Question about Garbage collection C# .NET

Question

I am experiencing problem in my application with OutOfMemoryException. My application can search for words within texts. When I start a long running process search to search about 2000 different texts for about 2175 different words the application will terminate at about 50 % through with a OutOfMemoryException (after about 6 hours of processing)

I have been trying to find the memory leak. I have an object graph like: (--> are references)

a static global application object (controller) --> an algorithm starter object --> text mining starter object --> text mining algorithm object (this object performs the searching).

The text mining starter object will start the text mining algorithm object's run()-method in a separate thread.

To try to fix the issue I have edited the code so that the text mining starter object will split the texts to search into several groups and initialize one text mining algorithm object for each group of texts sequentially (so when one text mining algorithm object is finished a new will be created to search the next group of texts). Here I set the previous text mining algorithm object to null. But this does not solve the issue.

When I create a new text mining algorithm object I have to give it some parameters. These are taken from properties of the previous text mining algorithm object before I set that object to null. Will this prevent garbage collection of the text mining algorithm object?

Here is the code for the creation of new text mining algorithm objects by the text mining algorithm starter:

    private void RunSeveralAlgorithmObjects()
    {

        IEnumerable<ILexiconEntry> currentEntries = allLexiconEntries.GetGroup(intCurrentAlgorithmObject, intNumberOfAlgorithmObjectsToUse);

        algorithm.LexiconEntries = currentEntries;
        algorithm.Run();

        intCurrentAlgorithmObject++;

        for (int i = 0; i < intNumberOfAlgorithmObjectsToUse - 1; i++)
        {
            algorithm = CreateNewAlgorithmObject();
            AddAlgorithmListeners();
            algorithm.Run();
            intCurrentAlgorithmObject++;
        }

    }

    private TextMiningAlgorithm CreateNewAlgorithmObject()
    {
        TextMiningAlgorithm newAlg = new TextMiningAlgorithm();

        newAlg.SortedTermStruct = algorithm.SortedTermStruct;
        newAlg.PreprocessedSynonyms = algorithm.PreprocessedSynonyms;
        newAlg.DistanceMeasure = algorithm.DistanceMeasure;
        newAlg.HitComparerMethod = algorithm.HitComparerMethod;
        newAlg.LexiconEntries = allLexiconEntries.GetGroup(intCurrentAlgorithmObject, intNumberOfAlgorithmObjectsToUse);
        newAlg.MaxTermPercentageDeviation = algorithm.MaxTermPercentageDeviation;
        newAlg.MaxWordPercentageDeviation = algorithm.MaxWordPercentageDeviation;
        newAlg.MinWordsPercentageHit = algorithm.MinWordsPercentageHit;
        newAlg.NumberOfThreads = algorithm.NumberOfThreads;
        newAlg.PermutationType = algorithm.PermutationType;
        newAlg.RemoveStopWords = algorithm.RemoveStopWords;
        newAlg.RestrictPartialTextMatches = algorithm.RestrictPartialTextMatches;
        newAlg.Soundex = algorithm.Soundex;
        newAlg.Stemming = algorithm.Stemming;
        newAlg.StopWords = algorithm.StopWords;
        newAlg.Synonyms = algorithm.Synonyms;
        newAlg.Terms = algorithm.Terms;
        newAlg.UseSynonyms = algorithm.UseSynonyms;

        algorithm = null;

        return newAlg;
    }

Here is the start of the thread that is running the whole search process:

            // Run the algorithm in it's own thread
            Thread algorithmThread = new Thread(new ThreadStart
                (RunSeveralAlgorithmObjects));
            algorithmThread.Start();

Can something here prevent the previous text mining algorithm object from being garbage collected?

Answer 1

I recommend first identifying what exactly is leaking. Then postulate a cause (such as references in event handlers).

To identify what is leaking:

1. Enable native debugging for the project. Properties -> Debug -> check Enable unmanaged code debugging.
2. Run the program. Since the memory leak is probably gradual, you probably don't need to let it run the whole 6 hours; just let it run for a while and then Debug -> Break All.
3. Bring up the Immediate window. Debug -> Windows -> Immediate

4. Type one of the following into the immediate window, depending on whether you're running 32 or 64 bit, .NET 2.0/3.0/3.5 or .NET 4.0:

   .load C:\WINDOWS\Microsoft.NET\Framework\v2.0.50727\sos.dll for 32-bit .NET 2.0-3.5

   .load C:\WINDOWS\Microsoft.NET\Framework\v4.0.30319\sos.dll for 32-bit .NET 4.0

   .load C:\WINDOWS\Microsoft.NET\Framework64\v2.0.50727\sos.dll for 64-bit .NET 2.0-3.5

   .load C:\WINDOWS\Microsoft.NET\Framework64\v4.0.30319\sos.dll for 64-bit .NET 4.0

5. You can now run SoS commands in the Immediate window. I recommend checking the output of !dumpheap -stat, and if that doesn't pinpoint the problem, check !finalizequeue.

Notes:

• Running the program the first time after enabling native debugging may take a long time (minutes) if you have VS set up to load symbols.
• The debugger commands that I recommended both start with ! (exclamation point).

These instructions are courtesy of the incredible Tess from Microsoft, and Mario Hewardt, author of Advanced .NET Debugging.

Once you've identified the leak in terms of which object is leaking, then postulate a cause and implement a fix. Then you can do these steps again to determine for sure whether or not the fix worked.

Answer 2

1) As I said in a comment, if you use events in your code (the AddAlgorithmListeners makes me suspect this), subscribing to an event can create a "hidden" dependency between objects which is easily forgotten. This dependency can mean that an object is not freed, because someone is still listening to one of it's events. Make sure you unsubscribe from all events when you no longer need to listen to them.

---

2) Also, I'd like to point you to one (probably not-so-off-topic) issue with your code:

private void RunSeveralAlgorithmObjects()
{
    ...
    algorithm.LexiconEntries = currentEntries;
    // ^ when/where is algorithm initialized?

    for (...)
    {
        algorithm = CreateNewAlgorithmObject();
        ....
    }
}

Is algoritm already initialized when this method is invoked? Otherwise, setting algorithm.LexiconEntries wouldn't seem like a valid thing to do. This means your method is dependent on some external state, which to me looks like a potential place for bugs creeping in your program logic.

If I understand it correctly, this object contains some state describing the algorithm, and CreateNewAlgorithmObject derives a new state for algorithm from the current state. If this was my code, I would make algorithm an explicit parameter to all your functions, as a signal that the method depends on this object. It would then no longer be hidden "external" state upon which your functions depend.

P.S.: If you don't want to go down that route, the other thing you could consider to make your code more consistent is to turn CreateNewAlgorithmObject into a void method and re-assign algorithm directly inside that method.

Answer 3

Is AddAlgorithmListeners attaching event handlers to events exposed by the algorithm object ? Are the listening objects living longer than the algorithm object - in which case they can continue to keep the algorithm object from being collected.

If yes, try unsubscribing events before you let the object go out of scope.

for (int i = 0; i < intNumberOfAlgorithmObjectsToUse - 1; i++)
        {
            algorithm = CreateNewAlgorithmObject();
            AddAlgorithmListeners();
            algorithm.Run();
            RemoveAlgoritmListeners();    // See if this fixes your issue.
            intCurrentAlgorithmObject++;
        }