On 30 Aug 2018, at 9:45, Michal Hocko wrote: > On Thu 30-08-18 09:22:21, Zi Yan wrote: >> On 30 Aug 2018, at 3:00, Michal Hocko wrote: >> >>> On Wed 29-08-18 18:54:23, Zi Yan wrote: >>> [...] >>>> I tested it against Linus’s tree with “memhog -r3 130g” in a two-socket machine with 128GB memory on >>>> each node and got the results below. I expect this test should fill one node, then fall back to the other. >>>> >>>> 1. madvise(MADV_HUGEPAGE) + defrag = {always, madvise, defer+madvise}: >>>> no swap, THPs are allocated in the fallback node. >>>> 2. madvise(MADV_HUGEPAGE) + defrag = defer: pages got swapped to the >>>> disk instead of being allocated in the fallback node. >>>> 3. no madvise, THP is on by default + defrag = {always, defer, >>>> defer+madvise}: pages got swapped to the disk instead of being >>>> allocated in the fallback node. >>>> 4. no madvise, THP is on by default + defrag = madvise: no swap, base >>>> pages are allocated in the fallback node. >>>> >>>> The result 2 and 3 seems unexpected, since pages should be allocated in the fallback node. >>>> >>>> The reason, as Andrea mentioned in his email, is that the combination >>>> of __THIS_NODE and __GFP_DIRECT_RECLAIM (plus __GFP_KSWAPD_RECLAIM >>>> from this experiment). >>> >>> But we do not set __GFP_THISNODE along with __GFP_DIRECT_RECLAIM AFAICS. >>> We do for __GFP_KSWAPD_RECLAIM though and I guess that it is expected to >>> see kswapd do the reclaim to balance the node. If the node is full of >>> anonymous pages then there is no other way than swap out. >> >> GFP_TRANSHUGE implies __GFP_DIRECT_RECLAIM. When no madvise is given, THP is on >> + defrag=always, gfp_mask has __GFP_THISNODE and __GFP_DIRECT_RECLAIM, so swapping >> can be triggered. > > Yes, but the setup tells that you are willing to pay price to get a THP. > defered=always uses that special __GFP_NORETRY (unless it is madvised > mapping) that should back off if the compaction failed recently. How > much that reduces the reclaim is not really clear to me right now to be > honest. > >> The key issue here is that “memhog -r3 130g” uses the default memory policy (MPOL_DEFAULT), >> which should allow page allocation fallback to other nodes, but as shown in >> result 3, swapping is triggered instead of page allocation fallback. > > Well, I guess this really depends. Fallback to a different node might be > seen as a bad thing and worse than the reclaim on the local node. > >>>> __THIS_NODE uses ZONELIST_NOFALLBACK, which >>>> removes the fallback possibility and __GFP_*_RECLAIM triggers page >>>> reclaim in the first page allocation node when fallback nodes are >>>> removed by ZONELIST_NOFALLBACK. >>> >>> Yes but the point is that the allocations which use __GFP_THISNODE are >>> optimistic so they shouldn't fallback to remote NUMA nodes. >> >> This can be achieved by using MPOL_BIND memory policy which restricts >> nodemask in struct alloc_context for user space memory allocations. > > Yes, but that requires and explicit NUMA handling. And we are trying to > handle those cases which do not really give a damn and just want to use > THP if it is available or try harder when they ask by using madvise. > >>>> IMHO, __THIS_NODE should not be used for user memory allocation at >>>> all, since it fights against most of memory policies. But kernel >>>> memory allocation would need it as a kernel MPOL_BIND memory policy. >>> >>> __GFP_THISNODE is indeed an ugliness. I would really love to get rid of >>> it here. But the problem is that optimistic THP allocations should >>> prefer a local node because a remote node might easily offset the >>> advantage of the THP. I do not have a great idea how to achieve that >>> without __GFP_THISNODE though. >> >> MPOL_PREFERRED memory policy can be used to achieve this optimistic >> THP allocation for user space. Even with the default memory policy, >> local memory node will be used first until it is full. It seems to >> me that __GFP_THISNODE is not necessary if a proper memory policy is >> used. >> >> Let me know if I miss anything. Thanks. > > You are missing that we are trying to define a sensible model for those > who do not really care about mempolicies. THP shouldn't cause more harm > than good for those. > > I wish we could come up with a remotely sane and comprehensible model. > That means that you know how hard the allocator tries to get a THP for > you depending on the defrag configuration, your memory policy and your > madvise setting. The easiest one I can think of is to > - always follow mempolicy when specified because you asked for it > explicitly > - stay node local and low latency for the light THP defrag mode (defrag, > madvise without hint and none) because THP is a nice to have > - if the defrag mode is always then you are willing to pay the latency > price but off-node might be still a no-no. > - allow fallback for madvised mappings because you really want THP. If > you care about specific numa placement then combine with the > mempolicy. > > As you can see I do not really mention anything about the direct reclaim > because that is just an implementation detail of the page allocator and > compaction interaction. > > Maybe you can formulate a saner matrix with all the available modes that > we have. > > Anyway, I guess we can agree that (almost) unconditional __GFP_THISNODE > is clearly wrong and we should address that first. Either Andrea's > option 2) patch or mine which does the similar thing except at the > proper layer (I believe). We can continue discussing other odd cases on > top I guess. Unless somebody has much brighter idea, of course. Thanks for your explanation. It makes sense to me. I am fine with your patch. You can add my Tested-by: Zi Yan , since my test result 1 shows that the problem mentioned in your changelog is solved. — Best Regards, Yan Zi