Li, Liang Z
2016-Mar-07 06:49 UTC
[Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization
> > No. And it's exactly what I mean. The ballooned memory is still > > processed during live migration without skipping. The live migration code is > in migration/ram.c. > > So if guest acknowledged VIRTIO_BALLOON_F_MUST_TELL_HOST, we can > teach qemu to skip these pages. > Want to write a patch to do this? >Yes, we really can teach qemu to skip these pages and it's not hard. The problem is the poor performance, this PV solution is aimed to make it more efficient and reduce the performance impact on guest.> > > > > > > > > The only advantage of ' inflating the balloon before live > > > > > > migration' is simple, > > > > > nothing more. > > > > > > > > > > That's a big advantage. Another one is that it does something > > > > > useful in real- world scenarios. > > > > > > > > > > > > > I don't think the heave performance impaction is something useful > > > > in real > > > world scenarios. > > > > > > > > Liang > > > > > Roman. > > > > > > So fix the performance then. You will have to try harder if you want > > > to convince people that the performance is due to bad host/guest > > > interface, and so we have to change *that*. > > > > > > > Actually, the PV solution is irrelevant with the balloon mechanism, I > > just use it to transfer information between host and guest. > > I am not sure if I should implement a new virtio device, and I want to > > get the answer from the community. > > In this RFC patch, to make things simple, I choose to extend the > > virtio-balloon and use the extended interface to transfer the request and > free_page_bimap content. > > > > I am not intend to change the current virtio-balloon implementation. > > > > Liang > > And the answer would depend on the answer to my question above. > Does balloon need an interface passing page bitmaps around?Yes, I need a new interface.> Does this speed up any operations?No, a new interface will not speed up anything, but it is the easiest way to solve the compatibility issue.> OTOH what if you use the regular balloon interface with your patches? >The regular balloon interfaces have their specific function and I can't use them in my patches. If using these regular interface, I have to do a lot of changes to keep the compatibility.> > > > -- > > > MST
Michael S. Tsirkin
2016-Mar-07 11:40 UTC
[Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization
On Mon, Mar 07, 2016 at 06:49:19AM +0000, Li, Liang Z wrote:> > > No. And it's exactly what I mean. The ballooned memory is still > > > processed during live migration without skipping. The live migration code is > > in migration/ram.c. > > > > So if guest acknowledged VIRTIO_BALLOON_F_MUST_TELL_HOST, we can > > teach qemu to skip these pages. > > Want to write a patch to do this? > > > > Yes, we really can teach qemu to skip these pages and it's not hard. > The problem is the poor performance, this PV solutionBalloon is always PV. And do not call patches solutions please.> is aimed to make it more > efficient and reduce the performance impact on guest.We need to get a bit beyond this. You are making multiple changes, it seems to make sense to split it all up, and analyse each change separately. If you don't this patchset will be stuck: as you have seen people aren't convinced it actually helps with real workloads.> > > > > > > > > > > The only advantage of ' inflating the balloon before live > > > > > > > migration' is simple, > > > > > > nothing more. > > > > > > > > > > > > That's a big advantage. Another one is that it does something > > > > > > useful in real- world scenarios. > > > > > > > > > > > > > > > > I don't think the heave performance impaction is something useful > > > > > in real > > > > world scenarios. > > > > > > > > > > Liang > > > > > > Roman. > > > > > > > > So fix the performance then. You will have to try harder if you want > > > > to convince people that the performance is due to bad host/guest > > > > interface, and so we have to change *that*. > > > > > > > > > > Actually, the PV solution is irrelevant with the balloon mechanism, I > > > just use it to transfer information between host and guest. > > > I am not sure if I should implement a new virtio device, and I want to > > > get the answer from the community. > > > In this RFC patch, to make things simple, I choose to extend the > > > virtio-balloon and use the extended interface to transfer the request and > > free_page_bimap content. > > > > > > I am not intend to change the current virtio-balloon implementation. > > > > > > Liang > > > > And the answer would depend on the answer to my question above. > > Does balloon need an interface passing page bitmaps around? > > Yes, I need a new interface.Possibly, but you will need to justify this at some level if you care about upstreaming your patches.