Getting Started with MPI

1
Getting Started with MPI

• Self Test with solution

2
Self Test

• Is a blocking send necessarily also synchronous?
• Yes
• No

3
Self Test

• Yes
• Incorrect - a synchronous send may be either
  blocking or nonblocking. Remember that the
  communication mode ("synchronous") implies a
  condition for the send to be complete (in this
  case, that the message is in the process of being
  received at its destination). A blocking
  synchronous send does not return until this
  condition is satisfied. A nonblocking synchronous
  send returns immediately, and you would then test
  later to check whether the completion condition
  has actually been satisfied.
• No
• That's correct!

4
Self Test

• Consider the following fragment of MPI
  pseudo-code
• ...
• x fun(y)
• MPI_SOME_SEND(the value of x to some other
  processor)
• x fun(z)
• ...
• where MPI_SOME_SEND is a generic send routine. In
  this case, it would be best to use
• A blocking send
• A nonblocking send

5
Self Test

• A blocking send
• Correct! This insures that the send is complete,
  and thus that it is safe to overwrite x on the
  sending processor. With a non-blocking send,
  there is no guarantee that the send will be
  complete before the subsequent update of x
  occurs. The value of x actually sent is thus
  unpredictable.
• A nonblocking send
• Incorrect. Remember that a non-blocking send
  returns immediately, before the send has
  necessarily completed. In this case, the contents
  of x might be changed by the following statement
  before the send actually occurs. To insure that
  the correct value of x is sent in this case, use
  a blocking send. An alternative would be to use a
  non-blocking send and add code to test for the
  completion of the send before actually updating x.

6
Self Test

• Which of the following is true for all send
  routines?
• It is always safe to overwrite the sent
  variable(s) on the sending processor after the
  send returns.
• Completion implies that the message has been
  received at its destination.
• It is always safe to overwrite the sent
  variable(s) on the sending processor after the
  send is complete.
• All of the above.
• None of the above.

7
Self Test

• It is always safe to overwrite the sent
  variable(s) on the sending processor after the
  send is complete.
• Correct

8
Matching Question

• Point-to-point communication
• Collective communication
• Communication mode
• Blocking send
• Synchronous send
• Broadcast
• Scatter
• Gather

• A send routine that does not return until it is
  complete
• Communication involving one or more groups of
  processes
• send routine that is not complete until receipt
  of the message at its destination has been
  acknowledged
• An operation in which one process sends the same
  data to several others
• Communication involving a single pair of
  processes
• An operation in which one process distributes
  different elements of a local array to several
  others
• An operation in which one process collects data
  from several others and assembles them in a local
  array
• Specification of the method of operation and
  completion criteria for a communication routine

9
Matching Question

• Correct Answers
• E
• B
• H
• A
• C
• D
• F
• G

10
Course Problem
11
Course Problem

• The initial problem implements a parallel search
  of an extremely large (several thousand elements)
  integer array. The program finds all occurrences
  of a certain integer, called the target, and
  writes all the array indices where the target was
  found to an output file. In addition, the program
  reads both the target value and all the array
  elements from an input file.
• Before writing a parallel version of a program,
  first write a serial version (that is, a version
  that runs on one processor). That is the task for
  this chapter. You can use C/C and should
  confirm that the program works by using a test
  input array.

12
Solution

• If you wish to confirm that these results are
  correct, you can run the serial code shown above
  using the input file "b.data".
• The results obtained from running this code are
  in the file "found.data" which contains the
  following
• 62
• 183
• 271
• 291
• 296