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

From: Michael R. Hines
Subject: Re: [Qemu-devel] [RFC PATCH RDMA support v5: 03/12] comprehensive protocol documentation
Date: Thu, 11 Apr 2013 13:49:34 -0400
User-agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20130106 Thunderbird/17.0.2

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
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

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