On Wed 12-09-18 09:54:17, Andrea Arcangeli wrote: > Hello, > > On Tue, Sep 11, 2018 at 01:56:13PM +0200, Michal Hocko wrote: > > Well, it seems that expectations differ for users. It seems that kvm > > users do not really agree with your interpretation. > > Like David also mentioned here: > > lkml.kernel.org/r/alpine.DEB.2.21.1808211021110.258924@xxxxxxxxxxxxxxxxxxxxxxxxx > > depends on the hardware what is a win, so there's no one size fits > all. > > For two sockets providing remote THP to KVM is likely a win, but > changing the defaults depending on boot-time NUMA topology makes > things less deterministic and it's also impossible to define an exact > break even point. > > > I do realize that this is a gray zone because nobody bothered to define > > the semantic since the MADV_HUGEPAGE has been introduced (a826e422420b4 > > is exceptionaly short of information). So we are left with more or less > > undefined behavior and define it properly now. As we can see this might > > regress in some workloads but I strongly suspect that an explicit > > binding sounds more logical approach than a thp specific mpol mode. If > > anything this should be a more generic memory policy basically saying > > that a zone/node reclaim mode should be enabled for the particular > > allocation. > > MADV_HUGEPAGE means the allocation is long lived, so the cost of > compaction is worth it in direct reclaim. Not much else. That is not > the problem. It seems there is no general agreement here. My understanding is that this means that the user really prefers THP for whatever reasons. > The problem is that even if you ignore the breakage and regression to > real life workloads, what is happening right now obviously would > require root privilege but MADV_HUEGPAGE requires no root privilege. I do not follow. > Swapping heavy because MADV_HUGEPAGE when there are gigabytes free on > other nodes and not even 4k would be swapped-out with THP turned off > in sysfs, is simply not possibly what MADV_HUGEPAGE could have been > about, and it's a kernel regression that never existed until that > commit that added __GFP_THISNODE to the default THP heuristic in > mempolicy. agreed > I think we should defer the problem of what is better between 4k NUMA > local or remote THP by default for later, I provided two options > myself because it didn't matter so much which option we picked in the > short term, as long as the bug was fixed. > > I wasn't particularly happy about your patch because it still swaps > with certain defrag settings which is still allowing things that > shouldn't happen without some kind of privileged capability. Well, I am not really sure about defrag=always. I would rather care about the default behavior to plug the regression first. And think about `always' mode on top. Or is this a no-go from your POV? -- Michal Hocko SUSE Labs
