How do Tasks in the Task Parallel Library affect A

Question

Before using the Task Parallel Library, I have often used CorrelationManager.ActivityId to keep track of tracing/error reporting with multiple threads.

ActivityId is stored in Thread Local Storage, so each thread get's its own copy. The idea is that when you fire up a thread (activity), you assign a new ActivityId. The ActivityId will be written to the logs with any other trace information, making it possible to single out the trace information for a single 'Activity'. This is really useful with WCF as the ActivityId can be carried over to the service component.

Here is an example of what I'm talking about:

static void Main(string[] args)
{
    ThreadPool.QueueUserWorkItem(new WaitCallback((o) =>
    {
        DoWork();
    }));
}

static void DoWork()
{
    try
    {
        Trace.CorrelationManager.ActivityId = Guid.NewGuid();
        //The functions below contain tracing which logs the ActivityID.
        CallFunction1();
        CallFunction2();
        CallFunction3();
    }
    catch (Exception ex)
    {
        Trace.Write(Trace.CorrelationManager.ActivityId + " " + ex.ToString());
    }
}

Now, with the TPL, my understanding is that multiple Tasks share Threads. Does this mean that ActivityId is prone to being reinitialized mid-task (by another task)? Is there a new mechanism to deal with activity tracing?

Answer 1

I ran some experiments and it turns out the assumption in my question is incorrect - multiple tasks created with the TPL do not run on the same thread at the same time.

ThreadLocalStorage is safe to use with TPL in .NET 4.0, since a thread can only be used by one task at a time.

The assumption that tasks can share threads concurrently was based on an interview I heard about c# 5.0 on DotNetRocks (sorry, I can't remember which show it was) - so my question may (or may not) become relevant soon.

My experiment starts a number of tasks, and records how many tasks ran, how long they took, and how many threads were consumed. The code is below if anyone would like to repeat it.

class Program
{
    static void Main(string[] args)
    {
        int totalThreads = 100;
        TaskCreationOptions taskCreationOpt = TaskCreationOptions.None;
        Task task = null;
        Stopwatch stopwatch = new Stopwatch();
        stopwatch.Start();
        Task[] allTasks = new Task[totalThreads];
        for (int i = 0; i < totalThreads; i++)
        {
            task = Task.Factory.StartNew(() =>
           {
               DoLongRunningWork();
           }, taskCreationOpt);

            allTasks[i] = task;
        }

        Task.WaitAll(allTasks);
        stopwatch.Stop();

        Console.WriteLine(String.Format("Completed {0} tasks in {1} milliseconds", totalThreads, stopwatch.ElapsedMilliseconds));
        Console.WriteLine(String.Format("Used {0} threads", threadIds.Count));
        Console.ReadKey();
    }


    private static List<int> threadIds = new List<int>();
    private static object locker = new object();
    private static void DoLongRunningWork()
    {
        lock (locker)
        {
            //Keep a record of the managed thread used.
            if (!threadIds.Contains(Thread.CurrentThread.ManagedThreadId))
                threadIds.Add(Thread.CurrentThread.ManagedThreadId);
        }
        Guid g1 = Guid.NewGuid();
        Trace.CorrelationManager.ActivityId = g1;
        Thread.Sleep(3000);
        Guid g2 = Trace.CorrelationManager.ActivityId;
        Debug.Assert(g1.Equals(g2));
    }
}

The output (of course this will depend on the machine) was:

Completed 100 tasks in 23097 milliseconds
Used 23 threads

Changing taskCreationOpt to TaskCreationOptions.LongRunning gave different results:

Completed 100 tasks in 3458 milliseconds 
Used 100 threads

Answer 2

Please forgive my posting this as an answer as it is not really answer to your question, however, it is related to your question since it deals with CorrelationManager behavior and threads/tasks/etc. I have been looking at using the CorrelationManager's LogicalOperationStack (and StartLogicalOperation/StopLogicalOperation methods) to provide additional context in multithreading scenarios.

I took your example and modified it slightly to add the ability to perform work in parallel using Parallel.For. Also, I use StartLogicalOperation/StopLogicalOperation to bracket (internally) DoLongRunningWork. Conceptually, DoLongRunningWork does something like this each time it is executed:

DoLongRunningWork
  StartLogicalOperation
  Thread.Sleep(3000)
  StopLogicalOperation

I have found that if I add these logical operations to your code (more or less as is), all of the logical operatins remain in sync (always the expected number of operations on stack and the values of the operations on the stack are always as expected).

In some of my own testing I found that this was not always the case. The logical operation stack was getting "corrupted". The best explanation I could come up with is that the "merging" back of the CallContext information into the "parent" thread context when the "child" thread exits was causing the "old" child thread context information (logical operation) to be "inherited" by another sibling child thread.

