On Mon, Nov 22, 2021 at 10:40:54AM -0800, Shakeel Butt wrote: > > Do we have a performance evaluation how much overhead is added e.g., for > > a single 4k MADV_DONTNEED call on a THP or on a MADV_DONTNEED call that > > covers the whole THP? > > I did a simple benchmark of madvise(MADV_DONTNEED) on 10000 THPs on > x86 for both settings you suggested. I don't see any statistically > significant difference with and without the patch. Let me know if you > want me to try something else. I'm a bit surprised that sync split thp didn't bring any extra overhead. "unmap whole thp" is understandable from that pov, because afaict that won't even trigger any thp split anyway even delayed, if this is the simplest case that only this process mapped this thp, and it mapped once. For "unmap 4k upon thp" IIUC that's the worst case and zapping 4k should be fast; while what I don't understand since thp split requires all hand-made work for copying thp flags into small pages and so on, so I thought there should at least be some overhead measured. Shakeel, could there be something overlooked in the test, or maybe it's me that overlooked? I had the same concern as what Kirill/Matthew raised in the other thread - I'm worried proactively splitting simply because any 4k page is zapped might quickly free up 2m thps in the system and I'm not sure whether it'll exaggerate the defragmentation of the system memory in general. I'm also not sure whether that's ideal for some very common workload that frequently uses DONTNEED to proactively drop some pages. To me, the old deffered-split has a point in that it'll only be done when at least the memory or cgroup is in low mem, that means we're in extreme cases so we'd better start to worry page allocation failures rather than number of thps and memory performance. v2 even added unmap() into account, so that'll further amplify that effect, imho. I'm wondering whether MADV_SPLIT would make more sense so as to keep the old DONTNEED/unmap behaviors, however before that I think I should understand the test results first, because besides 2m pages missing that'll be another important factor for "whether a new interface is more welcomed" from perf pov. Thanks, -- Peter Xu
