Why is the system so slow? is probably second on any system administrator's things-I-least-want-to-hear list (right after Why did the system crash again?!). Like system reliability, system performance is a topic that comes up only when there is a problem. Unfortunately, no one is likely to compliment or thank you for getting the most out of the system's resources.
System performance-related complaints can take on a variety of forms, ranging from sluggish interactive response time, to a job that takes too long to complete or is unable to run at all because of insufficient resources.
In general, system performance depends on how efficiently a system's resources are applied to the current demand for them by various jobs in the system. The most important system resources from a performance perspective are CPU, memory, and disk and network I/O, although sometimes other device I/O can also be relevant. How well a system performs at any given moment is the result of both the total demand for the various system resources and how well the competition among processes for them is being managed. Accordingly, performance problems can arise from a number of causes, including both a lack of needed resources and ineffective control over them. Addressing a performance problem involves identifying what these resources are and figuring out how to manage them more effectively.
As with most of life, performance tuning is much harder when you have ...