variant), but I don't know page-alloc/compaction good enough to Ack it. -- Kirill A. Shutemov
