On 03/19/2012 02:24 PM, Avi Kivity wrote: > On 03/19/2012 02:20 PM, Peter Zijlstra wrote: > > On Mon, 2012-03-19 at 13:42 +0200, Avi Kivity wrote: > > > It's the standard space/time tradeoff. Once solution wants more > > > storage, the other wants more faults. > > > > > > Note scanners can use A/D bits which are cheaper than faults. > > > > I'm not convinced.. the scanner will still consume time even if the > > system is perfectly balanced -- it has to in order to determine this. > > > > So sure, A/D/other page table magic can make scanners faster than faults > > however you only need faults when you're actually going to migrate a > > task. Whereas you always need to scan, even in the stable state. > > > > So while the per-instance times might be in favour of scanning, I'm > > thinking the accumulated time is in favour of faults. > > When you migrate a vnode, you don't need the faults at all. You know > exactly which pages need to be migrated, you can just queue them > immediately when you make that decision. > > The scanning therefore only needs to pick up the stragglers and can be > set to a very low frequency. Running the numbers, 4GB = 1Mpages, at 2us per page migration that's 2 seconds to migrate an entire process, perhaps 2x-3x that for kvm. So as long numa balancing happens at a lower frequency than once every few minutes, the gains should be higher than the loss. If those numbers are not too wrong then migrate on fault should be a win. -- error compiling committee.c: too many arguments to function -- 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/ . Fight unfair telecom internet charges in Canada: sign http://stopthemeter.ca/ Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>