[Beowulf] InfiniBand channel bundling?

[Mark Hahn]

> If you compare QDR devices to FDR devices, than FDR is showing lower
>latency.

in the paper referenced, that is not the case.  the numbers provided
are QDR 1.27 us, versus FDR 1.67 us.  although it's only 400ns,
it's still >30% slower, when one might expect a speed improvement.

> What you might heard is that the FDR switches are slightly higher
>latency than the QDR switches as they include new capabilities of link level
>retransmission and forward error correction, but overall end to end latency
>with FDR is lower.

that's interesting - do you mean that in order to achieve higher bandwidth,
the error rate becomes a problem, necessitating RT/FEC?  I guess it's 
obvious from the shrinkage of allowed passive/copper cable lengths
that SNR/BER is a big issue, but does this imply that going optical will
reduce the latency cost for FDR?

> The EDR switch latency is lower than the FDR switch and
> the QDR switch, so further latency decrease will be seen with EDR.

I'm puzzled by this, since at least marketing latencies of existing
switches are pretty low (170ns/hop for FDR).  how much can 170ns 
be improved?

thanks, mark hahn.