[Beowulf] InfiniBand channel bundling?

Mark Hahn hahn at mcmaster.ca
Thu Oct 30 22:17:49 PDT 2014

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

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.

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