EECE 432 / CS 442	Spring 1999
Homework Set 4

Due Wednesday, March 10, 1999

Communication on a message-passing machine is typically modeled as a
linear function of the message size. For instance, to send an "m"-byte
message from a source to destination processor takes

T(m) = L + (B * m) seconds

where "L" is the "latency" of the network, and "B" is the reciprocal
of the "bandwidth".

A common way of measuring communication time is by using the
"ping-pong" method:

  1) P0 starts the clock (see MPI_Wtime)
  2) send a message from processor P0 to P1
  3) P1 receives the message and immediately returns it to P0
  4) P0 receives the message and stops the clock

The approximate one-way time for the message is then half of the
elapsed time measured by processor P0. Typically, the ping-pong
communication is repeatedly, say, 100 times, and the final time is
divided by the same number of iterations, to average out any
anomalies.

(See Chapter 3 of Hwang and Xu for more details on the "ping-pong"
test.)

Write an MPI program that finds the latency and bandwidth parameters
for both the SP-2 and the Alta. First, calculate latency (L) by using
a short (1-byte) message. Then graph the bandwidth (1/B) for message
sizes of 1, 4, 16, 64, 256, 1024, 2048, 8192 bytes. Does the achieved
bandwidth change as a function of message size? 

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