On Tue, Dec 18, 2018 at 02:58:33PM +0100, Vlastimil Babka wrote: > On 12/18/18 2:51 PM, Mel Gorman wrote: > > On Tue, Dec 18, 2018 at 01:36:42PM +0100, Vlastimil Babka wrote: > >> On 12/15/18 12:03 AM, Mel Gorman wrote: > >>> When pageblocks get fragmented, watermarks are artifically boosted to pages > >>> are reclaimed to avoid further fragmentation events. However, compaction > >>> is often either fragmentation-neutral or moving movable pages away from > >>> unmovable/reclaimable pages. As the actual watermarks are preserved, > >>> allow compaction to ignore the boost factor. > >> > >> Right, I should have realized that when reviewing the boost patch. I > >> think it would be useful to do the same change in > >> __compaction_suitable() as well. Compaction has its own "gap". > >> > > > > That gap is somewhat static though so I'm a bit more wary of it. However, > > Well, watermark boost is dynamic, but based on allocations stealing from > other migratetypes, not reflecting compaction chances of success. > True. > > the check in __isolate_free_page looks too agressive. We isolate in > > units of COMPACT_CLUSTER_MAX yet the watermark check there is based on > > the allocation request. That means for THP that we check if 512 pages > > can be allocated when only somewhere between 1 and 32 is needed for that > > compaction cycle to complete. Adjusting that might be more appropriate? > > AFAIU the code in __isolate_free_page() reflects that if there's less > than 512 free pages gap, we might form a high-order page for THP but > won't be able to allocate it afterwards due to watermark. Yeah but it used to be a lot more important when watermark checking for high-orders was very different. Now, if the watermark is met for order-0 and a large enough free page is allocated, the allocation succeeds so it's a lot less relevant than it used to be. kswapd will still run in the background for order-0 if necessary so a heavy watermark check there doesn't really help. -- Mel Gorman SUSE Labs