> > Does this speed up any operations? > > No, a new interface will not speed up anything, but it is the easiest way to solve the compatibility issue.A bunch of new code is often easier to write than to figure out the old one, but if we keep piling it up we'll end up with an unmaintainable mess. So we are rather careful about adding new interfaces, and we try to make them generic sometimes even at cost of slight inefficiencies.> > OTOH what if you use the regular balloon interface with your patches? > > > > The regular balloon interfaces have their specific function and I can't use them in my patches. > If using these regular interface, I have to do a lot of changes to keep the compatibility.Why can't you? What exactly do we need to change? If we put things in terms of the balloon, that supports adding and removing pages. Using these terms, let's enumerate: - a new method (e.g. new virtqueue) that adds and immediately removes page in a balloon clearly, you can add then remove using the existing interfaces is a single command significantly faster than using existing two vqs? - a new kind of request that says "add (and immediately remove?) as many pages as you can" sounds rather benign - a new kind of message that adds multiple pages using a bitmap (instead of an address list) again, is this significantly faster? Does not look like compatibility is an issue, to me. At some level, your patches look like page hints. If we have more patches in mind that use page hints, then a new hint device might make sense. However, people experimented with page hints in the past, so far this always went nowhere. E.g. I CC Rick who saw some problems when page hints interact with huge pages. Rick, could you elaborate please? -- MST
Li, Liang Z
2016-Mar-07 15:06 UTC
[Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization
> Cc: Roman Kagan; Dr. David Alan Gilbert; ehabkost at redhat.com; > kvm at vger.kernel.org; quintela at redhat.com; linux-kernel at vger.kernel.org; > qemu-devel at nongnu.org; linux-mm at kvack.org; amit.shah at redhat.com; > pbonzini at redhat.com; akpm at linux-foundation.org; > virtualization at lists.linux-foundation.org; rth at twiddle.net; riel at redhat.com > Subject: Re: [Qemu-devel] [RFC qemu 0/4] A PV solution for live migration > optimization > > On Mon, Mar 07, 2016 at 06:49:19AM +0000, Li, Liang Z wrote: > > > > No. And it's exactly what I mean. The ballooned memory is still > > > > processed during live migration without skipping. The live > > > > migration code is > > > in migration/ram.c. > > > > > > So if guest acknowledged VIRTIO_BALLOON_F_MUST_TELL_HOST, we > can > > > teach qemu to skip these pages. > > > Want to write a patch to do this? > > > > > > > Yes, we really can teach qemu to skip these pages and it's not hard. > > The problem is the poor performance, this PV solution > > Balloon is always PV. And do not call patches solutions please. >OK.> > is aimed to make it more > > efficient and reduce the performance impact on guest. > > We need to get a bit beyond this. You are making multiple changes, it seems > to make sense to split it all up, and analyse each change separately. If you > don't this patchset will be stuck: as you have seen people aren't convinced it > actually helps with real workloads. >Really, changing the virtio spec must have good reasons.> > > > > > > > > > > > > The only advantage of ' inflating the balloon before live > > > > > > > > migration' is simple, > > > > > > > nothing more. > > > > > > > > > > > > > > That's a big advantage. Another one is that it does > > > > > > > something useful in real- world scenarios. > > > > > > > > > > > > > > > > > > > I don't think the heave performance impaction is something > > > > > > useful in real > > > > > world scenarios. > > > > > > > > > > > > Liang > > > > > > > Roman. > > > > > > > > > > So fix the performance then. You will have to try harder if you > > > > > want to convince people that the performance is due to bad > > > > > host/guest interface, and so we have to change *that*. > > > > > > > > > > > > > Actually, the PV solution is irrelevant with the balloon > > > > mechanism, I just use it to transfer information between host and > guest. > > > > I am not sure if I should implement a new virtio device, and I > > > > want to get the answer from the community. > > > > In this RFC patch, to make things simple, I choose to extend the > > > > virtio-balloon and use the extended interface to transfer the > > > > request and > > > free_page_bimap content. > > > > > > > > I am not