Develop Reference

Possible Race condition with ManualResetEvent

Problem:
I am trying to throw 6 threads from ThreadPool to work on individual tasks. Each task's ManualResetEvent is stored in a array of manual reset event. Number of thread corresponds to the index in the ManualResetEvent Array.
Now what happens is that once I have initiated these 6 threads I move out and wait for the threads to complete. Waiting for the thread is done in the main thread.
Now some times what happens is that my waiting logic doesn't return even after a long time (2 days that I have seen). Here is the code sample for thread wait logic
foreach (ManualResetEvent whandle in eventList)
{
try
{
whandle.WaitOne();
}
catch (Exception) { }
}
As per documentation of .WaitOne. It is sync call which makes the thread to not return if Set event is not received from the thread.
Sometimes my threads have less amount of work and they may even return before I reach the Wait logic. Is it possible that .WaitOne() will wait for the Set() event even if it was received in the past?
Is this a correct logic to wait for the all the threads to close?

I'm not directly answering this question. Here is what you should do:
Start tasks using Task.Factory.StartNew and use Task.WaitAll(Task[]) to wait for them. You do not have to deal with events that way. Exceptions will nicely propagate to the "forking" thread. You don't need the old ThreadPool API anymore.
Hope this helps.

(Note: I think your best bet is Parallel.Invoke() - see later in this answer.)
What you are doing will normally work fine, so the problem is likely to be that one of your threads is blocking for some reason.
You should be able to debug this readily enough - you can attach the debugger and break into the program and then look at the call stack to see which thread(s) are blocked. Be prepared for some head-scratching if you discover a race condition though!
Another thing to be aware of that you can't do the following:
myEvent.Set();
myEvent.Reset();
with nothing (or very little) between the .Set() and the .Reset(). If you do that when several threads are waiting on myEvent, some of them will miss the event being set! (This effect is not well documented on MSDN.)
By the way, you shouldn't ignore exceptions - always log them in some way, at the very least.
(This section doesn't answer the question, but it may provide some helpful information)
I also want to mention an alternative way to wait for the threads. Since you have a set of ManualResetEvents, you can copy them to a plain array and pass it to WaitHandle.WaitAll().
Your code could look a little like this:
WaitHandle.WaitAll(eventList.ToArray());
Another approach to waiting for all threads to finish is to use a CountdownEvent. It becomes signalled when a countdown reaches zero; you start the count at the number of threads, and each thread signals it when it exits. There's an example here.
Parallel.Invoke()
If your threads do not return values, and all you want to to is to launch them and then have the launching thread wait for them to exit, then I think Parallel.Invoke() will be the best way of all. It avoids you having to handle the synchronization yourself.
(Otherwise, as svick says in the comments above, use Task rather than the old thread classes.)

AutoResetEvent - two set calls quickly doesn't guarantee thread release - why?

I was reading AutoResetEvent documentation on MSDN and following warning kinda bothers me..
"Important:
There is no guarantee that every call to the Set method will release a thread. If two calls are too close together, so that the second call occurs before a thread has been released, only one thread is released. It is as if the second call did not happen. Also, if Set is called when there are no threads waiting and the AutoResetEvent is already signaled, the call has no effect."
But this warning basically kills the very reason to have such a thread synchronization techniques. For example I have a list which will hold jobs. And there is only one producer which will add jobs to the list. I have consumers (more than one), waiting to get the job from the list.. something like this..
Producer:
void AddJob(Job j)
{
lock(qLock)
{
jobQ.Enqueue(j);
}
newJobEvent.Set(); // newJobEvent is AutoResetEvent
}
Consumer
void Run()
{
while(canRun)
{
newJobEvent.WaitOne();
IJob job = null;
lock(qLock)
{
job = jobQ.Dequeue();
}
// process job
}
}
If the above warning is true, then if I enqueue two jobs very quickly, only one thread will pick up the job, isn't it? I was under the assumption that Set will be atomic, that is it does the following:
Set the event
If threads are waiting, pick one thread to wake up
reset the event
run the selected thread.
So I am basically confused about the warning in MSDN. is it a valid warning?

