3.9 Parallel Programming Tips

• Use the C shell command unlimit to bring the process stack limits to the maximum allowed on the system. In the event processes exceed the stack limits, a segmentation fault occurs.

  To set the system default stack size limit, use the dflssize keyword. To set the system maximum stack size limit, use the maxssiz keyword. For more information, refer to the Digital UNIX Programming Support Tools Manual.

• Performance on an SMP system may be limited by memory bandwidth. To test for a memory bandwidth limitation, run multiple copies of a program as a single processor application on an SMP system. It should take the same time for 2 independent copies to run on 2 processors as it takes for 1 copy to run on 1 processor. If 2 copies take longer to run, your system probably has a memory bandwidth bottleneck.

• Profile the subroutines in your parallel program with the Digital UNIX atom tool hiprof . Use the atom pixie tool to profile your program on the line level. For complete information, see the atom , hiprof , and pixie manpages.

• Use the Digital UNIX timer routines setitimer and getitimer to view the real elapsed time of code sections that run in less than 1 second.

• When you use PCF directives and you do not specify the KAP -concurrentize switch, you must add the Digital UNIX version-specific linker switches to the kf77 command line. Review Section 3.2 to view the linker switches for your version of Digital UNIX.

• To monitor CPU utilization per thread, use the following command:

  ps mOpcpu
  

• Experiment with each of the following switches to determine if they improve the run-time of your program.
  -fuse
  -fuselevel=1
  -ipa
  ipa_from_files=<file>,<file>
  -ipa_optimize=2

  However, the above switches may increase the amount of time and memory KAP needs to process your source files.