On Mon 12-11-18 14:26:41, Wei Yang wrote: > On Mon, Nov 12, 2018 at 09:09:26AM +0100, Michal Hocko wrote: > >On Mon 12-11-18 15:14:04, Wei Yang wrote: > >> Zone with no managed_pages doesn't contribute totalreserv_pages. And the > >> more nodes we have, the more empty zones there are. > >> > >> This patch skip the zones to save some cycles. > > > >What is the motivation for the patch? Does it really cause any > >measurable difference in performance? > > > > The motivation here is to reduce some unnecessary work. I have guessed so even though the changelog was quite modest on the motivation. > Based on my understanding, almost every node has empty zones, since > zones within a node are ordered in monotonic increasing memory address. Yes, this is likely the case. Btw. a check for populated_zone or for_each_populated_zone would suite much better. > The worst case is all zones has managed_pages. For example, there is > only one node, or configured to have only ZONE_NORMAL and > ZONE_MOVABLE. Otherwise, the more node/zone we have, the more empty > zones there are. > > I didn't have detail tests on this patch, since I don't have machine > with large numa nodes. While compared with the following ten lines of > code, this check to skip them is worthwhile to me. Well, the main question is whether the optimization is really worth it. There is not much work done for each zone. I haven't looked closer whether the patch is actually correct, it seems to be though, but optimizations without measurable effect tend to be not that attractive. -- Michal Hocko SUSE Labs
