On Wed, Dec 23, 2015 at 07:14:21AM +0100, Vlastimil Babka wrote: > On 22.12.2015 23:17, David Rientjes wrote: > > On Mon, 21 Dec 2015, Joonsoo Kim wrote: > > > >> Before vs After > >> Max: 1096 MB/s vs 1325 MB/s > >> Min: 635 MB/s 1015 MB/s > >> Avg: 899 MB/s 1194 MB/s > >> > >> Avg is improved by roughly 30% [2]. > >> > > > > Wow, ok! > > > > I'm wondering if it would be better to maintain this as a characteristic > > of each pageblock rather than each zone. Have you tried to introduce a > > couple new bits to pageblock_bits that would track (1) if a cached value > > makes sense and (2) if the pageblock is contiguous? On the first call to > > pageblock_pfn_to_page(), set the first bit, PB_cached, and set the second > > bit, PB_contiguous, iff it is contiguous. On subsequent calls, if > > PB_cached is true, then return PB_contiguous. On memory hot-add or > > remove (or init), clear PB_cached. > > I can imagine these bitmap operation to be as expensive as what > __pageblock_pfn_to_page() does (or close)? But if not, we could also just be a > bit smarter about PG_skip and check that before doing pfn_to_page. Although I don't think carefully, to get PB_xxx, we need to check pfn's zone and it requires pfn_valid() and pfn_to_page(). So, I guess that cost would be same or half compared to cost of __pageblock_pfn_to_page(). > > > What are the cases where pageblock_pfn_to_page() is used for a subset of > > the pageblock and the result would be problematic for compaction? I.e., > > do we actually care to use pageblocks that are not contiguous at all? > > The problematic pageblocks are those that have pages from more than one zone in > them, so we just skip them. Supposedly that can only happen by switching once > between two zones somewhere in the middle of the pageblock, so it's sufficient > to check first and last pfn and compare their zones. So using > pageblock_pfn_to_page() on a subset from compaction would be wrong. Holes (==no > pages) within pageblock is a different thing checked by pfn_valid_within() > (#defined out on archs where such holes cannot happen) when scanning the block. > > That's why I'm not entirely happy with how the patch conflates both the > first/last pfn's zone checks and pfn_valid_within() checks. Yes, a fully > contiguous zone does *imply* that pageblock_pfn_to_page() doesn't have to check > first/last pfn for a matching zone. But it's not *equality*. And any (now just > *potential*) user of pageblock_pfn_to_page() with pfn's different than > first/last pfn of a pageblock is likely wrong. Now, I understand your concern. What makes me mislead is that 3 of 4 callers to pageblock_pfn_to_page() in compaction.c could call it with non-pageblock boundary pfn. Maybe, they should be fixed first. Then, yes. I can separate first/last pfn's zone checks and pfn_valid_within() checks. If then, would you be entirely happy? :) Thanks. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>