ThreadLocal<> and memory leak

Question

.Net 4. ThreadLocal<> implements IDisposable. But it seems that calling Dispose() doesn't actually release references to thread local objects being held.

This code reproduces the problem:

using System;
using System.Collections.Generic;
using System.Collections.Concurrent;
using System.Linq;
using System.Threading;

namespace ConsoleApplication2
{
    class Program
    {
        class ThreadLocalData
        {
            // Allocate object in LOH
            public int[] data = new int[10 * 1024 * 1024];
        };

        static void Main(string[] args)
        {
            // Stores references to all thread local object that have been created
            var threadLocalInstances = new List<ThreadLocalData>();
            ThreadLocal<ThreadLocalData> threadLocal = new ThreadLocal<ThreadLocalData>(() =>
            {
                var ret = new ThreadLocalData();
                lock (threadLocalInstances)
                    threadLocalInstances.Add(ret);
                return ret;
            });
            // Do some multithreaded stuff
            int sum = Enumerable.Range(0, 100).AsParallel().Select(
                i => threadLocal.Value.data.Sum() + i).Sum();
            Console.WriteLine("Sum: {0}", sum);
            Console.WriteLine("Thread local instances: {0}", threadLocalInstances.Count);

            // Do our best to release ThreadLocal<> object
            threadLocal.Dispose();
            threadLocal = null;

            Console.Write("Press R to release memory blocks manually or another key to proceed: ");
            if (char.ToUpper(Console.ReadKey().KeyChar) == 'R')
            {
                foreach (var i in threadLocalInstances)
                    i.data = null;
            }
            // Make sure we don't keep the references to LOH objects
            threadLocalInstances = null;
            Console.WriteLine();

            // Collect the garbage
            GC.Collect();
            GC.WaitForPendingFinalizers();
            GC.Collect();

            Console.WriteLine("Garbage collected. Open Task Manager to see memory consumption.");
            Console.Write("Press any key to exit.");
            Console.ReadKey();
        }
    }
}

Thread local data stores a reference to a large object. GC doesn't collect these large objects if references are not nulled manually. I used Task Manager to observe memory consumption. I also run memory profiler. I made a snapshot after garbage was collected. The profiler showed that leaked object is rooted by GCHandle and was allocated with here:

mscorlib!System.Threading.ThreadLocal<T>.GenericHolder<U,V,W>.get_Boxed()
mscorlib!System.Threading.ThreadLocal<T>.get_Value()
ConsoleApplication2!ConsoleApplication2.Program.<>c__DisplayClass3.<Main>b__2( int ) Program.cs

That seems to be a flaw in ThreadLocal<> design. The trick with storing all allocated objects for further cleanup is ugly. Any ideas on how to work around that?

Answer 1

The memory has probably been garbage collected but the CLR process hasn't let go of it yet. It tends to hold onto allocated memory for a bit in case it needs it later so it doesn't have to do an expensive memory allocation.

Answer 2

Running on .Net 4.5 DP, I don't see any difference between pressing R or not in your application. If there actually was a memory leak in 4.0, it seems it was fixed.

(4.5 is a in-place update, so I can't test 4.0 on the same computer, sorry.)