Calculating PI, Part III

file: pi.py
1	import random
2	import mpi
3
4	inside = 0
5	nsamples = 120000/mpi.size
6
7	random.seed(mpi.rank)
8	for i in range(nsamples):
9	x = random.random();
10	y = random.random();
11	if (xx)+(yy)<1:
12	inside += 1
13
14	mypi = (4.0 * inside)/nsamples
15	if mpi.rank==0:
16	print "mypi =",mypi,"for rank",mpi.rank
17
18	pi = (1.0 / mpi.size) * mpi.allreduce(mypi, mpi.SUM)
19
20	if mpi.rank==0:
21	print "Computed value of pi on",mpi.size,"processors is",pi
22	print "Using ",mpi.size*nsamples,"samples."
> mpirun -np 4 pyMPI pi.py mypi = 3.1496 for rank 0 Computed value of pi on 4 processors is 3.13696666667 Using 120000 samples.

The above program puts a lot together and adds some new things.

The new things are

random.seed(mpi.rank): This initializes the random number generator, ensuring that it gives us a different sequence on each process.

mpi.allreduce(mpy, mpi.SUM): This provides us with a way of adding together the results obtained on all 4 processes. In essence, this program calculates pi by computing an average of the results obtained on each process individually.