[Beowulf] New HPCC results and the Myri viewpoint
patrick at myri.com
Thu Jul 21 01:37:27 EDT 2005
Stuart Midgley wrote:
> Actually, I tend to disagree with your comment here. The curve tells
> you one of the characteristics of the network, which is VERY useful in
> evaluating a network before you expend time/effort testing your code on
> it (assuming you know your code well). On its own (without lots of
> other micro benchmarks) I agree that it is useless.
Yes, Keith noted it also, it's useful to evaluate the receive rate of a
N-to-1 pattern. I meant that it's useless to optimize the send side in
> In my own experience, I tend to find that most codes are not latency
> sensitive (that is, QsNetII, Infinipath, Myricom etc are effectively
> the same, on a latency sense, to most codes)... until they try and
> scale to the 1000's of cpu's. All of a sudden simple things like
> barriers and synchronisation etc can become expensive on networks with
> higher latencies. Things that the software writer wasn't expensive
> start to dominate their code. Hence, the ping-pong latencies and ring
> latencies are useful in giving you an idea of how well the larger codes
> will scale.
In my experience, the main source of delay for synchronization points
when the number of nodes increase is jitter between computation phases:
one node will be late to enter the collective and delay the whole
sub-tree. The other source is contention in the fabric, specially at
1000's of nodes, which ring latency tests don't really exercise.
Ring latencies are a step in the good direction though, but it still
quite analytic IMHO.
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