C# .NET Garbage Collection not functioning?

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I am working on a relatively large solution in Visual Studio 2010. It has various projects, one of them being an XNA Game-project, and another one being an ASP.NET MVC 2-project.

With both projects I am facing the same issue: After starting them in debug mode, memory usage keeps rising. They start at 40 and 100MB memory usage respectively, but both climb to 1.5GB relatively quickly (10 and 30 minutes respectively). After that it would sometimes drop back down to close to the initial usage, and other times it would just throw OutOfMemoryExceptions.

Of course this would indicate severe memory leaks, so that is where I initially tried to spot the problem. After searching for leaks unsuccesfully, I tried calling GC.Collect() regularly (about once per 10 seconds). After introducing this "hack", memory usage stayed at 45 and 120MB respectively for 24 hours (until I stopped testing).

.NET's garbage collection is supposed to be "very good", but I can't help suspecting that it just doesn't do its job. I have used CLR Profiler in an attempt to troubleshoot the issue, and it showed that the XNA project seemed to have saved a lot of byte arrays I was indeed using, but to which the references should already be deleted, and therefore collected by the garbage collector.

Again, when I call GC.Collect() regularly, the memory usage issues seem to have gone. Does anyone know what could be causing this high memory usage? Is it possibly related to running in Debug mode?

garbage-collection .net c#
6 Answers

The GC doesn't take into account the unmanaged heap. If you are creating lots of objects that are merely wrappers in C# to larger unmanaged memory then your memory is being devoured but the GC can't make rational decisions based on this as it only see the managed heap.

You end up in a situation where the GC collector doesn't think you are short of memory because most of the things on your gen 1 heap are 8 byte references where in actual fact they are like icebergs at sea. Most of the memory is below!

You can make use of these GC calls:

System::GC::AddMemoryPressure(sizeOfField);
System::GC::RemoveMemoryPressure(sizeOfField);

These methods allow the garbage collector to see the unmanaged memory (if you provide it the right figures)

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