TABLE OF CONTENTS Parallel programming has been around for decades, though before the advent of multi-core processors, it was an esoteric discipline. Numerous programmers have tripped over the common stumbling blocks by now. By recognizing these problems you can avoid stumbling. Furthermore, it is important to understand the common problems before designing a parallel program, because many of the problems arise from the overall decomposition of the program, and cannot be easily patched later. This chapter surveys some of these common problems, their symptoms, and ways to circumvent them.
It may seem that if a little threading is good, then a lot must be better. In fact, having too many threads can seriously degrade program performance. The impact comes in two ways. First, partitioning a fixed amount of work among too many threads gives each thread too little work, so that the overhead of starting and terminating threads swamps the useful work. Second, having too many concurrent software threads incurs overhead from having to share fixed hardware resources.
When there are more software threads than hardware threads, the operating system typically resorts to round robin scheduling. The scheduler gives each software thread a short turn, called a time slice, to run on one of the hardware threads. When a software thread's time slice runs out, the scheduler preemptively suspends the thread in order to run another software thread on the same hardware thread. The software thread freezes in time until it gets another time slice.
Time slicing ensures...
