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Re: [Qemu-devel] [PATCH v3] Throttle-down guest when live migration does

From: Eric Blake
Subject: Re: [Qemu-devel] [PATCH v3] Throttle-down guest when live migration does not converge.
Date: Wed, 01 May 2013 06:38:24 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130402 Thunderbird/17.0.5

On 05/01/2013 06:22 AM, Chegu Vinod wrote:
> Busy enterprise workloads hosted on large sized VM's tend to dirty
> memory faster than the transfer rate achieved via live guest migration.
> Despite some good recent improvements (& using dedicated 10Gig NICs
> between hosts) the live migration does NOT converge.

> ---
> Changes from v2:
> - incorporated feedback from Orit, Juan and Eric
> - stop the throttling thread at the start of stage 3
> - rebased to latest qemu.git

> +++ b/qapi-schema.json
> @@ -600,9 +600,14 @@
>  #          loads, by sending compressed difference of the pages
>  #
>  # Since: 1.2
> +#
> +# @auto-converge: Migration supports automatic throttling down of guest
> +#          to force convergence. Disabled by default.
> +#
> +# Since: 1.6
>  ##

I've already argued that ALL new migration capabilities should be
disabled by default (see the thread on 'x-rdma-pin-all', which will be a
merge conflict if it gets applied before your patch).  So I don't think
that last sentence adds anything, and can be dropped.

I think this works, although it's the first instance of having two
top-level Since: tags on a single JSON entity.  I was envisioning:

@xbzrle: yadda... pages

@auto-convert: Migration supports... convergence (since 1.6)

Since: 1.2

to match the conventions elsewhere that the overall JSON entity (the
enum MigrationCapability) exists since 1.2, but the addition of
auto-convert happened in 1.6.

However, as nothing parses the .json file to turn it into formal docs
(yet), I'm not going to insist on a respin if this is the only problem
with your patch.  I'm not comfortable enough with my skills in reviewing
the rest of the patch, or I'd offer a reviewed-by.

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

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