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Application Responsiveness

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Joe works at Microsoft on concurrency programming models in the Common Language Runtime (CLR) team. He blogs regularly at

Thanks to innovation in both hardware graphics processors and client-side development frameworks, GUIs for Windows applications have become more and more visually stunning over time. But throughout the evolution of such frameworks, one problem hasn't gone away—poor responsiveness. Studies show that positive user experiences are linked to application responsiveness. Conversely, frustrating experiences are often caused by poor responsiveness. More often than not, the lack of responsiveness is due to a series of subtle (and sometimes accidental) design choices made during development. In this article, I examine the root of responsiveness problems, and then suggest best practices for improving applications.

Responsiveness: What Is It?

A responsive UI is snappy, responds to input promptly, and doesn't leave users hanging. You can think of response time as the delay between the time at which an event is generated and the time at which some acknowledgment is made. This principle applies as much to UI events as, say, server-side events. While I focus on GUIs here, many of the lessons learned are transferable to other event-based environments.

Throughput is typically defined as the amount of work performed in a given period of time, and is directly related to responsiveness. The maximum (best) responsiveness applications can reasonably exhibit is bounded by the maximum throughput for UI event processing. The more work that must be done to respond to any individual event, the higher the response latencies are apt to be. The lower the overall event-processing throughput, the worse the perceived responsiveness of an application. Responding to events in this context does not necessarily mean completely processing them, but rather simply giving the sender some acknowledgment of receipt. In cases where full processing takes time, it might also mean giving users an estimated completion time and ongoing progress using, say, progress bars.

But averages aren't sufficient. You must also consider worst-case scenarios. An application's degree of responsiveness must be predictable to maintain the perception of a responsive application. In other words, the maximum processing time for any event that might occur could severely impact the user's happiness with your UI. This is true even for events whose processing time is, on average, very short. Simply put, variable response times are problematic and can be almost as frustrating as applications that are all around sluggish. Understanding this is instrumental to building responsiveness into programs.

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