[openib-general] MPI Brodcast doubt

[Hal Rosenstock]

John,

On Mon, 2006-09-04 at 08:56, john t wrote:
> Hi,
>  
> I have 3 nodes connected via IB as shown below:
>  
> node1 ---> switch1 ---> node2
>                     |----------> node3
>  
> If node1 sends a brodcast message to node2 and node3, I want to know
> if the message is delivered to the switch twice (first time for node2
> and second time for node3) or just once (where switch will know by
> looking at some headers or so that its a brodcast message and will
> send it on all the outgoing ports) ?

Assuming nodes 1, 2, and 3 are part of the same multicast group, the
multicast send is sent once from node 1. When received at the switch, it
is replicated to all ports which have members in the same group (in this
case, nodes 2 and 3). The switch knows by the header (specifically the
LRH:DLID which is a multicast LID) and uses the MulticastForwardingTable
to determine on which ports to forward it. However, IB multicast is
unreliable so to create reliable multicast, it is sometimes "emulated"
in that the sender tracks the group members and may use serial unicast
sends or augment a multicast send with unicast sends to the receivers
and track their acknowledgements of receipt.

-- Hal

> Regards,
> John T.
> 
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