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Mastering Perl/Tk by Nancy Walsh, Stephen Lidie

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Computing π with Parallel Message Passing

Scientists demand the most powerful computing machinery available, whether they model global atmospheric conditions, decipher the human genome, study viscous fluid dynamics, visualize plasmas in 3 dimensions, or vibrate strings in 14. Besides tons of disk space (perhaps a terabyte for /tmp alone) and tremendous I/O and memory bandwidth, they want raw CPU power. Since no single CPU will ever suffice, scientists work with vector and massively parallel computers.

In parallel programming, we try to divide the work of a sequential program into portions so that many processors can work on the problem simultaneously. For example, this code[2] computes π by integrating the function 4 / (1 + x2) in the range -.5 ≤ x < .5:

my $intervals = 1_000;
my $h = 1.0 / $intervals;
my $sum = 0.0;

for(my $i = 1; $i <= $intervals; $i++) {
    my $x = $h * ($i - 0.5);
    $sum += 4.0 / (1.0 + $x*$x);
}

my $pi = $h * $sum;

The variable $intervals defines the granularity of the summation and, hence, the accuracy of the computation. To get a good result, the interval must be very finely divided, which, of course, increases the program’s running time. But suppose we parcel out the work to two processors, with each one integrating only half of the curve? If we then add the two partial sums, the value of π is the same, but the computational wall-clock time is halved. In fact, putting 10 processors to work completes the job an order of magnitude faster, because this problem ...

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