On Thu, Jun 23, 2011 at 7:28 AM, Rik van Riel <riel@xxxxxxxxxx> wrote: > On 06/22/2011 07:13 PM, Nai Xia wrote: >> >> 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? :-) > > That's not an argument. > > That is a "if I wave my hands vigorously enough, maybe people > will let my patch in without thinking about what I wrote" > style argument. Oh, NO, this is not what I meant. Really sorry if I made myself look so evil... I actually mean: "Skip or not, we agree on a point that will not harm most people, and provide another interface to let someon who _really_ want to take another way." I am by no means pushing the idea of "skipping" huge pages. I am just not sure about it and want to get a precise idea from you. And now I get it. > > I believe your optimization makes sense for 4kB pages, but > is going to be counter-productive for 2MB pages. > > Your approach of "make ksmd skip over more pages, so it uses > less CPU" is likely to reduce the effectiveness of ksm by not > sharing some pages. > > For 4kB pages that is fine, because you'll get around to them > eventually. > > However, the internal use of a 2MB page is likely to be quite > different. Chances are most 2MB pages will have actively used, > barely used and free pages inside. > > You absolutely want ksm to get at the barely used and free > sub-pages. Having just one actively used 4kB sub-page prevent > ksm from merging any of the other 511 sub-pages is a problem. No, no, I was just not sure about it. I meant we cannot satisfy all people but I was not sure which one is good for most of them. Sorry, again, if I didn't make it clear. Nai > > -- > 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
