On Wed 18-05-16 14:50:04, Mel Gorman wrote: > On Mon, May 16, 2016 at 11:25:05AM +0200, Michal Hocko wrote: > > On Tue 10-05-16 09:36:03, Vlastimil Babka wrote: > > > Compaction has been using watermark checks when deciding whether it was > > > successful, and whether compaction is at all suitable. There are few > > > problems > > > with these checks. > > > > > > - __compact_finished() uses low watermark in a check that has to pass if > > > the direct compaction is to finish and allocation should succeed. This > > > is > > > too pessimistic, as the allocation will typically use min watermark. It > > > may happen that during compaction, we drop below the low watermark (due > > > to > > > parallel activity), but still form the target high-order page. By > > > checking > > > against low watermark, we might needlessly continue compaction. After > > > this > > > patch, the check uses direct compactor's alloc_flags to determine the > > > watermark, which is effectively the min watermark. > > > > OK, this makes some sense. It would be great if we could have at least > > some clarification why the low wmark has been used previously. Probably > > Mel can remember? > > > > Two reasons -- it was a very rough estimate of whether enough pages are free > for compaction to have any chance. Secondly, it was to minimise the risk > that compaction would isolate so many pages that the zone was completely > depleted. This was a concern during the initial prototype of compaction. > > > > - __compaction_suitable() then checks the low watermark plus a (2 << > > > order) gap > > > to decide if there's enough free memory to perform compaction. This > > > check > > > > And this was a real head scratcher when I started looking into the > > compaction recently. Why do we need to be above low watermark to even > > start compaction. Compaction uses additional memory only for a short > > period of time and then releases the already migrated pages. > > > > Simply minimising the risk that compaction would deplete the entire > zone. Sure, it hands pages back shortly afterwards. At the time of the > initial prototype, page migration was severely broken and the system was > constantly crashing. The cautious checks were left in place after page > migration was fixed as there wasn't a compelling reason to remove them > at the time.
OK, then moving to min_wmark + bias from low_wmark should work, right? This would at least remove the discrepancy between the reclaim and compaction thresholds to some degree. Which is good IMHO. Thanks! -- Michal Hocko SUSE Labs
