Random number generation with OpenMP

In this parallel simulation using OpenMP, a coin is thrown many times and the number of tails is counted. The probability should be exactly 1/2.

This code is a backbone implementation of a parallelized loop using a thread-safe (reentrant) random number generator (drand48_r), uniformly distributed over the [0,1) interval, with OpenMP.

This backbone code (random_omp.c) is well-documented with comments and instructions, and can be used to perform other parallel simulations that are implemented over a loop and use random numbers.


The execution time is used to define the performance, measured in Mega trials per second. In an 8 processor, 16 core computer, running 4 threads and 2 millon tosses, the performance on average is about 250 megatrials per second and takes about 8 milliseconds.

$ ./random_omp 4 2000000
using OpenMP with 4/16 threads
thread: 0 seed: 1706223504
thread: 1 seed: 2092013418
thread: 3 seed: 569057117
thread: 2 seed: 509177014

number of tosses: 2000000
number of tails: 1000164

probability: 0.500082

execution time: 0.00802503 s
performance: 249.22 Mtrials/sec

With 2 billion tosses, the performance is on average 400 megatrials per second, and takes about 5 seconds. The following plot, using 16 threads, shows the well-known behavior of performance with increasing problem size.

The next plot shows the performance as a function of number of threads for 2 billion tosses, in an 8 processor, 16 core computer.

The speedup for this problem is ∼6.6, which gives an efficieny of 0.82. The parallel fraction of the problem is therefore about 97%.

Compilation and execution

Compile the code with,

$ gcc -Wall -o random_omp random_omp.c -fopenmp

and execute with the command line,

$ ./random_omp num_threads num_tries

where num_threads is the number of threads and num_tries is the number of coin tosses. To suppress the output to stderr,

$ ./random_omp num_threads num_tries 2>/dev/null


This software has been developed and tested using the GNU GCC compiler with OpenMP support. To determine if your GCC compiler has OpenMP support, execute the following command,

$ echo | cpp -fopenmp -dM | grep -i open

If the response is non-empty, for example,

#define _OPENMP 201511

the compiler has built-in OpenMP support. The _OPENMP preprocessor macro defines the year and month of the implemented OpenMP version, in decimal value format yyyymm.

Terms of Use and License

Before buying, please read our Terms of Use and License.

List of Files

Included in this software package are the following files:

  • random_omp.c (C code)
  • README (documentation)
  • terms.md (terms of use)
  • license.md (license)