Jeremy Fitzhardinge wrote: > Rik van Riel wrote: > >> Jeremy Fitzhardinge wrote: >> >>> Rik van Riel wrote: >>> >>>> Jeremy Fitzhardinge wrote: >>>> >>>> >>>>> That said, people have been looking at tracking block IO to work >>>>> out when it might be useful to try and share pages between guests >>>>> under Xen. >>>>> >>>> Tracking block IO seems like a bass-ackwards way to figure >>>> out what the contents of a memory page are. >>>> >>> Well, they're research projects, so nobody said that they're >>> necessarily useful results ;). I think the rationale is that, in >>> general, there aren't all that many sharable pages, and asize from >>> zero-pages, the bulk of them are the result of IO. >>> >> I'll give you a hint: Windows zeroes out freed pages. >> > > Right: "aside from zero-pages". If you exclude zero-pages from your > count of shared pages, the amount of sharing drops a lot. > > >> It should also be possible to hook up arch_free_page() so >> freed pages in Linux guests become sharable. >> >> Furthermore, every guest with the same OS version will be >> running the same system daemons, same glibc, etc. This >> means sharable pages from not just disk IO (probably from >> different disks anyway), >> > > Why? If you're starting a bunch of cookie-cutter guests, then you're > probably starting them from the same template image or COW block > devices. (Also, if you're wearing the cost of physical IO anyway, then > additional cost of hashing is relatively small.) > > >> but also in the BSS and possibly >> even on the heap. >> > > Well, maybe. Modern systems generally randomize memory layouts, so even > if they're semantically the same, the pointers will all have different > values. > > Other research into "sharing" mostly-similar pages is more promising for > that kind of case. > > >> Eventually. It starts out with hashing the first 128 (IIRC) >> bytes of page content and comparing the hashes. If that >> matches, it will do content comparison. >> The algorithm was changed quite a bit. Izik is planning to resubmit it any day now. >> Content comparison is done in the background on the host. >> I suspect (but have not checked) that it is somehow hooked >> up to the page reclaim code on the host. >> > > Yeah, that's the straightforward approach; there's about a research > project/year doing a Xen implementation, but they never seem to get very > good results aside from very artificial test conditions. > Actually we got really good results by using ksm along with kvm, running large amount of windows virtual machines. We can achieve over commit ratio of up to 400% of the host ram for VMs doing M$ office load. -dor _______________________________________________ Virtualization mailing list Virtualization@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/virtualization
