Threads don't switch instantaneously—after one thread blocks or yields to another, the kernel must save the settings before running another thread. The time to save this state and restore another is known as thread context-switch time. This context-switch time between threads is small, but important.
Figure 1. Thread context switching.In some cases, two or more threads may switch back and forth without actually accomplishing much. This is akin to two overly polite people each offering to let the other pass through a narrow door first—neither of them gets to where they're going on time (two aggressive people encounter a similar problem). This type of problem is exactly what the SAT can quickly and easily highlight. By showing the context-switch operations in conjunction with thread state transitions, you can quickly see why otherwise fast systems seem to “crawl.”