how-to-use-parallel-invoke-to-execute-parallel-operations.md

description	title	ms.date	helpviewer_keywords	ms.assetid
Learn more about: How to: Use parallel_invoke to Execute Parallel Operations	How to: Use parallel_invoke to Execute Parallel Operations	11/04/2016	parallel_invoke function, example calling multiple functions in parallel [Concurrency Runtime]	a6aea69b-d647-4b7e-bf3b-e6a6a9880072

How to: Use parallel_invoke to Execute Parallel Operations

This example shows how to use the concurrency::parallel_invoke algorithm to improve the performance of a program that performs multiple operations on a shared data source. Because no operations modify the source, they can be executed in parallel in a straightforward manner.

Example: Create, initialize, and perform operations on a variable

Consider the following code example that creates a variable of type MyDataType, calls a function to initialize that variable, and then performs multiple lengthy operations on that data.

[!code-cppconcrt-parallel-word-mining#1]

If the lengthy_operation1, lengthy_operation2, and lengthy_operation3 functions do not modify the MyDataType variable, these functions can be executed in parallel without additional modifications.

Example: Run previous example in parallel

The following example modifies the previous example to run in parallel. The parallel_invoke algorithm executes each task in parallel and returns after all tasks are finished.

[!code-cppconcrt-parallel-word-mining#2]

Example: Perform multiple operations on a downloaded file

The following example downloads The Iliad by Homer from gutenberg.org and performs multiple operations on that file. The example first performs these operations serially and then performs the same operations in parallel.

[!code-cppconcrt-parallel-word-mining#3]

This example produces the following sample output.

Downloading 'The Iliad'...

Running serial version... took 953 ms.
Running parallel version... took 656 ms.

The most common words that have five or more letters are:
    their (953)
    shall (444)
    which (431)
    great (398)
    Hector (349)
    Achilles (309)
    through (301)
    these (268)
    chief (259)
The longest sequence of words that have the same first letter is:
    through the tempest to the tented
The following palindromes appear in the text:
    spots stops
    speed deeps
    keels sleek

This example uses the parallel_invoke algorithm to call multiple functions that act on the same data source. You can use the parallel_invoke algorithm to call any set of functions in parallel, not only those that act on the same data.

Because the parallel_invoke algorithm calls each work function in parallel, its performance is bounded by the function that takes the longest time to finish (that is, if the runtime processes each function on a separate processor). If this example performs more tasks in parallel than the number of available processors, multiple tasks can run on each processor. In this case, performance is bounded by the group of tasks that takes the longest time to finish.

Because this example performs three tasks in parallel, you should not expect performance to scale on computers that have more than three processors. To improve performance more, you can break the longest-running tasks into smaller tasks and run those tasks in parallel.

You can use the parallel_invoke algorithm instead of the concurrency::task_group and concurrency::structured_task_group classes if you do not require support for cancellation. For an example that compares the usage of the parallel_invoke algorithm versus task groups, see How to: Use parallel_invoke to Write a Parallel Sort Routine.

Compiling the Code

To compile the code, copy it and then paste it in a Visual Studio project, or paste it in a file that is named parallel-word-mining.cpp and then run the following command in a Visual Studio Command Prompt window.

cl.exe /EHsc /MD /DUNICODE /D_AFXDLL parallel-word-mining.cpp

See also

Parallel Algorithms
parallel_invoke Function