Question

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Possible memoryleak in ConcurrentBag?

Edit1:

The actual question is. Can you confirm this or is my sample wrong and I am missing somthing obvious?

I have thought that ConcurrentBag is simpy a replacement for an unorderd list. But I was wrong. ConcurrentBag does add itself to as ThreadLocal to the creating thread which does basically cause a memory leak.

   class Program
    {
        static void Main(string[] args)
        {
            var start = GC.GetTotalMemory(true);
            new Program().Start(args);
            Console.WriteLine("Diff: {0:N0} bytes", GC.GetTotalMemory(true) - start);
            GC.Collect();
            GC.WaitForPendingFinalizers();
            GC.Collect();
            Thread.Sleep(5000);
        }

        private void Start(string[] args)
        {
            for (int i = 0; i < 1000; i++)
            { 
                var bag = new ConcurrentBag<byte>();
                bag.Add(1);
                byte by;
                while (bag.TryTake(out by)) ;
            }
        }

I can make Diff 250 KB or 100 GB depending on how much data I add to the bags. The data nor the bags go away.

When I break into this with Windbg and I do a
!DumpHeap -type Concurrent

….

000007ff00046858        1           24 System.Threading.ThreadLocal`1+GenericHolder`3[[System.Collections.Concurrent.ConcurrentBag`1+ThreadLocalList[[System.Byte, mscorlib]], System],[System.Threading.ThreadLocal`1+C0[[System.Collections.Concurrent.ConcurrentBag`1+ThreadLocalList[[System.Byte, mscorlib]], System]], mscorlib],[System.Threading.ThreadLocal`1+C0[[System.Collections.Concurrent.ConcurrentBag`1+ThreadLocalList[[System.Byte, mscorlib]], System]], mscorlib],[System.Threading.ThreadLocal`1+C0[[System.Collections.Concurrent.ConcurrentBag`1+ThreadLocalList[[System.Byte, mscorlib]], System]], mscorlib]]
000007feed812648        2           64 System.Collections.Concurrent.ConcurrentStack`1[[System.Int32, mscorlib]]
000007feece41528        1          112 System.Collections.Concurrent.CDSCollectionETWBCLProvider
000007ff000469e0     1000        32000 System.Threading.ThreadLocal`1+Boxed[[System.Collections.Concurrent.ConcurrentBag`1+ThreadLocalList[[System.Byte, mscorlib]], System]]
000007feed815900     1000        32000 System.Collections.Concurrent.ConcurrentStack`1+Node[[System.Int32, mscorlib]]
000007ff00045530     1000        72000 System.Collections.Concurrent.ConcurrentBag`1+ThreadLocalList[[System.Byte, mscorlib]]

When I do create an empty ConcurrentBag to let some worker threads add data to it ConcurrentBag and its data will be there as long as the creating thread is still alive.

This way I got a several GB memory leak. I did “fix” this by using a List and locks. ConcurrentBag may be fast but it is useless as simple replacement for a List with the same object lifetime.

If I ever create a ConcurrentBag on the main thread I will keep it as long as the thread is alive. This is not something I would expect and it can cause major pain.

Answer 1

You are right that ConcurrentBag creates a ThreadLocal copy, in fact they are optimized for scenarios where the same thread is reading and writing the data to the bag: “… ConcurrentBag is a thread-safe bag implementation, optimized for scenarios where the same thread will be both producing and consuming data stored in the bag.”

On the otherhand, I do not see a strange behaviour here; the thread lives and the concurrent bag lives. When thread finishes GC will do it’s job.