.net - Garbage Collection does not reduce current memory usage - in release mode. Why?

Question

I built a quick program that needed to loop through an enormous log file (a couple of million records) and find various bits and pieces from inside. Because the volume of data was so huge, I have been curious to watch my Windows Task Manager performance tab and see how much CPU and memory is being used.

After the program successfully gets my results, the CPU usage goes right down, obviously. But after watching my memory usage slowly go up to several gigabytes during execution, it stays the same.

I have tried to call GC.Collect() at the end of my function, tried setting things to null, tried to run the program in Release mode (I heard that GC.Collect() might not work as I want it to in debug mode).

The memory usage goes right down if I close the program, but I can't figure out why I can't clean my app up during the apps lifetime. At the end of the day this is a throwaway app, but I'm just curious to know what I'm missing i.e. what is holding on to all this memory.

Thoughts?

Answer 1

Calling GC.Collect() is only a request to collect garbage, it's not an order, so the CLR may choose to ignore the request if it sees fit.

For example, if there's a lot of garbage that would cause a noticable delay whilst collecting, but there's still free memory to service subsequent allocations, then the GC may opt to ignore your request.