On Wed, Jul 31, 2013 at 11:34:37AM +0200, Peter Zijlstra wrote: > On Wed, Jul 31, 2013 at 10:29:38AM +0100, Mel Gorman wrote: > > > Hurmph I just stumbled upon this PMD 'trick' and I'm not at all sure I > > > like it. If an application would pre-fault/initialize its memory with > > > the main thread we'll collapse it into a PMDs and forever thereafter (by > > > virtue of do_pmd_numa_page()) they'll all stay the same. Resulting in > > > PMD granularity. > > > > > > > Potentially yes. When that PMD trick was introduced it was because the cost > > of faults was very high due to a high scanning rate. The trick mitigated > > worse-case scenarios until faults were properly accounted for and the scan > > rates were better controlled. As these *should* be addressed by the series > > I think I will be adding a patch to kick away this PMD crutch and see how > > it looks in profiles. > > I've been thinking on this a bit and I think we should split these and > thp pages when we get shared faults from different nodes on them and > refuse thp collapses when the pages are on different nodes. > Agreed, I reached the same conclusion when thinking about THP false sharing just before I went on holiday. The first prototype patch was a bit messy and performed very badly so "Handle false sharing of THP" was chucked onto the TODO pile to worry about when I got back. It also collided a little with the PMD handling of base pages which is another reason to get rid of that. > With the exception that when we introduce the interleave mempolicies we > should define 'different node' as being outside of the interleave mask. Understood. -- 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>