Kevin Schroeder has a new post to his site looking at the cost of logging in web applications. It's not so much about the theory behind what to log and when to log as it is the actual performance hit various kinds of logging take on your app.
So, I was having a discussion with a person I respect about logging and they noted that often logging poses a prohibitive cost from a performance perspective. This seemed a little odd to me and so I decided to run a quick series of benchmarks on my own system.
His tests ran through the types of logging available to PHP developers (like "debug", "notice" or "info") and wrote out the same message to a file ten thousand times each. He just did a simple microtime-based benchmark and graphed out the overall time it took. The differences between the tops of the bars is the time it took for that specific level of logging. He dropped it down to a more realistic level and notes that it only took "1/1000ths of a second per request."