Whenever a system resource, such as a CPU or a particular
disk, is occupied by a transaction or query, the resource 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 after it obtains
the resource.
U
is the utilization for the resource (expressed as a decimal).
As Service
Time for a Single Component as a Function of Resource Utilization 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 1: 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.
Important: Monitor any
system resource that shows a utilization of over 70 percent or any
resource that exhibits symptoms of overuse as described in the following
sections.
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.
