On Tue, Oct 09, 2018 at 04:25:10PM +0200, Michal Hocko wrote: > On Tue 09-10-18 14:00:34, Mel Gorman wrote: > > On Tue, Oct 09, 2018 at 02:27:45PM +0200, Michal Hocko wrote: > > > [Sorry for being slow in responding but I was mostly offline last few > > > days] > > > > > > On Tue 09-10-18 10:48:25, Mel Gorman wrote: > > > [...] > > > > This goes back to my point that the MADV_HUGEPAGE hint should not make > > > > promises about locality and that introducing MADV_LOCAL for specialised > > > > libraries may be more appropriate with the initial semantic being how it > > > > treats MADV_HUGEPAGE regions. > > > > > > I agree with your other points and not going to repeat them. I am not > > > sure madvise s the best API for the purpose though. We are talking about > > > memory policy here and there is an existing api for that so I would > > > _prefer_ to reuse it for this purpose. > > > > > > > I flip-flopped on that one in my head multiple times on the basis of > > how strict it should be. Memory policies tend to be black or white -- > > bind here, interleave there, etc. It wasn't clear to me what the best > > policy would be to describe "allocate local as best as you can but allow > > fallbacks if necessary". MPOL_PREFERRED is not black and white. In fact I asked David earlier if MPOL_PREFERRED could check if it would already be a good fit for this. Still the point is it requires privilege (and for a good reason). > I was thinking about MPOL_NODE_PROXIMITY with the following semantic: > - try hard to allocate from a local or very close numa node(s) even when > that requires expensive operations like the memory reclaim/compaction > before falling back to other more distant numa nodes. If MPOL_PREFERRED can't work something like this could be added. I think "madvise vs mbind" is more an issue of "no-permission vs permission" required. And if the processes ends up swapping out all other process with their memory already allocated in the node, I think some permission is correct to be required, in which case an mbind looks a better fit. MPOL_PREFERRED also looks a first candidate for investigation as it's already not black and white and allows spillover and may already do the right thing in fact if set on top of MADV_HUGEPAGE. Thanks, Andrea
