On Thu, Mar 24, 2016 at 01:19:40AM +0000, Li, Liang Z wrote: > > > > > 2. Why not use virtio-balloon > > > > > Actually, the virtio-balloon can do the similar thing by inflating > > > > > the balloon before live migration, but its performance is no good, > > > > > for an 8GB idle guest just boots, it takes about 5.7 Sec to > > > > > inflate the balloon to 7GB, but it only takes 25ms to get a valid > > > > > free page bitmap from the guest. There are some of reasons for > > > > > the bad performance of > > > > > vitio-balloon: > > > > > a. allocating pages (5%, 304ms) > > > > > > > > Interesting. This is definitely worth improving in guest kernel. > > > > Also, will it be faster if we allocate and pass to guest huge pages instead? > > > > Might speed up madvise as well. > > > > > > Maybe. > > > > > > > > b. sending PFNs to host (71%, 4194ms) > > > > > > > > OK, so we probably should teach balloon to pass huge lists in bitmaps. > > > > Will be benefitial for regular balloon operation, as well. > > > > > > > > > > Agree. Current balloon just send 256 PFNs a time, that's too few and > > > lead to too many times of virtio transmission, that's the main reason for the > > bad performance. > > > Change the VIRTIO_BALLOON_ARRAY_PFNS_MAX to a large value can > > improve > > > the performance significant. Maybe we should increase it before doing > > > the further optimization, do you think so ? > > > > We could push it up a bit higher: 256 is 1kbyte in size, so we can make it 3x > > bigger and still fit struct virtio_balloon is a single page. But if we are going to > > add the bitmap variant anyway, we probably shouldn't bother. > > > > > > > c. address translation and madvise() operation (24%, 1423ms) > > > > > > > > How is this split between translation and madvise? I suspect it's > > > > mostly madvise since you need translation when using bitmap as well. > > > > Correct? Could you measure this please? Also, what if we use the > > > > new MADV_FREE instead? By how much would this help? > > > > > > > For the current balloon, address translation is needed. > > > But for live migration, there is no need to do address translation. > > > > Well you need ram address in order to clear the dirty bit. > > How would you get it without translation? > > > > If you means that kind of address translation, yes, it need. > What I want to say is, filter out the free page can be done by bitmap operation. > > Liang OK so I see that your patches use block->offset in struct RAMBlock to look up bits in guest-supplied bitmap. I don't think that's guaranteed to work. -- MST -- 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
