On Thu, Feb 25, 2016 at 05:12:19PM +0000, Mel Gorman wrote: > some cases, this will reduce THP usage but the benefit of THP is hard to > measure and not a universal win where as a stall to reclaim/compaction is It depends on the workload: with virtual machines THP is essential from the start without having to wait half a khugepaged cycle in average, especially on large systems. We see this effect for example in postcopy live migraiton where --postcopy-after-precopy is essential to reach peak performance during database workloads in guest, immediately after postcopy completes. With --postcopy-after-precopy only those pages that may be triggering userfaults will need to be collapsed with khugepaged and all the rest that was previously passed over with precopy has an high probability to be immediately THP backed also thanks to defrag/direct-compaction. Failing at starting the destination node largely THP backed is very visible in benchmark (even if a full precopy pass is done first). Later on the performance increases again as khugepaged fixes things, but it takes some time. So unless we've a very good kcompatd or a workqueue doing the job of providing enough THP for page faults, I'm skeptical of this. If something I'd rather set defrag to "madvise" as qemu and other loads that critically need THP or they run literally half as slow (for example with enterprise workloads in the guest) will still be able not to rely solely on khugepaged which can takes some time to act. So your benchmark would run the same but the VM could still start fully THP backed. Another problem is that khugepaged isn't able to collapse shared readonly anon pages, mostly because of the rmap complexities. I agree with Kirill we should be looking into how make this work, although I doubt the simpler refcounting is going to help much in this regard as the problem is in dealing with rmap, not so much with refcounts. -- 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>