Messages in this thread |  | | | Date | Mon, 19 Mar 2012 17:44:42 +0200 | | From | Avi Kivity <> | | Subject | Re: [RFC][PATCH 00/26] sched/numa |
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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
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