Even if the warning isn't true and Set is atomic, why would you use an AutoResetEvent here? Let's say you have some producers queue up 3 events in row and there's one consumer. After processing the 2nd job, the consumer blocks and never processes the third.
I would use a ReaderWriterLockSlim for this type of synchronization. Basically, you need multiple producers to be able to have write locks, but you don't want consumers to lock out producers for a long time while they are only reading the queue size.

The message on MSDN is a valid message indeed. What's happening internally is something like this:
Thread A waits for the event
Thread B sets the event
[If thread A is in spinlock]
[yes] Thread a detects that the event is set, unsets it and resumes its work
[no] The event will tell thread A to wake up, once woken, thread A will unset the event resume its work.
Note that the internal logic is not synchronous since Thread B doesn't wait for Thread A to continue its business. You can make this synchronous by introducing a temporary ManualResetEvent that thread A has to signal once it continues its work and on which Thread B has to wait. This is not done by default due to the inner working of the windows threading model. I guess the documentation is misleading but correct for saying that the Set method only releases one or more waiting threads.
Alternatively i would suggest you to look at the BlockingCollection class in the System.Collections.Concurrent namespace of the BCL introduced in .NET 4.0 which does exactly what you are trying to do

Monitor.Wait, Condition variable

Given a following code snippet(found in somewhere while learning threading).
public class BlockingQueue<T>
{
private readonly object sync = new object();
private readonly Queue<T> queue;
public BlockingQueue()
{
queue = new Queue<T>();
}
public void Enqueue(T item)
{
lock (sync)
{
queue.Enqueue(item);
Monitor.PulseAll(sync);
}
}
public T Dequeue()
{
lock (sync)
{
while (queue.Count == 0)
Monitor.Wait(sync);
return queue.Dequeue();
}
}
}
What I want to understand is ,
Why is there a while loop ?
while (queue.Count == 0)
Monitor.Wait(sync);
and what is wrong with the,
if(queue.Count == 0)
Monitor.Wait(sync);
In fact, all the time when I see the similar code I found using while loop, can anyone please help me understand the use of one above another.
Thank you.

You need to understand what Pulse, PulseAll, and Wait are doing. The Monitor maintains two queues: the waiting queue and the ready queue. When a thread calls Wait it is moved into the waiting queue. When a thread calls Pulse it moves one and only one thread from the waiting queue to the ready queue. When a thread calls PulseAll it moves all threads from the waiting queue to the ready queue. Threads in the ready queue are eligible to reacquire the lock at any moment, but only after the current holder releases it of course.
Based on this knowledge it is fairly easy to understand why you must recheck the queue count when using PulseAll. It is because all dequeueing threads will eventually wake and will want to attempt to extract an item from queue. But, what if there is only one item in the queue to begin with? Obviously, we must recheck the queue count to avoid dequeueing an empty queue.
So what would be the conclusion if you had used Pulse instead of PulseAll? There would still be a problem with the simple if check. The reason is because a thread from the ready queue is not necessarily going to be the next thread to acquire the lock. That is because the Monitor does not give preference to a Wait call above an Enter call.
The while loop is a fairly standard pattern when using Monitor.Wait. This is because pulsing a thread does not have semantic meaning by itself. It is only a signal that the lock state has changed. When threads wake up after blocking on Wait they should recheck the same condition that was originally used to block the thread to see if the thread can now proceed. Sometimes it cannot and so it should block some more.
The best rule of thumb here is that if there is doubt about whether to use an if check or a while check then always choose a while loop because it is safer. In fact, I would take this to the extreme and suggest to always use a while loop because there is no inherent advantage in using the simpler if check and because the if check is almost always the wrong choice anyway. A similar rule holds for choosing whether to use Pulse or PulseAll. If there is doubt about which one to use then always choose PulseAll.

you have to keep checking whether the queue is still empty or not. Using only if would only check it once, wait for a while, then a dequeue. What if at that time the queue is still empty? BANG! queue underflow error...

with if condition when something released the lock the queue.Count == 0 will not check again and maybe a queue underflow error so we have to check the condition every time because of concurrency and this is called Spinning

