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 replacement, then output them in various formats. Naturally, this means a lot of strings, and so doing memory tracing shows that I have a lot of strings, which is exactly what I'd expect. The structure of the program is a series of classes (each in their own thread, because I'm an idiot) that acts on an object that represents each file in memory. (Each object has an input queue that uses a lock on both ends. While this means I get to run this simple processing in parallel, it also means I have multiple 2MB objects sitting in memory.) Each object's structure is defined by a schema object.

My processing classes raise events when they've done their processing and pass a reference to the large object that holds all my strings to add it to the next processing object's queue. Replacing the event with a function call to add to the queue does not stop the leak. One of the output formats requires me to use an unmanaged object. Implementing Dispose() on the class does not stop the leak. I've replaced all the references to the schema object with an index name. No dice. I got no idea what's causing it, and no idea where to look. The memory trace doesn't help because all I see are a bunch of strings being created, and I don't see where the references are sticking in memory.

We're pretty much going to give up and roll back at this point, but I have a pathological need to know exactly how I messed this up. I know Stack Overflow can't exactly comb my code, but what strategies can you suggest for tracking this leak down? I'm probably going to do this in my own time, so any approach is viable.

Answer 1

One technique I would try is to systematically reduce the amount of code you need to demonstrate the problem without making the problem go away. This is informally known as "divide and conquer" and is a powerful debugging technique. Once you have a small example that demonstrates the same problem, it will be much easier for you to understand. Perhaps the memory problem will become clearer at that point.

Answer 2

There is only one person who can help you. That person's name is Tess Ferrandez. (hushed silence)

But seriously. read her blog (the first article is pretty pertinent). Seeing how she debugs this stuff will give you a lot of deep insight into knowing what's going on with your problem.