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

[Avi Kivity]

On 08/08/2011 05:15 PM, Anthony Liguori wrote:
> > > 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).

libvirt isn't an exec helper.

> 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).

If we have a shared object helper, the thread should be maintained by 
qemu proper, not the plugin.

I wouldn't call it "migration transport", but instead a 
compression/decompression plugin.

I don't think it merits a plugin at all though.  There's limited scope 
for compression and it best sits in qemu proper.  If anything, it needs 
to be more integrated (for example turning itself off if it doesn't 
match enough).

--
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