Numerical Computing with Modern Fortran

Richard J. Hanson and Tim Hopkins

Chapter 15: OpenMP in Fortran

Source Code:

These examples illustrate a number of issues that are intended as helpers when getting started using OpenMP. The Fortran compiler must be configured to process OpenMP directives, i.e. those source lines starting with '!$OMP'.

Example1 uses Example1.f90
Two threads are used. There is a danger that thread 1 uses x before thread 0 defines it to have the value 5. This has not happened here, but it could.
Example2 uses Example2.f90, set_precision.f90
Four threads are used. The question is: Which thread processes a do loop index with the upper limit of value n = 101?
Example3 uses Example3.f90, set_precision.f90
Thread 0 asks for input. Each of the alternate threads wait for a (random) fraction of a second before pausing on thread 0 to complete the input. When input is completed all threads print a message. This uses a VOLATILE routine argument to support the pausing step.
Example4 uses Example4.f90, set_precision.f90
A sum of weighted vector dot or inner products is computed. The vectors are random and the generator is entered in a critical section so that the results are repeatable. The outer sum of the weighted dot products uses an ATOMIC designation so that a race condition will not occur.
Example5 uses Example5.f90, set_precision.f90
A parallel do loop records the loop index associated with each thread number + 1. These values here match, but that is not assured.
Example6 uses Example6.f90, set_precision.f90
A subprogram needs work space. This is allocated in the main program and passed to the threads. Each thread uses its index to construct a pointer that uses a specified part of the global work space for its local work space.
dotp uses dotp.f90, set_precision.f90
A loop for computing a dot product has a race condition. The accumulation step is invalid. This is a purposeful error and it is fixed in the example dotpAtomic.

dotpAtomic use dotpAtomic.f90, set_precision.f90

By using an ATOMIC designation before the accumulation step, the race condition of example dotp is eliminated.

Sample Output from Example1 using 2 threads

Race condition: Thread:     1, x =    5
After barrier:  Thread:     1, x =    5
After barrier:  Thread:     0, x =    5

Sample Output from Example2 using 4 threads

Thread   3 processes index   101

Sample Output from Example3 using 16 threads

Enter an integer -- 1
Thread  10 waited  0.345 secs before (Volatile) FLAG was tested
Thread   9 waited  0.833 secs before (Volatile) FLAG was tested
Thread  14 waited  0.701 secs before (Volatile) FLAG was tested
Thread  15 waited  0.735 secs before (Volatile) FLAG was tested
Thread  11 waited  0.871 secs before (Volatile) FLAG was tested
Thread   4 waited  0.963 secs before (Volatile) FLAG was tested
Thread   6 waited  0.335 secs before (Volatile) FLAG was tested
Thread   2 waited  0.353 secs before (Volatile) FLAG was tested
Thread  13 waited  0.888 secs before (Volatile) FLAG was tested
Thread   1 waited  0.025 secs before (Volatile) FLAG was tested
Thread   8 waited  0.796 secs before (Volatile) FLAG was tested
Thread  12 waited  0.090 secs before (Volatile) FLAG was tested
Thread   7 waited  0.915 secs before (Volatile) FLAG was tested
Thread   0 did no waiting after input.
Thread   5 waited  0.838 secs before (Volatile) FLAG was tested
Thread   3 waited  0.667 secs before (Volatile) FLAG was tested

Sample Output from Example4 using 16 threads

Weighted dot product =  0.1296754E+04

Sample Output from Example5 using 16 threads

Natural Numbers for Each Thread =
    1  1.00
    2  2.00
    3  3.00
    4  4.00
    5  5.00
    6  6.00
    7  7.00
    8  8.00
    9  9.00
   10 10.00
   11 11.00
   12 12.00
   13 13.00
   14 14.00
   15 15.00
   16 16.00

Sample Output from Example6 using 16 threads

If no "Error" messages, work assignment succeeded.

Sample Output from dotp using 16 threads

Dot product - n*6 =  -0.2060E+05 Thread number =   0
 Race condition, stop!

Sample Output from dotpAtomic using 16 threads

No race condition with ATOMIC construct!

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