On 01/23/2015 08:59 AM, Dr. David Alan Gilbert wrote:
> * 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)

Then it probably needs to be exposed through a command-line parameter,
so that -incoming gains the ability to specify parameters as part of
starting up qemu.

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