On Tue, Nov 13, 2018 at 10:07:58AM +0100, Michal Hocko wrote: >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. Got your point. Let me take a look to see if I could make a cleanup. > >-- >Michal Hocko >SUSE Labs -- Wei Yang Help you, Help me
