On Wed, 10 Oct 2018, David Rientjes wrote: > > 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. > > > > We would never want to thrash the local node for hugepages because there > is no guarantee that any swapping is useful. On COMPACT_SKIPPED due to > low memory, we have very clear evidence that pageblocks are already > sufficiently fragmented by unmovable pages such that compaction itself, > even with abundant free memory, fails to free an entire pageblock due to > the allocator's preference to fragment pageblocks of fallback migratetypes > over returning remote free memory. > > As I've stated, we do not want to reclaim pointlessly when compaction is > unable to access the freed memory or there is no guarantee it can free an > entire pageblock. Doing so allows thrashing of the local node, or remote > nodes if __GFP_THISNODE is removed, and the hugepage still cannot be > allocated. If this proposed mbind() that requires permissions is geared > to me as the user, I'm afraid the details of what leads to the thrashing > are not well understood because I certainly would never use this. > At the risk of beating a dead horse that has already been beaten, what are the plans for this patch when the merge window opens? It would be rather unfortunate for us to start incurring a 14% increase in access latency and 40% increase in fault latency. Would it be possible to test with my patch[*] that does not try reclaim to address the thrashing issue? If that is satisfactory, I don't have a strong preference if it is done with a hardcoded pageblock_order and __GFP_NORETRY check or a new __GFP_COMPACT_ONLY flag. I think the second issue of faulting remote thp by removing __GFP_THISNODE needs supporting evidence that shows some platforms benefit from this (and not with numa=fake on the command line :). [*] https://marc.info/?l=linux-kernel&m=153903127717471
