Resource Utilization

Whenever a system resource, such as a CPU or a particular disk, is occupied by a transaction or query, it is unavailable for processing other requests. Pending requests must wait for the resources to become available before they can complete. When a component is too busy to keep up with all its requests, the overused component becomes a bottleneck in the flow of activity. The higher the percentage of time that the resource is occupied, the longer each operation must wait for its turn.

You can use the following formula to estimate the service time for a request based on the overall utilization of the component that services the request. The expected service time includes the time that is spent both waiting for and using the resource in question. Think of service time as that portion of the response time accounted for by a single component within your computer, as the following formula shows:

S= P/(1-U)

S

is the expected service time.

P

is the processing time that the operation requires once it obtains the resource.

U

is the utilization for the resource (expressed as a decimal).

As Figure 2 shows, the service time for a single component increases dramatically as the utilization increases beyond 70 percent. For instance, if a transaction requires 1 second of processing by a given component, you can expect it to take 2 seconds on a component at 50 percent utilization and 5 seconds on a component at 80 percent utilization. When utilization for the resource reaches 90 percent, you can expect the transaction to take 10 seconds to make its way through that component.

Figure 2. Service Time for a Single Component as a Function of Resource Utilization

If the average response time for a typical transaction soars from 2 or 3 seconds to 10 seconds or more, users are certain to notice and complain.

When you consider resource utilization, also consider whether increasing the page size of a standard or temporary dbspace is beneficial in your environment. If you want a longer key length than is available for the default page size of a standard or temporary dbspace, you can increase the page size.