Why on Unix it could go wrong is because of the spurious wake up, possibility caused by OS signals. It is a side effect that is not guaranteed to never happen on windows as well. This is not a legacy, it is how OS works. If Monitors are implemented in terms of Condition Variable, that is.
def : a spurious wake up is a re-scheduling of a sleeping thread on a condition variable wait site, that was not triggered by an action coming from the current program threads (like Pulse()).
This inconvenience could be masked in managed languages by, e.g. the queues. So before going out of the Wait() function, the framework could check that this running thread is actually really being requested for scheduling, if it does not find itself in a run queue it can go back to sleep. Hiding the problem.

if (queue.Count == 0)
will do.
Using while loop pattern for "wait for and check condition" context is a legacy leftover, I think. Because non-Windows, non-.NET monitor variables can be triggered without actual Pulse.
In .NET, you private monitor variable cannot be triggered without Queue filling so you don't need to worry about queue underflow after monitor waiting. But, it is really not bad habit to use while loop for "wait for and check condition".

Efficient Cancel Event Implementation for Number Crunching Threads?

What is the most efficient way to create a “cancel” event in a C# program that is crunching a large set of data in a loop on a separate thread?
For now, I am simply using a cancel event that is triggered from my UI thread, which subsequently calls an “onCancel” function on the number crunching thread. That cancel function sets a variable to “true”, which the crunch loop checks periodically, e.g.
Class Cruncher {
private bool cancel = false;
public cruncher()
{
crunch();
}
private void crunch()
{
while(conditions AND !cancel) { crunch; }
dispose_resources;
}
private void onCancel()
{
cancel = true;
}
}
While I am not checking the cancel variable as often as my example above (and not actually performing a NOT cancel), I would still like to optimize this crunch method as much as possible. Any examples where this is done more efficiently would be very nice to see.

The cancel event/flag should be a volatile... I asked a very similar question to yours: Is it safe to use a boolean flag to stop a thread from running in C#
I would also recommend that when you cancel your threads you wait for all of them to cancel by using something similar to the C# version of CountDownLatch. It's useful when you want to guarantee that the thread is canceled.

It will ultimately always result in something like this - although it's important that you make your cancel variable volatile, as otherwise the worker threads may not see the change from the cancelling thread.
You've got to check something periodically unless you want to go the more drastic route of interrupting the thread (which I don't recommend). Checking a single Boolean flag isn't likely to be exactly costly... if you can do a reasonable chunk of work in each iteration of the loop (enough to dwarf the cost of the check) then that's fine.
If you ever need to perform any waiting, however (in the worker thread), then you may be able to improve matters, by using a form of waiting (e.g. Monitor.Wait) which allows the cancelling thread to wake any waiting threads up early. That won't make normal operation more efficient, but it will allow the threads to terminate more quickly in the event of cancellation.

Especially since it's UI-triggered, I would recommend just leveraging the BackgroundWorker that's already in the framework, especially since it'll nicely have the progress and done events happen on the UI thread for you (so you don't have to invoke it over yourself).
Then you can just use the CancelAsync() call. Admittedly, it's not much different than what you're already doing, just done in the framework already (and including the thread synchronization logic)
As Jon mentioned, you're still going to want to do cooperative cancellation (checking CancellationPending in your DoWork for use of BackgroundWorker) since the 'interrupt/abort the thread' option is something you want to avoid if possible.
If in .NET 4 you can use TPL and the new Cancellation support, but again it's focused on cooperative cancellation.

I recommend using the unified cancellation model that was introduced in .NET 4.0 (if .NET 4.0 is an option).
It is very efficient, and allows integrated cancellation with Task objects and Parallel LINQ.

i would do it the same way. i would also add Thread.Sleep in to the loop to yield control to the main thread.
http://msdn.microsoft.com/en-us/library/7a2f3ay4%28VS.80%29.aspx

How can I check if a function is being called on a particular Thread?

