Re: [Qemu-devel] [PATCH 0/17 v2] Localhost migration with side channel for ram

[Anthony Liguori]

On Oct 25, 2013 8:30 AM, "Paolo Bonzini" <address@hidden> wrote:
>
> Il 25/10/2013 06:58, Lei Li ha scritto:
> > Right now just has inaccurate numbers without the new vmsplice, which
> > based on
> > the result from info migrate, as the guest ram size increases, although the
> > 'total time' is number of times less compared with the current live
> > migration, but the 'downtime' performs badly.
>
> Of course.
> >
> > For a 1GB ram guest,
> >
> > total time: 702 milliseconds
> > downtime: 692 milliseconds
> >
> > And when the ram size of guest increasesexponentially, those numbers are
> > proportional to it.
> >
> > I will make a list of the performance with the new vmsplice later, I am
> > sure it'd be much better than this at least.
>
> Yes, please.  Is the memory usage is still 2x without vmsplice?
>
> I think you have a nice proof of concept, but on the other hand this
> probably needs to be coupled with some kind of postcopy live migration,
> that is:
>
> * the source starts sending data
>
> * but the destination starts running immediately
>
> * if the machine needs a page that is missing, the destination asks the
> source to send it
>
> * as soon as it arrives, the destination can restart
>
> Using postcopy is problematic for reliability: if the destination fails,
> the virtual machine is lost because the source doesn't have the latest
> content of memory.  However, this is a much, much smaller problem for
> live QEMU upgrade where the network cannot fail.
>
> If you do this, you can achieve pretty much instantaneous live upgrade,
> well within your original 200 ms goals.  

This is actually a very nice justification for post copy.

Regards,

Anthony Liguori

But the flipping code with
> vmsplice should be needed anyway to avoid doubling memory usage, and
> it's looking pretty good in this version already!  I'm relieved that the
> RDMA code was designed right!
>
> Paolo
>
>