Times have changed. Multi-core CPUs have become the norm and multi-threading has been replaced by asynchronous programming. You think you know everything about async/await... until something goes wrong. While debugging synchronous code can be straightforward, investigating an asynchronous deadlock or race condition proves to be surprisingly tricky. In this talk, follow us through real-life examples and investigations to cover the main asynchronous code patterns that can go wrong. You will tumble on deadlock and understand the reasons behind ThreadPool thread starvation. In addition to WinDbg magic to follow async/await chains, Visual Studio goodies won't be forgotten to quickly analyze hundreds of call stacks or tasks status.

1. Organized by Donating to
R&Devents@criteo.com
criteo.com
Medium.com/criteo-labs
@CriteoEng #NYANconf
Debugging asynchronous scenarios
by Christophe Nasarre
Kevin Gosse
NYAN conference

2. First case: a service refuses to stop
• Still in running state in Windows Services panel

3. In production → take a memory snaphot
procdump -ma <pid>

4. Parallel Stack in Visual Studio
• Yes: VS is able to load a memory dump
• This is a nice way to visually see what is going on
→ We are waiting for ClusterClient.Dispose() to end

5. In production → take a memory snaphot
procdump -ma <pid>
Which foreground thread is still running?
what ClusterClient.Dispose() is waiting for?
Look at the Code Luke!

6. ActionBlock internals
Task ProcessMessage(TInput message)
{
...
}

7. In production → take a memory snaphot
procdump -ma <pid>
Which foreground thread is still running?
what ClusterClient.Dispose() is waiting for?
Look at the Code Luke!
Look for _agent state

8. Task ContinueWith(Action<Task> nextAction,…)
{
Task task = new ContinuationTaskFromTask<TResult>
(this, nextAction,…);
base.ContinueWithCore(task, …);
return task;
}
ContinueWith internals (1|3)

9. internal void ContinueWithCore(Task continuationTask, …)
{
TaskContinuation taskContinuation =
new StandardTaskContinuation(continuationTask, …);
…
if (!continuationTask.IsCompleted)
{
// add task to m_continuationObject
if (!AddTaskContinuation(taskContinuation, …))
{
taskContinuation.Run(this, …);
}
}
}
ContinueWith internals (2|3)

10. ContinueWith internals (3|3)

11. async Task<long> AAA(CancellationToken token)
{
Stopwatch tick = new Stopwatch();
tick.Start();
await BBB(token);
tick.Stop();
return tick.ElapsedMillisecond;
}
async/await Internals (1|2)

12. async/await Internals (2|2)
async Task AAA()
{
await BBB();
...
}
async Task BBB()
{
...
}
AsyncMethodBuilderCore+MoveNextRunner
Action
TaskSchedulerAwaitTaskContinuation**
Task (returned by BBB)
AAA State machine
Task (returned by AAA)

13. In production → take a memory snaphot
procdump -ma <pid>
Which foreground thread is still running?
what ClusterClient.Dispose() is waiting for?
Look at the Code Luke!
Look for _agent state
→ Exception broke the responses ActionBlock

14. BONUS: more continuations
• A few other continuation scenarios that you may encounter
✓ Task.Delay
✓ Task.WhenAny
✓ Special cases

15. Why a List<object> as continuation?
Task DoStuffAsync()
{
var task = SendAsync();
task.ContinueWith(t => LogStuff(t));
return task;
}
// user code
await DoStuffAsync();
DoSomethingSynchronously()
Task
m_continuationObject
nullStandardTaskContinuation
List<object>
StandardTaskContinuation
*TaskContinuation

16. Why a empty List<object> as continuation?
async Task DoStuffAsync()
{
var T1 = Task.Run(…);
var T2 = Task.Run(…);
await Task.WhenAny(T1, T2);
… // T2 ends first
}
T1
m_continuationObject
null
T2
m_continuationObject
null
CompleteOnInvokePromise
CompleteOnInvokePromise
empty List<object>object

17. Investigation 1 - key takeaways
1. Thread call stacks do not give the full picture
• Even Visual Studio parallel stacks is not enough
2. Require clear understanding of Task internals
• m_continuationObject and state machines
3. Start from the blocked task and follow the reverse references chain
• sosex!refs is your friend

18. Symptoms: 0% CPU and thread count raises

19. In production → take a memory snaphot
procdump -ma <pid>
look at call stacks in Visual Studio

20. In production → take a memory snaphot
procdump -ma <pid>
look at call stacks in Visual Studio
→ what are those tasks (we are waiting for) doing?

21. In production → take a memory snaphot
procdump -ma <pid>
look at call stacks in Visual Studio
→ what are those tasks (we are waiting for) doing?
look at tasks in WinDBG
→ no deadlock but everything is blocked…

22. ThreadPool internals
static void ProcessRequest()
{
var task = CallbackAsync();
task.Wait();
}
R
C

23. ThreadPool internals
ThreadPool
R
R

24. ThreadPool internals
ThreadPool
R R
R
R
R
R
C C

25. ThreadPool internals
ThreadPool
R R
R
R
C
C
R
R

26. ThreadPool internals
ThreadPool
R R
R
R
C
C
R R
R
R
R
R
C C
R
R
R
R

27. DEMO
Simple ThreadPool starvation code

28. Thread 1 Thread 2
ThreadPool internals
Global queue Local queue Local queue
Task 1
Task 2
Task 5
Task 4
Task 6Task 3

29. ThreadPool internals
Global queue Local queue
C
Thread 1
Local queue
C
Thread 2
Local queue
C
Thread 3
R
R
R
R

30. In production → take a memory snaphot
procdump -ma <pid>
look at call stacks in Visual Studio
→ what are those tasks (we are waiting for) doing?
look at tasks in WinDBG
→ no deadlock but everything is blocked…
→ ThreadPool is starved

31. Investigation 2 - key takeaways
1. Waiting synchronously on a Task is dangerous
2. ThreadPool scheduling is unfair
3. 0% CPU + increasing thread count = sign of ThreadPool starvation

32. Conclusion
• Understand the underlying data structures
• Think of causality chains instead of threads call stack
• Visual Studio is your friend
• Parallel Stacks to get the big picture
• WinDBG is your true ally
• Use and abuse of sosex !refs
• You knew that waiting on tasks is bad
• Now you know why

33. Resources
Criteo blog series
• http://labs.criteo.com/
• https://medium.com/@kevingosse
• https://medium.com/@chnasarre
Debugging extensions
• https://github.com/chrisnas/DebuggingExtensions (aka Grand Son Of Strike)
Contacts
• Kevin Gosse @kookiz
• Christophe Nasarre @chnasarre