General Comments about Homework 2

    All Problems
	Testing. Use corner cases and random numbers.

        C and Unix are not topics of this class. They are just
        tools we happen to use. Feel free to ask questions on the
        mailing list or to me. The tools should not be in the way
        of our progress.

        However, I will continue to mark poor indentation, and
        questionable C practices, such as compile time warnings. If
        you are not sure about something, or can't solve a C or Unix
        problem, ask. The rest of the class and I will be happy to
        assist before you turn in your homework.


    Problem 1
	Will discuss in class.

    Problem 2
	The answer is No. A lot of people got it right, but I am
	not totally convinced everybody understood exactly why. I'll
	explain in class.

	During the initial broadcast, a rank may get done early.
	Imagine we are doing a broadcast by sending values around a
	ring. When a rank receives the value from its left neighbor,
	it can send it on to its right neighbor and then proceed
	to the reduction. If one of the ranks comes later to
	the broadcast, it will delay ranks that are after it in
	the rings, but earlier ranks can already proceed to the
	reduction.

	Some ranks in the reduction, only need to submit their
	value and can go on. If an early rank from the broadcast
	happens to be one of those, it can go through the "barrier"
	very quickly, long before some other ranks have even reached
	the broadcast.

    Problem 3
	Some students assumed that MPI_Send() or MPI_Isend() followed
	by an MPI_Wait() would block. These sends can finish, and
	usually do with short messages, before the corresponding
	receive starts.

    Problem 4
        Problem statement is not very clear. Also, it should refer
        to the root node, not rank 0. My intent was that each rank
        submits a single number, and the reduction function makes
        sure the values are in rank order.

        The problem can only be solved if we make the INVALID
        assumption that MPI evaluates the expression from right
        to left. That is usually the case, but not mandated by
        the standard.

        I apologize for the confusion. Everybody who tried their
        hands on a solution got 20 points.

    Problem 5
        The specification for this program was not all that clear
        either. Most of you made reasonable assumptions for the
        unspecified parts.