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

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] [RFC PATCH RDMA support v5: 03/12] comprehensive protocol documentation
Date: Fri, 12 Apr 2013 13:48:02 +0300

On Thu, Apr 11, 2013 at 04:33:03PM -0400, Michael R. Hines wrote:
> On 04/11/2013 03:15 PM, Michael S. Tsirkin wrote:
> >On Thu, Apr 11, 2013 at 01:49:34PM -0400, Michael R. Hines wrote:
> >>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".
> >The problem is bad performance, isn't it?
> >If it wasn't we'd use chunk based all the time.
> >
> >>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.
> >What gave you that idea? Not at all.  It is based on the idea
> >of doing control actions in parallel with data transfers,
> >so that control latency does not degrade performance.
> Again, this parallelization is trying to solve a problem that
> doesn't exist.
> As I've described before, I re-executed the worst-case memory stress hog
> tests with RDMA *after* the bulk-phase round completes and determined
> that RDMA throughput remains unaffected because most of the memory
> was already registered in advance.
> >>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
> >What is the bottleneck for chunk-based? Can you tell me that?  Find out,
> >and you will maybe see pipelining will help.
> >
> >Basically to me, when you describe the protocol in detail the problems
> >become apparent.
> >
> >I think you worry too much about what the guest does, what APIs are
> >exposed from the migration core and the specifics of the workload. Build
> >a sane protocol for data transfers and layer the workload on top.
> >
> What is the point in enhancing a protocol to solve a problem will
> never be manifested?
> We're trying to overlap two *completely different use cases* that
> are completely unrelated:
> 1. Static overcommit
> 2. Dynamic, fine-grained overcommit (at small time scales... seconds
> or minutes)
> #1 Happens all the time. Cram a bunch of virtual machines with fixed
> workloads
> and fixed writable working sets into the same place, and you're good to go.
> #2 never happens. Ever. It just doesn't happen, and the enhancements you've
> described are trying to protect against #2, when we should really be
> focused on #1.
> It is not standard practice for a workload to expect high overcommit
> performance
> in the *middle* of a relocation and nobody in the industry that I
> have met over the
> years has expressed any such desire to do so.

Depends on who you talk to I guess.  Almost everyone
overcommits to some level. They might not know it.
It depends on the amount of overcommit.  You pin all (at least non zero)
memory eventually, breaking memory overcommit completely. If I
overcommit by 4kilobytes do you expect performance to go completely
down? It does not make sense.

> Workloads just don't behave that way.
> Dynamic registration does an excellent job at overcommitment for #1
> because most
> of the registrations are done at the very beginning and can be
> further optimized to
> cause little or no performance loss by simply issuing the
> registrations before the
> migration ever begins.

How does it? You pin all VM's memory eventually.
You said your tests have the OOM killer triggering.

> Performance for #2 even with dynamic registration is excellent and I am not
> experiencing any problems associated with it.

Well previously you said the reverse. You keep vaguely speaking about
performance.  We care about these metrics:

        1. total migration time: measured by:

         ssh dest qemu -incoming &;echo migrate > monitor

        2.  min allowed downtime that lets migration converge

        3. average host CPU utilization during migration,
           on source and destination

        4. max real memory used by qemu

Can you fill this table for TCP, and two protocol versions?

If dynamic works as well as static, this is a good reason to drop the
static one.  As the next step, fix the dynamic to unregister
memory (this is required for _GIFT anyway). When you do this
it is possible that pipelining is required.

> So, we're discussing a non-issue.
> - Michael

There are two issues.

1.  You have two protocols already and this does not make sense in
version 1 of the patch.  You said dynamic is slow so I pointed out ways
to improve it. Now you says it's as fast as static?  so drop static
then. At no point does it make sense to have management commands to play
with low level protocol details.

> Overcommit has two

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