On Wed, Jun 22, 2011 at 11:39 PM, Rik van Riel <riel@xxxxxxxxxx> wrote: > On 06/22/2011 07:19 AM, Izik Eidus wrote: > >> So what we say here is: it is better to have little junk in the unstable >> tree that get flushed eventualy anyway, instead of make the guest >> slower.... >> this race is something that does not reflect accurate of ksm anyway due >> to the full memcmp that we will eventualy perform... > > With 2MB pages, I am not convinced they will get "flushed eventually", > because there is a good chance at least one of the 4kB pages inside > a 2MB page is in active use at all times. > > I worry that the proposed changes may end up effectively preventing > KSM from scanning inside 2MB pages, when even one 4kB page inside > is in active use. This could mean increased swapping on systems > that run low on memory, which can be a much larger performance penalty > than ksmd CPU use. > > We need to scan inside 2MB pages when memory runs low, regardless > of the accessed or dirty bits. I agree on this point. Dirty bit , young bit, is by no means accurate. Even on 4kB pages, there is always a chance that the pte are dirty but the contents are actually the same. Yeah, the whole optimization contains trade-offs and trades-offs always have the possibilities to annoy someone. Just like page-bit-relying LRU approximations none of them is perfect too. But I think it can benefit some people. So maybe we could just provide a generic balanced solution but provide fine tuning interfaces to make sure tha when it really gets in the way of someone, he has a way to walk around. Do you agree on my argument? :-) > > -- > All rights reversed > -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
