Sorting Algorithms: Merge Sort

Merge sort algorithm is a comparison-based sorting algorithm. In this sample, we use top-down implementation, which recursively splits list into two halves (called sublists) until size of list is 1. Then merge these two sublists and produce a sorted list. This sample could run in serial, or in parallel with Intel® Cilk™ Plus keywords cilk_spawn and cilk_sync. For more details about merge sort algorithm and top-down implementation, please refer to http://en.wikipedia.org/wiki/Merge_sort.

• System Requirements
• Hardware:
  • Any Intel processor with multiple cores
  For Microsoft* Windows*:
  • Microsoft* Visual Studio 2010* or 2012* standard edition or above
  • Intel® C++ Composer XE 2013 SP1 for Windows (visit the Release Notes for supported OSes)
  For Linux*:
  • GNU* GCC* 4.5 or newer
  • Intel® C++ Composer XE 2013 SP1 for Linux* (visit the Release Notes for supported OSes)

Code Change Highlights:

• cilk_spawn
• serial version:

  void merge_sort(int a[], int tmp_a[], int first, int last)
  {
      if (first < last) {
          int middle = (first + last + 1) / 2;
          merge_sort(a, tmp_a, first, middle - 1);
          merge_sort(a, tmp_a, middle,last);
          merge(a, tmp_a, first, middle, last);
      }
  }

  cilk_spawn version:

  void merge_sort(int a[], int tmp_a[], int first, int last)
  {
      if (first < last) {
          int middle = (first + last + 1) / 2;
          cilk_spawn merge_sort(a, tmp_a, first, middle - 1);
          merge_sort(a, tmp_a, middle,last);
          cilk_sync;
          merge(a, tmp_a, first, middle, last);
      }
  }

Performance Data:

Note: Modified Speedup shows performance speedup with respect to serial implementation.

Build Instructions:

• For Microsoft Visual Studio* 2010 and 2012 users:
• Open the solution .sln file
  [Optional] To collect performance numbers (will run example 5 times and take average time):

  • Project Properties -> C/C++ -> Preprocessor -> Preprocessor Definitions: add PERF_NUM
  Choose a configuration (for best performance, choose a release configuration):

  • Intel-debug and Intel-release: uses Intel® C++ compiler
  • VSC-debug and VSC-release: uses Visual C++ compiler (only linear/scalar will run)

• For Windows* Command Line users:
• Enable your particular compiler environment
  for Intel® C++ Compiler:

  • open the appropriate Intel C++ compiler command prompt
  • navigate to project folder
  • compile with Build.bat [perf_num]
  • perf_num: collect performance numbers (will run example 5 times and take average time)
  • to run: Build.bat run [help|0|1|2|3|4]
  for Visual C++ Compiler (only linear/scalar will run):

  • open the appropriate MicrosoftVisual Studio* 2010 or 2012 command prompt
  • navigate to project folder
  • to compile: Build.bat [perf_num]
    • perf_num: collect performance numbers (will run example 5 times and take average time)
  • to run: Build.bat run

• For Linux* or OS X* users:
• set the icc environment: source <icc-install-dir>/bin/compilervars.sh {ia32|intel64}
  navigate to project folder
  for Intel® C++ compiler:

  • to compile: make [icpc] [perf_num=1]
    • perf_num=1: collect performance numbers (will run example 5 times and take average time)
  • to run: make run [option=help|0|1|2|3|4]
  for gcc (only linear/scalar will run):

  • compile with make gcc [perf_num=1]
    • perf_num=1: collect performance numbers (will run example 5 times and take average time)
  • to run: make run