Performance Measures

• Response time
• Responsiveness
• Latency
• Throughput
• Load
• Load sensitivity
• Capacity
• Scalability

In this terminology, performance is either throughput or response time, whichever matters more to you. It can sometimes be difficult to talk about performance when a technique improves throughput but decreases response time. From a user’s perspective responsiveness may be more important than response time, so improving responsiveness at a cost of response time or throughput will increase performance.

The problem is that design decisions don’t affect all of these performance factors equally.

Response time

Amount of time it takes for the system to process a request from the outside. This may be a UI action or a server API call.

Responsiveness

How quickly a system acknowledges a request as opposed to processing it. This is important in many systems because users may become frustrated if a system has low responsiveness, even if its response time is good.

If your system waits during the whole request, then your responsiveness and response time are the same. For example, providing a progress bar during a file copy improves the responsiveness of your user interface, even though it doesn’t improve response time.

Latency

Minimum time required to get any form of response, even if the work to be done is nonexistent.

This is the reason why you should minimize remote calls. If I ask a program to do nothing, but to tell me when it’s done doing nothing, then I should get an almost instantaneous response if the program runs on my local computer. However, if the program runs on a remote computer, I may get a few seconds just because of the time take for the request and resposne to make their way across the wire.

Troughput

How much stuff you can do in a given amount of time. If you’re timing the copying of a file, throughput might be measured in bytes per second. For enterprise applications a typical measure is transactions per second (tps).

Generally, you should aim for maximal throughput with acceptable latency.

Load

How much stress a system is under, which might be measured in how many users are currently connected to it. The load is usually in context for some other measurement, such as a response time. Thus, you may say that the response time for some request is 0.5 seconds with 10 users and 2 seconds with 20 users.

Load sensitivity

Is an expression of how the response time varies with the load.

Capacity

Indication of maximum effective throughput or load. This might be an absolute maximum or a point at which performance dips below an acceptable threshold.

Scalability

Measure of how adding resources affects performance. A scalable system is one that allows you to add hardware and get a commensurate performance improvement, such as doubling how many servers you have to double your throughput.

Vertical scalability or scaling up, means adding more power to a single server, such as more memory. Horizontally scalability or scaling out means adding more servers.