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This chapter is from the book

Example: Parallel Pattern Searching

Now is the time to put Windows processes to the test. This example, grepMP, creates processes to search for patterns in files, one process per search file. The simple pattern search program is modeled after the UNIX grep utility, although the technique would apply to any program that uses standard output. The search program should be regarded as a black box and is simply an executable program to be controlled by a parent process; however, the project and executable (grep.exe) are in the Examples file.

The command line to the program is of the form

grepMP pattern F1 F2 ... FN

The program, Program 6-1, performs the following processing:

  • Each input file, F1 to FN, is searched using a separate process running the same executable. The program creates a command line of the form grep pattern FK.
  • The temporary file handle, specified to be inheritable, is assigned to the hStdOutput field in the new process's start-up information structure.
  • Using WaitForMultipleObjects, the program waits for all search processes to complete.
  • As soon as all searches are complete, the results (temporary files) are displayed in order, one at a time. A process to execute the cat utility (Program 2-3) outputs the temporary file.
  • WaitForMultipleObjects is limited to MAXIMUM_WAIT_OBJECTS (64) handles, so the program calls it multiple times.
  • The program uses the grep process exit code to determine whether a specific process detected the pattern.

Figure 6-3 shows the processing performed by Program 6-1, and Run 6-1 shows program execution and timing results.

Figure 6-3

Figure 6-3 File Searching Using Multiple Processes

Run 6-1

Run 6-1 grepMP Parallel Searching

Run 6-1 shows grepMP execution for large and small files, and the run contrasts sequential grep execution with parallel grepMP execution to perform the same task. The test computer has four processors; a single or dual processor computer will give different timing results. Notes after the run explain the test operation and results.

Run 6-1 uses files and obtains results as follows:

  • The small file test searches two Examples files, Presidents.txt and Monarchs.txt, which contain names of U.S. presidents and English monarchs, along with their dates of birth, death, and term in office. The "i" at the right end of each line is a visual cue and has no other meaning. The same is true of the "x" at the end of the randfile-generated files.
  • The large file test searches four randfile-generated files, each with 10 million 64-byte records. The search is for a specific record number (1234562), and each file has a different random key (the first 8 bytes).
  • grepMP is more than four times faster than four sequential grep executions (Real Time is 15 seconds compared to 77 seconds), so the multiple processes gain even more performance than expected, despite the process creation overhead.
  • timep is Program 6-2, the next example. Notice, however, that the grepMP system time is zero, as the time applies to grepMP itself, not the grep processes that it creates.
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