If I have Thread A which is the main Application Thread and a secondary Thread. How can I check if a function is being called within Thread B?
Basically I am trying to implement the following code snippit:
public void ensureRunningOnCorrectThread()
{
if( function is being called within ThreadB )
{
performIO()
}
else
{
// call performIO so that it is called (invoked?) on ThreadB
}
}
Is there a way to perform this functionality within C# or is there a better way of looking at the problem?
EDIT 1
I have noticed the following within the MSDN documentation, although Im a dit dubious as to whether or not its a good thing to be doing! :
// if function is being called within ThreadB
if( System.Threading.Thread.CurrentThread.Equals(ThreadB) )
{
}
EDIT 2
I realise that Im looking at this problem in the wrong way (thanks to the answers below who helped me see this) all I care about is that the IO does not happen on ThreadA. This means that it could happen on ThreadB or indeed anyother Thread e.g. a BackgroundWorker. I have decided that creating a new BackgroundWorker within the else portion of the above f statement ensures that the IO is performed in a non-blocking fashion. Im not entirely sure that this is the best solution to my problem, however it appears to work!

Here's one way to do it:
if (System.Threading.Thread.CurrentThread.ManagedThreadId == ThreadB.ManagedThreadId)
...
I don't know enough about .NET's Thread class implementation to know if the comparison above is equivalent to Equals() or not, but in absence of this knowledge, comparing the IDs is a safe bet.
There may be a better (where better = easier, faster, etc.) way to accomplish what you're trying to do, depending on a few things like:
what kind of app (ASP.NET, WinForms, console, etc.) are you building?
why do you want to enforce I/O on only one thread?
what kind of I/O is this? (e.g. writes to one file? network I/O constrained to one socket? etc.)
what are your performance constraints relative to cost of locking, number of concurrent worker threads, etc?
whether the "else" clause in your code needs to be blocking, fire-and-forget, or something more sophisticated
how you want to deal with timeouts, deadlocks, etc.
Adding this info to your question would be helpful, although if yours is a WinForms app and you're talking about user-facing GUI I/O, you can skip the other questions since the scenario is obvious.
Keep in mind that // call performIO so that it is called (invoked?) on ThreadB implementation will vary depending on whether this is WinForms, ASP.NET, console, etc.
If WinForms, check out this CodeProject post for a cool way to handle it. Also see MSDN for how this is usually handled using InvokeRequired.
If Console or generalized server app (no GUI), you'll need to figure out how to let the main thread know that it has work waiting-- and you may want to consider an alternate implementation which has a I/O worker thread or thread pool which just sits around executing queued I/O requests that you queue to it. Or you might want to consider synchronizing your I/O requests (easier) instead of marshalling calls over to one thread (harder).
If ASP.NET, you're probably implementing this in the wrong way. It's usually more effective to use ASP.NET async pages and/or to (per above) synchronize snchronizing to your I/O using lock{} or another synchronization method.

What you are trying to do is the opposite of what the InvokeRequired property of a windows form control does, so if it's a window form application, you could just use the property of your main form:
if (InvokeRequired) {
// running in a separate thread
} else {
// running in the main thread, so needs to send the task to the worker thread
}

The else part of your snippet, Invoking PerformIO on ThreadB is only going to work when ThreadB is the Main thread running a Messageloop.
So maybe you should rethink what you are doing here, it is not a normal construction.

Does your secondary thread do anything else besides the performIO() function? If not, then an easy way to do this is to use a System.Threading.ManualResetEvent. Have the secondary thread sit in a while loop waiting for the event to be set. When the event is signaled, the secondary thread can perform the I/O processing. To signal the event, have the main thread call the Set() method of the event object.
using System.Threading;
static void Main(string[] args)
{
ManualResetEvent processEvent = new ManualResetEvent(false);
Thread thread = new Thread(delegate() {
while (processEvent.WaitOne()) {
performIO();
processEvent.Reset(); // reset for next pass...
}
});
thread.Name = "I/O Processing Thread"; // name the thread
thread.Start();
// Do GUI stuff...
// When time to perform the IO processing, signal the event.
processEvent.Set();
}
Also, as an aside, get into the habit of naming any System.Threading.Thread objects as they are created. When you create the secondary thread, set the thread name via the Name property. This will help you when looking at the Threads window in Debug sessions, and it also allows you to print the thread name to the console or the Output window if the thread identity is ever in doubt.