On Mon, Jul 24, 2023 at 03:47:50PM -0400, Peter Xu wrote: > On Mon, Jul 24, 2023 at 07:04:29PM +0100, Daniel P. Berrangé wrote: > > On Mon, Jul 24, 2023 at 01:07:55PM -0400, Peter Xu wrote: > > > Migration bandwidth is a very important value to live migration. It's > > > because it's one of the major factors that we'll make decision on when to > > > switchover to destination in a precopy process. > > > > To elaborate on this for those reading along... > > > > QEMU takes maxmimum downtime limit and multiplies by its estimate > > of bandwidth. This gives a figure for the amount of data QEMU thinks > > it can transfer within the downtime period. > > > > QEMU compares this figure to the amount of data that is still pending > > at the end of an iteration. > > > > > This value is currently estimated by QEMU during the whole live migration > > > process by monitoring how fast we were sending the data. This can be the > > > most accurate bandwidth if in the ideal world, where we're always feeding > > > unlimited data to the migration channel, and then it'll be limited to the > > > bandwidth that is available. > > > > The QEMU estimate for available bandwidth will definitely be wrong, > > potentially by orders of magnitude, if QEMU has a max bandwidth limit > > set, as in that case it is never trying to push the peak rates available > > from the NICs/network fabric. > > > > > The issue is QEMU itself may not be able to avoid those uncertainties on > > > measuing the real "available migration bandwidth". At least not something > > > I can think of so far. > > > > IIUC, you can query the NIC properties to find the hardware transfer > > rate of the NICs. That doesn't imply apps can actually reach that > > rate in practice - it has a decent chance of being a over-estimate > > of bandwidth, possibly very very much over. > > > > Is such an over estimate better or worse than QEMU's current > > under-estimate ? It depends on the POV. > > > > From the POV of QEMU, over-estimating means means it'll be not > > be throttling as much as it should. That's not a downside of > > migration - it makes it more likely for migration to complete :-) > > Heh. :) > > > > > From the POV of non-QEMU apps though, if QEMU over-estimates, > > it'll mean other apps get starved of network bandwidth. > > > > Overall I agree, there's no obvious way QEMU can ever come up > > with a reliable estimate for bandwidth available. > > > > > One way to fix this is when the user is fully aware of the available > > > bandwidth, then we can allow the user to help providing an accurate value. > > > > > > For example, if the user has a dedicated channel of 10Gbps for migration > > > for this specific VM, the user can specify this bandwidth so QEMU can > > > always do the calculation based on this fact, trusting the user as long as > > > specified. > > > > I can see that in theory, but when considering a non-trivial > > deployments of QEMU, I wonder if the user can really have any > > such certainty of what is truely avaialble. It would need > > global awareness of the whole network of hosts & workloads. > > Indeed it may or may not be easy always. > > The good thing about this parameter is we always use the old estimation if > the user can't specify anything valid, so this is always optional not > required. > > It solves the cases where the user can still specify accurately on the bw - > our QE team has already verified that it worked for us on GPU tests, where > it used to not be able to migrate at all with any sane downtime specified. > I should have attached a Tested-By from Zhiyi but since this is not exactly > the patch he was using I didn't. > > > > > > When the user wants to have migration only use 5Gbps out of that 10Gbps, > > > one can set max-bandwidth to 5Gbps, along with available-bandwidth to > > > 5Gbps > > > so it'll never use over 5Gbps too (so the user can have the rest 5Gbps for > > > other things). So it can be useful even if the network is not dedicated, > > > but as long as the user can know a solid value. > > > > > > A new parameter "available-bandwidth" is introduced just for this. So when > > > the user specified this parameter, instead of trusting the estimated value > > > from QEMU itself (based on the QEMUFile send speed), let's trust the user > > > more. > > > > I feel like rather than "available-bandwidth", we should call > > it "max-convergance-bandwidth". > > > > To me that name would better reflect the fact that this isn't > > really required to be a measure of how much NIC bandwidth is > > available. It is merely an expression of a different bandwidth > > limit to apply during switch over. > > > > IOW > > > > * max-bandwidth: limit during pre-copy main transfer > > * max-convergance-bandwidth: limit during pre-copy switch-over > > * max-postcopy-banwidth: limit during post-copy phase > > I worry the new name suggested is not straightforward enough at the 1st > glance, even to me as a developer. > > "available-bandwidth" doesn't even bind that value to "convergence" at all, > even though it was for solving this specific problem here. I wanted to make > this parameter sololy for the admin to answer the question "how much > bandwidth is available to QEMU migration in general?" That's pretty much > straightforward IMHO. With that, it's pretty sane to consider using all we > have during switchover (aka, unlimited bandwidth, as fast as possible). > > Maybe at some point we can even leverage this information for other purpose > rather than making the migration converge.
The flipside is that the semantics & limits we want for convergance are already known to be different from what we wanted for pre-copy and post-copy. With that existing practice, it is probably more likely that we would not want to re-use the same setting for different cases, which makes me think a specifically targetted parameter is better. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
