> 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. It's part of the bitmap operation, because the latest change of the ram_list.dirty_memory. Why do you think so? Could you tell me the reason? Liang -- 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
