[openib-general] IPoIB broadcast MC group membership

[amith rajith mamidala]

Hi,

> * MVAPICH uses a multicast group for some MPI collectives
>   * This can be turned off by setting env var DISABLE_HARDWARE_MCST

MVAPICH uses h/w multicast for MPI_Bcast. We plan to extend this to other
collectives in future,

Thanks,
Amith

>
> * An IB multicast group has to use ports of the same speed
>   * This one was a surprise to me
>
> Ergo, when you mix 1X, 4X SDR, and 4X DDR hosts, it behaves
> differently from a homogeneous network.
>
> -- greg
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