joezuntz · February 5, 2015 15:41 · Feb 5, 2015
diff --git a/mpi4py_trivial.py b/mpi4py_trivial.py
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+#Suppose you have a collection of tasks, which in this example I'll assume is just running a function f.
+#If these tasks are completely separate and independent the most then you can parallelize them easily.
+#In this gist I'll show the simplest possible way to do this using mpi4py.
+#There are better ways to do this, in particular if the tasks vary significantly in time taken to run.
+
+import mpi4py.MPI
+
+def f(i):
+  "A fake task - in this case let just open a file and write a number to it"
+  #open file with name based on task number
+  f=open("%d.txt"%i,"w")
+  #write some info to it
+  f.write("%d * 10 = %d\n"%(i, 10*i))
+  #close the file
+  f.close()
+
+#A list of all the tasks to do.  In your case you will probably build this task list in a more complex way.
+#You don't even need to build it in advance for this approach to work
+task_list = range(100)
+
+#main program loop.  This is the unparallelized verion, for comparison
+for task in task_list:
+  f(task)
+
+#And now moving on the parallel version
+
+#mpi4py has the notion of a "communicator" - a collection of processors
+#all operating together, usually on the same program.  Each processor 
+#in the communicator is identified by a number, its rank,  We'll use that
+#number to split the tasks
+
+#find out which number processor this particular instance is,
+#and how many there are in total
+rank = mpi4py.MPI.COMM_WORLD.Get_rank()
+size = mpi4py.MPI.COMM_WORLD.Get_size()
+
+#parallelized version
+#the enumerate function gives us a number i in addition
+#to the task.  (In this specific case i is the same as task!  But that's
+#not true usually)
+for i,task in enumerate(task_list):
+  #This is how we split up the jobs.
+  #The % sign is a modulus, and the "continue" means
+  #"skip the rest of this bit and go to the next time
+  #through the loop"
+  # If we had e.g. 4 processors, this would mean
+  # that proc zero did tasks 0, 4, 8, 12, 16, ...
+  # and proc one did tasks 1, 5, 9, 13, 17, ...
+  # and do on.
+  if i%size!=rank: continue
+  print "Task number %d (%d) being done by processor %d of %d" % (i, task, rank, size)
+  f(task)
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