Re: [Qemu-devel] [RFC PATCH v2 11/12] mc: introduce new capabilities to control micro-checkpointing

[Eric Blake]

On 02/18/2014 01:50 AM, address@hidden wrote:
> From: "Michael R. Hines" <address@hidden>
> 
> New capabilities include the use of RDMA acceleration,
> use of network buffering, and keepalive support, as documented
> in patch #1.
> 
> Signed-off-by: Michael R. Hines <address@hidden>
> ---
>  qapi-schema.json | 36 +++++++++++++++++++++++++++++++++++-
>  1 file changed, 35 insertions(+), 1 deletion(-)
> 

> +#          Only for performance testing. (Since 2.x)
> +#
> +# @mc-rdma-copy: MC requires creating a local-memory checkpoint before
> +#          transmission to the destination. This requires heavy use of 
> +#          memcpy() which dominates the processor pipeline. This option 
> +#          makes use of *local* RDMA to perform the copy instead of the CPU.
> +#          Enabled by default only if the migration transport is RDMA.
> +#          Disabled by default otherwise. (Since 2.x)

How does that work?  If I query migration capabilities before requesting
a migration, what state am I going to read?  Is there coupling where I
would observe the state of this flag change merely because I did some
other action?  And if so, then how do I know that explicitly setting
this flag won't be undone by similar coupling?

It sounds like you are describing a tri-state option (unspecified so
default to migration transport, explicitly disabled, explicitly
enabled); but that doesn't work for something that only lists boolean
capabilities.  The only way around that is to have 2 separate
capabilities (one on whether to base decision on transport or to honor
override, and the other to provide the override value which is ignored
when defaulting by transport).

> +#
> +# @rdma-keepalive: RDMA connections do not timeout by themselves if a peer
> +#         has disconnected prematurely or failed. User-level keepalives
> +#         allow the migration to abort cleanly if there is a problem with the
> +#         destination host. For debugging, this can be problematic as
> +#         the keepalive may cause the peer to abort prematurely if we are
> +#         at a GDB breakpoint, for example.
> +#         Enabled by default. (Since 2.x)

Enabled-by-default is an interesting choice, but I suppose it is okay.


> +#
>  # Since: 1.2
>  ##
>  { 'enum': 'MigrationCapability',
> -  'data': ['xbzrle', 'x-rdma-pin-all', 'auto-converge', 'zero-blocks'] }
> +  'data': ['xbzrle', 
> +           'rdma-pin-all', 
> +           'auto-converge', 
> +           'zero-blocks',
> +           'mc', 
> +           'mc-net-disable',
> +           'mc-rdma-copy',
> +           'rdma-keepalive'
> +          ] }
>  
>  ##
>  # @MigrationCapabilityStatus
> 

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

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