If I/O wait is having a noticeable impact on performance, it is clearly an issue, however not all wait for I/O is cause for concern. Given that CPUs are much faster than other resources, the CPU must wait for these slower resources some percentage of the time.
When tuning a system, it is desirable to have some idle time available, but is not a requirement. It is possible to use 100 percent of the CPU time on a two-CPU system with two processes. This does not mean that the system was performing poorly; rather it means the system is running as fast as possible. In performance tuning, you try to push the bottleneck to the fastest resource, the CPU. Under ideal circumstances, CPU activity is divided into 70 percent user time, 20 percent system time, 0 percent wait for I/O time, and 10 percent idle time.
The ratio of user time to system time should be approximately 3 to 1. However, this ratio varies widely when user time is below 20 percent of total CPU usage, as system time tends to consume a much larger portion. In some cases, system time is greater than user time due to poor allocation of resources. However, as you increase user time through performance tuning, system time should level off at one-third or less of user time. This can be determined by looking at your CPU resources from monitoring tool.
Indications that you are out of CPU resources might be masking an issue with another resource. In most cases it is a disk issue. If you see a CPU bottleneck, first determine that you do not have a runaway process, and then make sure that your other resources are working efficiently.
