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. The default value is 75. 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 versions and functionality:

• Default: -par_threshold is not specified in the command line, which is the same as when -par_threshold0 is specified. The loops get auto-parallelized regardless of computation work volume, that is, parallelize always.
• -par_threshold100 - loops get auto-parallelized only if profitable parallel execution is almost certain.
• The intermediate 1 to 99 values represent the percentage probability for profitable speed-up. For example, n=50 would mean: parallelize only if there is a 50% probability of the code speeding up if executed in parallel.
• The default value of n is n=75 (or -par_threshold75). When -par_threshold is used on the command line without a number, the default value passed is 75.

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

The example below shows output generated by -par_report3:

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

Sample Ouput

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 to eliminate assumed side-effects done to function calls