Auto-parallelization Threshold Control and Diagnostics

Threshold Control

The -par_threshold[n] option sets a threshold for the auto-parallelization of loops based on the probability of profitable execution of the loop in parallel. The value of n can be from 0 to 100. This option is used for loops whose computation work volume cannot be determined at compile time. The threshold is usually relevant when the loop trip count is unknown at compile time.

The -par_threshold[n] option has the following functionality:

• -par_threshold100 is executed by default, so loops get auto-parallelized only if profitable parallel execution is almost certain.
• If you specify -par_threshold with designating a value for n, the compiler uses the default value n=100.
• The intermediate 1 to 99 values represent the percentage probability for profitable speed-up. For example, n=50 directs the compiler to parallelize only if there is a 50% probability of the code speeding up if executed in parallel.

The compiler applies a heuristic that tries to balance the overhead of creating multiple threads versus the amount of work available to be shared amongst the threads.

Diagnostics

The -par_report{0|1|2|3} option controls the auto-parallelizer's diagnostic levels 0, 1, 2, or 3 as follows:

• -par_report0 = no diagnostic information is displayed.
• -par_report1 = indicates loops successfully auto-parallelized (default). Issues a "LOOP AUTO-PARALLELIZED" message for parallel loops.
• -par_report2 = indicates successfully auto-parallelized loops as well as unsuccessful loops.
• -par_report3 = same as 2 plus additional information about any proven or assumed dependencies inhibiting auto-parallelization (reasons for not parallelizing).

Example of Parallelization Diagnostics Report

This example shows output generated by -par_report3:

prompt>icpc -c -parallel -par_report3 prog.cpp

Sample Output

where the program prog.cpp is as follows:

Sample prog.c

Troubleshooting Tips

• Use -par_threshold0 to see if the compiler assumed there was not enough computational work
• Use -par_report3 to view diagnostics
• Use -ipo[value] to eliminate assumed side-effects done to function calls