Re: [Qemu-devel] [v3 12/13] migration: Add command to set migration parameter

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Eric Blake (eblake@redhat.com) wrote:
> On 01/23/2015 06:48 AM, Dr. David Alan Gilbert wrote:
> > * Liang Li (liang.z.li@intel.com) wrote:
> >> Add the qmp and hmp commands to tune the parameters used in live
> >> migration.
> > 
> > If I understand correctly on the destination side we need to set the number of
> > decompression threads very early on an incoming migration - I'm not
> > clear how early that needs to be - especially if you're using fd: so it's
> > not waiting for a connect ?
> > 
> > Eric: How would libvirt do that?
> 
> Libvirt does some handshaking before starting migration.  The source
> would advertise that "I'm capable of threaded migration; and plan to use
> X send and Y receive threads"; the destination would either reply that
> threads are unsupported or set the receive thread count at that point,
> then the source would know if it can enable threads and set the send
> thread count, before letting migration start.  I don't see any problems
> with the current design of starting things up.

Patch 3 calls migrate_decompress_threads_create from process_incoming_migration_co,
if you're using fd based incoming migration then I think that gets called
before libvirt would have a chance to connect to the monitor and
call the parameter setting to say how many threads it wants.
(A tcp incoming migration wouldn't have that problem because it
waits until the TCP accept before calling process_incoming_migration_co)

Dave

> 
> -- 
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
> 


--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK