Li, Liang Z
2016-Dec-16 00:48 UTC
[Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration
> Subject: Re: [Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for > fast (de)inflating & fast live migration > > On 12/14/2016 12:59 AM, Li, Liang Z wrote: > >> Subject: Re: [Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon > >> for fast (de)inflating & fast live migration > >> > >> On 12/08/2016 08:45 PM, Li, Liang Z wrote: > >>> What's the conclusion of your discussion? It seems you want some > >>> statistic before deciding whether to ripping the bitmap from the > >>> ABI, am I right? > >> > >> I think Andrea and David feel pretty strongly that we should remove > >> the bitmap, unless we have some data to support keeping it. I don't > >> feel as strongly about it, but I think their critique of it is pretty > >> valid. I think the consensus is that the bitmap needs to go. > >> > >> The only real question IMNHO is whether we should do a power-of-2 or > >> a length. But, if we have 12 bits, then the argument for doing > >> length is pretty strong. We don't need anywhere near 12 bits if doing > power-of-2. > > > > Just found the MAX_ORDER should be limited to 12 if use length instead > > of order, If the MAX_ORDER is configured to a value bigger than 12, it > > will make things more complex to handle this case. > > > > If use order, we need to break a large memory range whose length is > > not the power of 2 into several small ranges, it also make the code complex. > > I can't imagine it makes the code that much more complex. It adds a for loop. > Right? >Yes, just a little. :)> > It seems we leave too many bit for the pfn, and the bits leave for > > length is not enough, How about keep 45 bits for the pfn and 19 bits > > for length, 45 bits for pfn can cover 57 bits physical address, that should be > enough in the near feature. > > > > What's your opinion? > > I still think 'order' makes a lot of sense. But, as you say, 57 bits is enough for > x86 for a while. Other architectures.... who knows?Yes.
Dave Hansen
2016-Dec-16 01:09 UTC
[Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration
On 12/15/2016 04:48 PM, Li, Liang Z wrote:>>> It seems we leave too many bit for the pfn, and the bits leave for >>> length is not enough, How about keep 45 bits for the pfn and 19 bits >>> for length, 45 bits for pfn can cover 57 bits physical address, that should be >> enough in the near feature. >>> What's your opinion? >> I still think 'order' makes a lot of sense. But, as you say, 57 bits is enough for >> x86 for a while. Other architectures.... who knows?Thinking about this some more... There are really only two cases that matter: 4k pages and "much bigger" ones. Squeezing each 4k page into 8 bytes of metadata helps guarantee that this scheme won't regress over the old scheme in any cases. For bigger ranges, 8 vs 16 bytes means *nothing*. And 16 bytes will be as good or better than the old scheme for everything which is >4k. How about this: * 52 bits of 'pfn', 5 bits of 'order', 7 bits of 'length' * One special 'length' value to mean "actual length in next 8 bytes" That should be pretty simple to produce and decode. We have two record sizes, but I think it is manageable.
Li, Liang Z
2016-Dec-16 01:38 UTC
[Qemu-devel] [PATCH kernel v5 0/5] Extend virtio-balloon for fast (de)inflating & fast live migration
> On 12/15/2016 04:48 PM, Li, Liang Z wrote: > >>> It seems we leave too many bit for the pfn, and the bits leave for > >>> length is not enough, How about keep 45 bits for the pfn and 19 bits > >>> for length, 45 bits for pfn can cover 57 bits physical address, that > >>> should be > >> enough in the near feature. > >>> What's your opinion? > >> I still think 'order' makes a lot of sense. But, as you say, 57 bits > >> is enough for > >> x86 for a while. Other architectures.... who knows? > > Thinking about this some more... There are really only two cases that > matter: 4k pages and "much bigger" ones. > > Squeezing each 4k page into 8 bytes of metadata helps guarantee that this > scheme won't regress over the old scheme in any cases. For bigger ranges, 8 > vs 16 bytes means *nothing*. And 16 bytes will be as good or better than > the old scheme for everything which is >4k. > > How about this: > * 52 bits of 'pfn', 5 bits of 'order', 7 bits of 'length' > * One special 'length' value to mean "actual length in next 8 bytes" > > That should be pretty simple to produce and decode. We have two record > sizes, but I think it is manageable.It works, Now that we intend to use another 8 bytes for length Why not: Use 52 bits for 'pfn', 12 bits for 'length', when the 12 bits is not long enough for the 'length' Set the 'length' to a special value to indicate the "actual length in next 8 bytes". That will be much more simple. Right? Liang
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