Re: [Qemu-devel] [PATCH v4] XBZRLE delta for live migration of large memory apps

[Anthony Liguori]

On 08/08/2011 08:51 AM, Avi Kivity wrote:
> On 08/08/2011 04:29 PM, Anthony Liguori wrote:
> > One thing that strikes me about this algorithm is that it's very good
> > for a particular type of workload--shockingly good really.
>
> Poking bytes at random places in memory is fairly generic. If you have a
> lot of small objects, and modify a subset of them, this is the pattern
> you get.
>
> > I think workload aware migration compression is possible for a lot of
> > different types of workloads. That makes me a bit wary of QEMU growing
> > quite a lot of compression mechanisms.
> >
> > It makes me think that this logic may really belong at a higher level
> > where more information is known about the workload. For instance, I
> > can imagine XBZRLE living in something like libvirt.
>
> A better model would be plugin based.

exec helpers are plugins.  They just live in a different address space 
and a channel to exchange data (pipe).

If we did .so plugins, which I'm really not opposed to, I'd want the 
interface to be something like:

typedef struct MigrationTransportClass
{
   ssize_t (*writev)(MigrationTransport *obj,
                     struct iovec *iov,
                     int iovcnt);
} MigrationTransportClass;

I think it's useful to use an interface like this because it makes it 
easy to put the transport in a dedicated thread that didn't hold 
qemu_mutex (which is sort of equivalent to using a fork'd helper but is 
zero-copy at the expense of less isolation).

Regards,

Anthony Liguori

>