When a user is waiting for the application to respond and there are available computing resources, then parallelism is being limited and serialization has raised its ugly head. Serialization is death to performance maximization!

While related, serialization and queuing are distinct, and you don't want to confuse the two. Queuing occurs when a process waits. But a process can wait because of serialization issues, not just because all available resources are busy. Also, there are times when all efforts to parallelize a process have failed, resulting in idle resources while the process is running. Let's take a closer look at each of these situations.

Figure 3-1 can help differentiate serialization and queuing. In Figure 3-1, the outlined circles represent a CPU. In capacity planning, we call these servers, because they service transactions. The solid circles represent a process. The horizontal outlined rectangle represents the single queue, where all processes enter the system.