Question

I experience strange memory leak in computation expensive content-based image retrieval (CBIR) .NET application

The concept is that there is service class with thread loop which captures images from some source and then passes them to image tagging thread for annotation.

Image tags are queried from repository by the service class at specified time intervals and stored in its in-memory cache (Dictionary) to avoid frequent db hits.

The classes in the project are:

class Tag
{
    public Guid Id { get; set; }        // tag id
    public string Name { get; set; }    // tag name: e.g. 'sky','forest','road',...
    public byte[] Jpeg { get; set; }    // tag jpeg image patch sample
}

class IRepository
{
    public IEnumerable<Tag> FindAll();
}

class Service
{        
    private IDictionary<Guid, Tag> Cache { get; set; }  // to avoid frequent db reads
    // image capture background worker (ICBW)
    // image annotation background worker (IABW)
}

class Image
{
    public byte[] Jpeg { get; set; }
    public IEnumerable<Tag> Tags { get; set; }
}

ICBW worker captures jpeg image from some image source and passes it to IABW worker for annotation. IABW worker first tries to update Cache if time has come and then annotates the image by some algorithm creating Image object and attaching Tags to it then storing it to annotation repository.

Service cache update snippet in IABW worker is:

IEnumerable<Tag> tags = repository.FindAll();
Cache.Clear();
tags.ForEach(t => Cache.Add(t.Id, t));

IABW is called many times a second and is pretty processor extensive.

While running it for days I found memory increase in task manager. Using Perfmon to watch for Process/Private Bytes and .NET Memory/Bytes in all heaps I found them both increasing over the time.

Experimenting with the application I found that Cache update is the problem. If it is not updated there is no problem with the mem increase. But if the Cache update is as frequent as once in 1-5 minutes application gets ouf of mem pretty fast.

What might be the reason of that mem leak? Image objects are created quite often containing references to Tag objects in Cache. I presume when the Cache dictionary is created those references somehow are not garbage collected in the future.

Does it need to explicitly null managed byte[] objects to avoid memory leak e.g. by implementing Tag, Image as IDisposable?

Edit: 4 aug 2001, addition of the buggy code snippet causing quick mem leak.

static void Main(string[] args)
{
    while (!Console.KeyAvailable)
    {
        IEnumerable<byte[]> data = CreateEnumeration(100);
        PinEntries(data);
        Thread.Sleep(900);
        Console.Write(String.Format("gc mem: {0}\r", GC.GetTotalMemory(true)));
    }
}

static IEnumerable<byte[]> CreateEnumeration(int size)
{
    Random random = new Random();
    IList<byte[]> data = new List<byte[]>();
    for (int i = 0; i < size; i++)
    {
        byte[] vector = new byte[12345];
        random.NextBytes(vector);
        data.Add(vector);
    }
    return data;
}

static void PinEntries(IEnumerable<byte[]> data)
{
    var handles = data.Select(d => GCHandle.Alloc(d, GCHandleType.Pinned));
    var ptrs = handles.Select(h => h.AddrOfPinnedObject());
    IntPtr[] dataPtrs = ptrs.ToArray();
    Thread.Sleep(100); // unmanaged function call taking byte** data
    handles.ToList().ForEach(h => h.Free());
}

Answer 1

No, you don’t need to set anything to null or dispose of anything if it’s just memory as you’ve shown.

I suggest you get hold of a good profiler to work out where the leak is. Do you have anything non-memory-related that you might be failing to dispose of, e.g. loading a GDI+ image to get the bytes?