Strategies For Tracking Down Memory Leaks When You've Done Everything Wrong

Question

My program, alas, has a memory leak somewhere, but I\'ll be damned if I know what it is.

Its job is to read in a bunch of ~2MB files, do some parsing and string repl

Answer 1

Be careful how you define "leak". "Uses more memory" or even "uses too much memory" is not the same as "memory leak". This is especially true in a garbage-collected environment. It may simply be that GC hasn't needed to collect the extra memory you're seeing used. Also be careful about the difference between virtual memory use and physical memory use.

Finally not all "memory leaks" are caused by "memory" sorts of issues. I was once told (not asked) to fix an urgent memory leak that was causing IIS to restart frequently. In fact, I did profiling and found I was using a lot of strings through the StringBuilder class. I implemented an object pool (from an MSDN article) for the StringBuilders, and memory usage went down substantially.

IIS still restarted just as frequently. This was because there was no memory leak. Instead, there was unmanaged code that claimed to be thread-safe but was not. Using it in a web service (multiple threads) caused it to write all over the C Runtime Library heap. Since nobody was looking for unmanaged exceptions, nobody saw this until I happened to do some profiling with AQtime from Automated QA. It happens to have an events window, that happened to display the cries of pain from the C Runtime Library.

Placed locks around the calls to the unmanaged code, and the "memory leak" went away.