Intel® Advisor User Guide

ID 766448
Date 12/16/2022
Public

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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*