User Guide

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Limit the Number of Threads Used by Parallel Frameworks

Intel® Advisor
tools are designed to collect data and analyze
serial
programs.
Before
you use the
Intel Advisor
to examine a partially parallel program, modify your program so it runs as a serial program with a single thread within each parallel site.

Run Your Program as a Serial Program

To run the current version of your program as a serial program, you need to limit the number of threads to 1. To run your program with a single thread:
  • With
    Intel® oneAPI Threading Building Blocks (oneTBB)
    , in the main thread create a
    tbb::task_scheduler_init init(1);
    object for the lifetime of the program and run the executable again. For example:
    int main() { tbb::task_scheduler_init init(1); // ...rest of program... return 0; }
    The effect of
    task_scheduler_init
    applies separately to each user-created thread. So if the program creates threads elsewhere, you need to create a
    tbb::task_scheduler_init init(1);
    for that thread's lifetime as well. Use of certain
    oneTBB
    features can prevent the program from running serially. For more information, see the
    oneTBB
    documentation.
  • With OpenMP*, do one of the following:
    • Set the OpenMP* environment variable
      OMP_NUM_THREADS
      to 1 before you run the program.
    • Omit the compiler option that enables recognition of OpenMP pragmas and directives. On Windows* OS, omit
      /Qopenmp
      , and on Linux* OS omit
      -openmp
      .
For more information, see your compiler documentation.
If you cannot remove the parallelism, you should add annotations to mark the parallel code regions and learn how parallel code will impact
Intel Advisor
tool reports.

Product and Performance Information

1

Performance varies by use, configuration and other factors. Learn more at www.Intel.com/PerformanceIndex.