[Beowulf] Why is communication so expensive for very small messages?
patrick at myri.com
Tue Apr 24 12:54:42 PDT 2007
Jonathan Boyle wrote:
> For 2 processor blocking calls, mpptest indicates a latency of about 30
> However when I measure communication times in my own program using a loop as
If you don't want flow control problems, you need to do a pingpong (each
node send and recv alternatively) or do explicit flow control using a
window nad ack message. MPI_Send() can return as soon as the data is
buffered (ie copied somewhere on the send side), and that is much faster
than the network itself, so the somewhere will fill up eventually.
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