On Tue 13-11-18 08:16:44, Wei Yang wrote: > On Tue, Nov 13, 2018 at 09:08:34AM +0100, Michal Hocko wrote: > >On Tue 13-11-18 01:39:42, Wei Yang wrote: > >> On Mon, Nov 12, 2018 at 03:40:20PM +0100, Michal Hocko wrote: > >> >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. > >> > > >> > >> Hmm... maybe not exact. > >> > >> populated_zone checks zone->present_pages > >> managed_zone checks zone->managed_pages > >> > >> As the comment of managed_zone says, this one records the pages managed > >> by buddy system. And when we look at the usage of totalreserve_pages, it > >> is only used in page allocation. And finally, *max* is checked with > >> managed_pages instead of present_pages. > >> > >> Because of this, managed_zone is more accurate at this place. Is my > >> understanding correct? > > > >OK, fair enough. There is a certain discrepancy here. You are right that > >we do not care about pages out of the page allocator scope (e.g. early > >bootmem allocations, struct pages) but this is likely what other callers > >of populated_zone are looking for as well. It seems that managed pages > >counter which only came in later was not considered in other places. > > > >That being said this asks for a cleanup of some sort. And I think such a > >cleanup wold be appreciated much more than an optimization of an unknown > >effect and wonder why this check is used here and not at other places. > > You are right. There are three pages(spanned, managed, present) in a > zone, which is a little confusing. > > So you are willing to get rid of present_pages, if I am right? No, I believe we want all three of them. But reviewing for_each_populated_zone users and explicit checks for present/managed pages and unify them would be a step forward both a more optimal code and more maintainable code. I haven't checked but for_each_populated_zone would seem like a proper user for managed page counter. But that really requires to review all current users. -- Michal Hocko SUSE Labs
