Performance objectives

After defining the workload that your system will have to process, you can choose performance criteria and set performance objectives based on those criteria.

The overall performance criteria of computer systems are response time and throughput.

Response time is the elapsed time between when a request is submitted and when the response from that request is returned. Examples include:

• The amount of time a database query takes
• The amount of time it takes to echo characters to the terminal
• The amount of time it takes to access a Web page

Throughput is a measure of the amount of work that can be accomplished over some unit of time. Examples include:

• Database transactions per minute
• Kilobytes of a file transferred per second
• Kilobytes of a file read or written per second
• Web server hits per minute

The relationship between these metrics is complex. Sometimes you can have higher throughput at the cost of response time or better response time at the cost of throughput. In other situations, a single change can improve both. Acceptable performance is based on reasonable throughput combined with reasonable response time.

In planning for or tuning any system, make sure that you have clear objectives for both response time and throughput when processing the specified workload. Otherwise, you risk spending analysis time and resource dollars improving an aspect of system performance that is of secondary importance.