intend to change the current virtio-balloon implementation. > > > > > > > > Liang > > > > > > And the answer would depend on the answer to my question above. > > > Does balloon need an interface passing page bitmaps around? > > > > Yes, I need a new interface. > > Possibly, but you will need to justify this at some level if you care about > upstreaming your patches. > > > > Does this speed up any operations? > > > > No, a new interface will not speed up anything, but it is the easiest way to > solve the compatibility issue. > > A bunch of new code is often easier to write than to figure out the old one, > but if we keep piling it up we'll end up with an unmaintainable mess. So we > are rather careful about adding new interfaces, and we try to make them > generic sometimes even at cost of slight inefficiencies. > > > > OTOH what if you use the regular balloon interface with your patches? > > > > > > > The regular balloon interfaces have their specific function and I can't use > them in my patches. > > If using these regular interface, I have to do a lot of changes to keep the > compatibility. > > Why can't you? > > What exactly do we need to change? > > If we put things in terms of the balloon, that supports adding and removing > pages. > > Using these terms, let's enumerate: > - a new method (e.g. new virtqueue) that adds and immediately removes > page in a balloon > clearly, you can add then remove using the existing interfaces > is a single command significantly faster than using existing two vqs? > - a new kind of request that says "add (and immediately remove?) as many > pages as you can" > sounds rather benign > - a new kind of message that adds multiple pages using a bitmap > (instead of an address list) > again, is this significantly faster?More of less faster because of less data traffic. I didn't measure this, I will do it and take a deep look at the way you suggest if we choose to make use of the virtio-balloon interface.> > Does not look like compatibility is an issue, to me. > > > At some level, your patches look like page hints. > If we have more patches in mind that use page hints, then a new hint device > might make sense. >Yes, I have ever considered to implement a new device, use the virtio-balloon to transfer the free pages information which is irrelevant with the balloon mechanism is some more or less confusing.> However, people experimented with page hints in the past, so far this always > went nowhere. E.g. I CC Rick who saw some problems when page hints > interact with huge pages. Rick, could you elaborate please? >Thanks a lot. Can't wait to know the problems. Liang> > -- > MST
Roman Kagan
2016-Mar-09 14:28 UTC
[Qemu-devel] [RFC qemu 0/4] A PV solution for live migration optimization
On Mon, Mar 07, 2016 at 01:40:06PM +0200, Michael S. Tsirkin wrote:> On Mon, Mar 07, 2016 at 06:49:19AM +0000, Li, Liang Z wrote: > > > > No. And it's exactly what I mean. The ballooned memory is still > > > > processed during live migration without skipping. The live migration code is > > > in migration/ram.c. > > > > > > So if guest acknowledged VIRTIO_BALLOON_F_MUST_TELL_HOST, we can > > > teach qemu to skip these pages. > > > Want to write a patch to do this? > > > > > > > Yes, we really can teach qemu to skip these pages and it's not hard. > > The problem is the poor performance, this PV solution > > Balloon is always PV. And do not call patches solutions please. > > > is aimed to make it more > > efficient and reduce the performance impact on guest. > > We need to get a bit beyond this. You are making multiple > changes, it seems to make sense to split it all up, and analyse each > change separately.Couldn't agree more. There are three stages in this optimization: 1) choosing which pages to skip 2) communicating them from guest to host 3) skip transferring uninteresting pages to the remote side on migration For (3) there seems to be a low-hanging fruit to amend migration/ram.c:iz_zero_range() to consult /proc/self/pagemap. This would work for guest RAM that hasn't been touched yet or which has been ballooned out. For (1) I've been trying to make a point that skipping clean pages is much more likely to result in noticable benefit than free pages only. As for (2), we do seem to have a problem with the existing balloon: according to your measurements it's very slow; besides, I guess it plays badly with transparent huge pages (as both the guest and the host work with one 4k page at a time). This is a problem for other use cases of balloon (e.g. as a facility for resource management); tackling that appears a more natural application for optimization efforts. Thanks, Roman.
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