Troubleshooting a memory leak

Got called into a memory leak troubleshooting. The application was leaking memory at 1mb per second. In memory profiler, most of the growing memory are used by byte[] and RuntimeMethodHandle (growing at around a million per minute). Initially I thought it is a disposing problem, but the memory occupied by disposable objects does not increase over time.

Looking at the RuntimeMethodHandle instances, I found that it is closely associated with DynamicResolver, whose instance count grows in a much smaller but steady rate. This leads me to kissdodog’s post Windbg + .Net .NET Memory Profiler 排查内存泄露 but kissdodog only located the problem but didn’t give a cause (some coworker got the blame and fixed it).

Then I want to see if there’s any GDI leak and added handle columns to task manager, surprisingly there’s a handle leak growing at 1/2 per second, but then there is no memory map in the code. Under Process Explorer’s handle view,  the leaking handles are revealed as user tokens, which is even more puzzling because the code doesn’t deal with windows logins. But at least this gives me some clear indicator to divide and conquer.

By commenting out portions of code (more like a human binary search) and watching Task Manager religiously, I found the issue is caused by creating XslCompiledTransform in a loop, something KB316775 cited as a design limitation that the dynamic assemblies can’t be unloaded. Obviously this isn’t fixed even in .Net 4.8. The KB was no longer published by Microsoft and I had to read it off a mirror.

Once the problem is located the fix is easy, making those XslCompiledTransform objects singleton solved the issue.


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