Memory leak?

Question

Hi everyone,
I created some C# code to change rename variable names in my large C# file (file size is near 50 M). I used regular expressions to perform this task. Resulted code looks like this one: 

           //Replace each variable with new name
            foreach (string varname in varNames)
            {
                string newName = GenerateVariableName();
                code = Regex.Replace(code, string.Format(@"\b{0}\b", varname), newName);
                GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
                GC.WaitForPendingFinalizers();
                GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
            }
            GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
            GC.WaitForPendingFinalizers();
            GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);

Variable code contains input C# code as string, it contains up to 50000000 unicode characters, so it should need 2 times more bytes (100 M) to be stored in memory. I performing garbage collection regularly during this operation, that is why I suppose that program should need up to 100 M of memory when this method is finished.
However, program memory usage grows up to 1,5 G when this code is finished, and memory usage never decreased back to 100 M or even 500 M, even when I write something like this:
            code = null;
            GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);            
            GC.WaitForPendingFinalizers();
            GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
So even when I remove resulted generated code, memory never becomes released.
What is possible cause of such problem?

Answer 1

I'd say use a profiler to see what/who is keeping the reference on the data , if you can't afford one this article explains how to do it yourself.

Answer 2

 It is really good article and I was able to decrease memory leak a lot (its  cause was Regex static methods calls caching).
However, memory leak was not removed fully. I see strange things now.
1. When I enter !dumpheap -stat I see:
790fd8c4 8309 160335872 System.String
Well, it looks like the cause is a lot of not released strings.
2. I make !dumpheap -mt 790fd8c4
18011000 790fd8c4 53036360
3. At last, I make following !gcroot
!gcroot 18011000
Note: Roots found on stacks may be false positives. Run "!help gcroot" for
more info.
Error during command: Warning. Extension is using a callback which Visual Studio does not implement.
Scan Thread 3236 OSTHread ca4
Scan Thread 3816 OSTHread ee8
Scan Thread 1812 OSTHread 714
Scan Thread 3888 OSTHread f30
As I understand, such result means that there are not any references to target string... however, it is never removed when I make GC.Collect() calls...
What is possible cause?
Does string.Format method have any caching? The cause of error is supposed to be caching strings by some types like Regex, string.Format, ect. I cleared Regex cache by setting CacheSize to 0, but some strings are still not removed.

Answer 3

Do you define the "code" variable as global or static? if so, define it as local variable.

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Answer 4

Hello Vitally

If GC is not effective for you, play a bit with System.Diagnostics.Process.GetCurrentProcess().MaxWorkingSet and System.Diagnostics.Process.GetCurrentProcess().MinWorkingSet.
Changing this properties will decrease imediatelly memory consumption.

Hope this helps :)

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