Re: [Qemu-devel] [RFC PATCH RDMA support v5: 03/12] comprehensive protocol documentation

["Michael R. Hines" <mrhines@linux.vnet.ibm.com>]

On 04/11/2013 10:56 AM, Michael S. Tsirkin wrote:
> On Thu, Apr 11, 2013 at 04:50:21PM +0200, Paolo Bonzini wrote:
>> Il 11/04/2013 16:37, Michael S. Tsirkin ha scritto:
>>> pg1 ->  pin -> req -> res -> rdma -> done
>>>          pg2 ->  pin -> req -> res -> rdma -> done
>>>                  pg3 -> pin -> req -> res -> rdma -> done
>>>                         pg4 -> pin -> req -> res -> rdma -> done
>>>                                pg4 -> pin -> req -> res -> rdma -> done
>>>
>>> It's like a assembly line see?  So while software does the registration
>>> roundtrip dance, hardware is processing rdma requests for previous
>>> chunks.
>> Does this only affects the implementation, or also the wire protocol?
> It affects the wire protocol.

I *do* believe chunked registration was a *very* useful request by
the community, and I want to thank you for convincing me to implement it.

But, with all due respect, pipelining is a "solution looking for a problem".

Improving the protocol does not help the behavior of any well-known 
workloads,
because it is based on the idea the the memory footprint of a VM would
*rapidly* shrink and contract up and down during the steady-state iteration
rounds while the migration is taking place.

This simply does not happen - workloads don't behave that way - they either
grow really big or they grow really small and they settle that way for a 
reasonable
amount of time before the load on the application changes at a future 
point in time.

- Michael