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Designing Threaded Programs

Pthreads Overview: Designing Threaded Programs

Parallel Programming

On modern, multi-core machines, pthreads are ideally suited for parallel programming, and whatever applies to parallel programming in general, applies to parallel pthreads programs.

There are many considerations for designing parallel programs, such as:

Covering these topics is beyond the scope of this tutorial, however interested readers can obtain a quick overview in the Introduction to Parallel Computing tutorial.

In general though, in order for a program to take advantage of Pthreads, it must be able to be organized into discrete, independent tasks which can execute concurrently. For example, if routine1 and routine2 can be interchanged, interleaved and/or overlapped in real time, they are candidates for threading.

concurrent

Programs having the following characteristics may be well suited for pthreads:

Several common models for threaded programs exist:

Shared Memory Model

All threads have access to the same global, shared memory.

Threads also have their own private data.

Programmers are responsible for synchronizing access (protecting) globally shared data.

sharedMemoryModel

Thread-safeness

Thread-safeness: in a nutshell, refers to an application’s ability to execute multiple threads simultaneously without “clobbering” shared data or creating “race” conditions.

For example, suppose that your application creates several threads, each of which makes a call to the same library routine:

threadUnsafe

The implication to users of external library routines is that if you aren’t 100% certain the routine is thread-safe, then you take your chances with problems that could arise.

Recommendation: Be careful if your application uses libraries or other objects that don’t explicitly guarantee thread-safeness. When in doubt, assume that they are not thread-safe until proven otherwise. This can be done by “serializing” the calls to the uncertain routine, etc.

Thread Limits

Although the Pthreads API is an ANSI/IEEE standard, implementations can, and usually do, vary in ways not specified by the standard. Because of this, a program that runs fine on one platform, may fail or produce wrong results on another platform. For example, the maximum number of threads permitted, and the default thread stack size are two important limits to consider when designing your program.

Several thread limits are discussed in more detail later in this tutorial.