Friday, 26 June 2015

A simple class wrapper for parallelism in C++

Concurrency can be extremely complicated, and causes problems that will haunt you in your dreams. The classical libraries in C/C++ don't protect you from this horror in any way, and you will have to figure it out yourself. Parallelism is supposed to be a lot easier, but C/C++ does not have standard libraries like---for instance---Pythons parallelism modules. The boost libraries, however, provide an extensive interface for concurrency and parallelism.
If you don't want to use boost, don't panic. There are other options. The POSIX pthread library provides a down-to-the-metal concurrency model. It took me a while to find out what it is all about, and I haven't successfully applied all it's possibilities. What I have managed to apply, is a so-called "worker-pool" model. This is one of the easier concurrency applications (it falls under the parallelism category) of the pthread library, but can be quite useful. Here I will demonstrate a "wrapper" that C++ classes can inherit.

Suppose that you have a class Fun, that needs to do some computations. We declare Fun in---say---Fun.hpp:
Fun inherits the worker pool functionality from the class "WorkerPoolWrapper" (imported from WorkerPoolWrapper.hpp). The class WorkerPoolWrapper has a "pure virtual" member executeJob(void* ). You, the user, must specify what you want to compute in your own definition of the executeJob method. Besides implementing executeJob, you must also initiate the worker pool, and somewhere before Fun's destuctor returns, the threads must be "joined", i.e., all worker threads must finish their execution. In this case, I use the constructor and destructor of Fun to accomplish these things:
The methods initWorkerPool and waitForWorkerPoolToExit are inherited from WorkerPoolWrapper. Lets use Fun to compute the number of primes pi(n) below a number n. We overload operator() as follows:
Notice that this implementation of pi(n) is not the most efficient one. It checks for every integer i between 0 and n whether i is prime or not. This prime test is performed by executeJob. In the background, WorkerPoolWrapper has a list of jobs that have to be executed. Jobs can be added to this job list using addNewJob(void* ). Once executed, the result of a job must somehow be stored in the job again. Above, the number pi is the sum of the array ns, which makes sense when we look at the implementation of executeJob:
Hence, executeJob transforms the number i pointed to by job into 0 if i is composit, or 1 if i is prime, such that the sum of the i's equals pi(n). Before we gathered the results in Fun::operator(), we called syncWorkerThreads(). This method lets the program halt until every job in the job list has been executed.
Using the functionoid Fun now works as follows:
The class WorkerPoolWrapper is declared here:
And the members are defined here: The credits for combining pthread with C++ classes go to Jeremy Friesner.

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