On 12/14/18 10:04 PM, David Rientjes wrote: > On Wed, 12 Dec 2018, Vlastimil Babka wrote: ... > Reclaim likely could be deterministically useful if we consider a redesign > of how migration sources and targets are determined in compaction. > > Has anybody tried a migration scanner that isn't linearly based, rather > finding the highest-order free page of the same migratetype, iterating the > pages of its pageblock, and using this to determine whether the actual > migration will be worthwhile or not? Not exactly that AFAIK, but a year ago in my series [1] patch 6 made migration scanner 'prescan' the block of requested order before actually trying to isolate anything for migration. > I could imagine pageblock_skip being > repurposed for this as the heuristic. > > Finding migration targets would be more tricky, but if we iterate the > pages of the pageblock for low-order free pages and find them to be mostly > used, that seems more appropriate than just pushing all memory to the end > of the zone? Agree. That was patch 8/8 of the same series [1]. > It would be interesting to know if anybody has tried using the per-zone > free_area's to determine migration targets and set a bit if it should be > considered a migration source or a migration target. If all pages for a > pageblock are not on free_areas, they are fully used. Repurposing/adding a new pageblock bit was in my mind to help multiple compactors not undo each other's work in the scheme where there's no free page scanner, but I didn't implement it yet. >>> otherwise we fail and defer because it wasn't able >>> to make a hugepage available. >> >> Note that THP fault compaction doesn't actually defer itself, which I >> think is a weakness of the current implementation and hope that patch 3 >> in my series from yesterday [1] can address that. Because defering is >> the general feedback mechanism that we have for suppressing compaction >> (and thus associated reclaim) in cases it fails for any reason, not just >> the one you mention. Instead of inspecting failure conditions in detail, >> which would be costly, it's a simple statistical approach. And when >> compaction is improved to fail less, defering automatically also happens >> less. >> > > I couldn't get the link to work, unfortunately, I don't think the patch > series made it to LKML :/ I do see it archived for linux-mm, though, so > I'll take a look, thanks! Yeah I forgot to Cc: LKML, but you were also in direct To: so you should have received them directly. Also the abovementioned series, but that's year ago. My fault for not returning to it after being done with the Meltdown fun. I hope to do that soon. [1] https://marc.info/?l=linux-mm&m=151315560308753 >> [1] https://lkml.kernel.org/r/20181211142941.20500-1-vbabka@xxxxxxx >>
