On Wed, May 18, 2016 at 04:27:53PM +0200, Michal Hocko wrote: > > > > - __compaction_suitable() then checks the low watermark plus a (2 << order) gap > > > > to decide if there's enough free memory to perform compaction. This check > > > > > > And this was a real head scratcher when I started looking into the > > > compaction recently. Why do we need to be above low watermark to even > > > start compaction. Compaction uses additional memory only for a short > > > period of time and then releases the already migrated pages. > > > > > > > Simply minimising the risk that compaction would deplete the entire > > zone. Sure, it hands pages back shortly afterwards. At the time of the > > initial prototype, page migration was severely broken and the system was > > constantly crashing. The cautious checks were left in place after page > > migration was fixed as there wasn't a compelling reason to remove them > > at the time. > > OK, then moving to min_wmark + bias from low_wmark should work, right? Yes. I did recall there was another reason but it's marginal. I didn't want compaction isolation free pages to artifically push a process into direct reclaim but given that we are likely under memory pressure at that time anyway, it's unlikely that compaction is the sole reason processes are entering direct reclaim. -- Mel Gorman SUSE Labs -- 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>