The problem might also be related to the fact that Parallel.For apparently uses the main thread (at least in the example code, as written) as one of the "worker threads" (or whatever they should be called in the parallel domain). Whenever DoLongRunningWork is executed, a new logical operation is started (at the beginning) and stopped (at the end) (that is, pushed onto the LogicalOperationStack and popped back off of it). If the main thread already has a logical operation in effect and if DoLongRunningWork executes ON THE MAIN THREAD, then a new logical operation is started so the main thread's LogicalOperationStack now has TWO operations. Any subsequent executions of DoLongRunningWork (as long as this "iteration" of DoLongRunningWork is executing on the main thread) will (apparently) inherit the main thread's LogicalOperationStack (which now has two operations on it, rather than just the one expected operation).

It took me a long time to figure out why the behavior of the LogicalOperationStack was different in my example than in my modified version of your example. Finally I saw that in my code I had bracketed the entire program in a logical operation, whereas in my modified version of your test program I did not. The implication is that in my test program, each time my "work" was performed (analogous to DoLongRunningWork), there was already a logical operation in effect. In my modified version of your test program, I had not bracketed the entire program in a logical operation.

So, when I modified your test program to bracket the entire program in a logical operation AND if I am using Parallel.For, I ran into exactly the same problem.

Using the conceptual model above, this will run successfully:

Parallel.For
  DoLongRunningWork
    StartLogicalOperation
    Sleep(3000)
    StopLogicalOperation

While this will eventually assert due to an apparently out of sync LogicalOperationStack:

StartLogicalOperation
Parallel.For
  DoLongRunningWork
    StartLogicalOperation
    Sleep(3000)
    StopLogicalOperation
StopLogicalOperation

Here is my sample program. It is similar to yours in that it has a DoLongRunningWork method that manipulates the ActivityId as well as the LogicalOperationStack. I also have two flavors of kicking of DoLongRunningWork. One flavor uses Tasks one uses Parallel.For. Each flavor can also be executed such that the whole parallelized operation is enclosed in a logical operation or not. So, there are a total of 4 ways to execute the parallel operation. To try each one, simply uncomment the desired "Use..." method, recompile, and run. UseTasks, UseTasks(true), and UseParallelFor should all run to completion. UseParallelFor(true) will assert at some point because the LogicalOperationStack does not have the expected number of entries.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Diagnostics;
using System.Threading;
using System.Threading.Tasks;

namespace CorrelationManagerParallelTest
{
  class Program 
  {     
    static void Main(string[] args)     
    { 
      //UseParallelFor(true) will assert because LogicalOperationStack will not have expected
      //number of entries, all others will run to completion.

      UseTasks(); //Equivalent to original test program with only the parallelized
                      //operation bracketed in logical operation.
      ////UseTasks(true); //Bracket entire UseTasks method in logical operation
      ////UseParallelFor();  //Equivalent to original test program, but use Parallel.For
                             //rather than Tasks.  Bracket only the parallelized
                             //operation in logical operation.
      ////UseParallelFor(true); //Bracket entire UseParallelFor method in logical operation
    }       

    private static List<int> threadIds = new List<int>();     
    private static object locker = new object();     

    private static int mainThreadId = Thread.CurrentThread.ManagedThreadId;

    private static int mainThreadUsedInDelegate = 0;

    // baseCount is the expected number of entries in the LogicalOperationStack
    // at the time that DoLongRunningWork starts.  If the entire operation is bracketed
    // externally by Start/StopLogicalOperation, then baseCount will be 1.  Otherwise,
    // it will be 0.
    private static void DoLongRunningWork(int baseCount)     
    {
      lock (locker)
      {
        //Keep a record of the managed thread used.             
        if (!threadIds.Contains(Thread.CurrentThread.ManagedThreadId))
          threadIds.Add(Thread.CurrentThread.ManagedThreadId);

        if (Thread.CurrentThread.ManagedThreadId == mainThreadId)
        {
          mainThreadUsedInDelegate++;
        }
      }         

      Guid lo1 = Guid.NewGuid();
      Trace.CorrelationManager.StartLogicalOperation(lo1);

      Guid g1 = Guid.NewGuid();         
      Trace.CorrelationManager.ActivityId = g1;

      Thread.Sleep(3000);         

      Guid g2 = Trace.CorrelationManager.ActivityId;
      Debug.Assert(g1.Equals(g2));

