"You're assuming that performance -- or, more precisely, CPU usage -- is important; in many cases, reliability (and being able to track down bugs after a crash) are far more important than CPU usage."
I work on a real-time embedded medical device, where both performance and reliability are vital. We've got constrained resources, and the system must be extremely responsive.
Our logging scheme is pretty cool. It's written so that two computers can log to a single hard drive, and each logging statement must define a log level. So, for example, if I'm writing GUI code, I can log to log_level_gui_info, log_level_gui_debug, log_level_gui_error, or any of a number of more specific log levels.
The idea is
So, we have two ways to filter out extraneous logging that we don't care about (one actually keeps the logging from happening, and one just filters it out during analysis), and we can log as freely as we like as long as we're smart about which levels we're using.
As much faith as we all have in our own code, nothing's as frustrating as trying to analyze a log that came in from the field where there's just no information about what went wrong.
FORTUNE'S FUN FACTS TO KNOW AND TELL: A firefly is not a fly, but a beetle.