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? >-- >Michal Hocko >SUSE Labs -- Wei Yang Help you, Help me