      //This assert, LogicalOperation.Count, will eventually fail if there is a logical operation
      //in effect when the Parallel.For operation was started.
      Debug.Assert(Trace.CorrelationManager.LogicalOperationStack.Count == baseCount + 1, string.Format("MainThread = {0}, Thread = {1}, Count = {2}, ExpectedCount = {3}", mainThreadId, Thread.CurrentThread.ManagedThreadId, Trace.CorrelationManager.LogicalOperationStack.Count, baseCount + 1));
      Debug.Assert(Trace.CorrelationManager.LogicalOperationStack.Peek().Equals(lo1), string.Format("MainThread = {0}, Thread = {1}, Count = {2}, ExpectedCount = {3}", mainThreadId, Thread.CurrentThread.ManagedThreadId, Trace.CorrelationManager.LogicalOperationStack.Peek(), lo1));

      Trace.CorrelationManager.StopLogicalOperation();
    } 

    private static void UseTasks(bool encloseInLogicalOperation = false)
    {
      int totalThreads = 100;
      TaskCreationOptions taskCreationOpt = TaskCreationOptions.None;
      Task task = null;
      Stopwatch stopwatch = new Stopwatch();
      stopwatch.Start();

      if (encloseInLogicalOperation)
      {
        Trace.CorrelationManager.StartLogicalOperation();
      }

      Task[] allTasks = new Task[totalThreads];
      for (int i = 0; i < totalThreads; i++)
      {
        task = Task.Factory.StartNew(() =>
        {
          DoLongRunningWork(encloseInLogicalOperation ? 1 : 0);
        }, taskCreationOpt);
        allTasks[i] = task;
      }
      Task.WaitAll(allTasks);

      if (encloseInLogicalOperation)
      {
        Trace.CorrelationManager.StopLogicalOperation();
      }

      stopwatch.Stop();
      Console.WriteLine(String.Format("Completed {0} tasks in {1} milliseconds", totalThreads, stopwatch.ElapsedMilliseconds));
      Console.WriteLine(String.Format("Used {0} threads", threadIds.Count));
      Console.WriteLine(String.Format("Main thread used in delegate {0} times", mainThreadUsedInDelegate));

      Console.ReadKey();
    }

    private static void UseParallelFor(bool encloseInLogicalOperation = false)
    {
      int totalThreads = 100;
      Stopwatch stopwatch = new Stopwatch();
      stopwatch.Start();

      if (encloseInLogicalOperation)
      {
        Trace.CorrelationManager.StartLogicalOperation();
      }

      Parallel.For(0, totalThreads, i =>
      {
        DoLongRunningWork(encloseInLogicalOperation ? 1 : 0);
      });

      if (encloseInLogicalOperation)
      {
        Trace.CorrelationManager.StopLogicalOperation();
      }

      stopwatch.Stop();
      Console.WriteLine(String.Format("Completed {0} tasks in {1} milliseconds", totalThreads, stopwatch.ElapsedMilliseconds));
      Console.WriteLine(String.Format("Used {0} threads", threadIds.Count));
      Console.WriteLine(String.Format("Main thread used in delegate {0} times", mainThreadUsedInDelegate));

      Console.ReadKey();
    }

  } 
}

This whole issue of if LogicalOperationStack can be used with Parallel.For (and/or other threading/Task constructs) or how it can be used probably merits its own question. Maybe I will post a question. In the meantime, I wonder if you have any thoughts on this (or, I wonder if you had considered using LogicalOperationStack since ActivityId appears to be safe).

[EDIT]

See my answer to this question for more information about using LogicalOperationStack and/or CallContext.LogicalSetData with some of the various Thread/ThreadPool/Task/Parallel contstructs.

See also my question here on SO about LogicalOperationStack and Parallel extensions: Is CorrelationManager.LogicalOperationStack compatible with Parallel.For, Tasks, Threads, etc

Finally, see also my question here on Microsoft's Parallel Extensions forum: http://social.msdn.microsoft.com/Forums/en-US/parallelextensions/thread/7c5c3051-133b-4814-9db0-fc0039b4f9d9

In my testing it looks like Trace.CorrelationManager.LogicalOperationStack can become corrupted when using Parallel.For or Parallel.Invoke IF you start a logical operation in the main thread and then start/stop logical operations in the delegate. In my tests (see either of the two links above) the LogicalOperationStack should always have exactly 2 entries when DoLongRunningWork is executing (if I start a logical operation in the main thread before kicking of DoLongRunningWork using various techniques). So, by "corrupted" I mean that the LogicalOperationStack will eventually have many more than 2 entries.

From what I can tell, this is probably because Parallel.For and Parallel.Invoke use the main thread as one of the "worker" threads to perform the DoLongRunningWork action.

Using a stack stored in CallContext.LogicalSetData to mimic the behavior of the LogicalOperationStack (similar to log4net's LogicalThreadContext.Stacks which is stored via CallContext.SetData) yields even worse results. If I am using such a stack to maintain context, it becomes corrupted (i.e. does not have the expected number of entries) in almost all of the scenarios where I have a "logical operation" in the main thread and a logical operation in each iteration/execution of the DoLongRunningWork delegate.