Re: [PATCH vfio 0/7] Enhances the vfio-virtio driver to support live migration

[Alex Williamson <alex.williamson@redhat.com>]

On Mon, 28 Oct 2024 13:23:54 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Mon, Oct 28, 2024 at 10:13:48AM -0600, Alex Williamson wrote:
> 
> > If the virtio spec doesn't support partial contexts, what makes it
> > beneficial here?    
> 
> It stil lets the receiver 'warm up', like allocating memory and
> approximately sizing things.
> 
> > If it is beneficial, why is it beneficial to send initial data more than
> > once?    
> 
> I guess because it is allowed to change and the benefit is highest
> when the pre copy data closely matches the final data..

It would be useful to see actual data here.  For instance, what is the
latency advantage to allocating anything in the warm-up and what's the
probability that allocation is simply refreshed versus starting over?

Re-sending the initial data up to some arbitrary cap sounds more like
we're making a policy decision in the driver to consume more migration
bandwidth for some unknown latency trade-off at stop-copy.  I wonder if
that advantage disappears if the pre-copy data is at all stale relative
to the current device state.  Thanks,

Alex