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Progress bars

© July 2005 Tony Lawrence

We all know that software progress bars have to be taken with a large helping of salt. It's not at all unusual for the bar to have no discernible relationship to reality, and this is particularly apt to be true for operating system installations and software startups. If any percentage indication is shown, it probably cannot be correlated to required time. The percentage is likely to hang in the same place for frustratingly long periods of time or suddenly jump large increments for no apparent reason. Part of this is because for complex tasks, it can be very difficult to estimate completion time, and part of it is surely just programmer laziness.

Mac OS X displays a "Starting MAc OS X" graphic with a lovely blue progress bar that crawls along quite convincingly. In fact, though, it's totally fake. A program called "WaitingForLoginWindow" draws that display, but it isn't really measuring anything - it's just a bar to keep you happy while OS X starts. Well, that's not entirely true. Actually, it's drawing a bar that is calculated to last approximately the same length of time it ran on the last startup. What happens is that the Login Window process kills off this bar display when it is ready to let you log in. The bar stores how long it ran in /var/db/loginwindow.boottime and then goes away. Next time it runs, it uses that stored value to decide how quickly to move the bar. Cute, isn't it?

Well, apparently there is reason and precedence for this deception: it makes users happy even if it is meaningless.. The Placebo Mini-Pattern explains why this is useful and generally a good idea.


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