threading - Monitor.Wait/Pulse

  • Thread starter Thread starter Perecli Manole
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Perecli Manole

I am having some strange thread synchronization problems that require me to
better understand the intricacies of Monitor.Wait/Pulse.

I have 3 threads. Thread 1 does a Monitor.Wait in a SyncLock block
protecting a resource. Thread 2 and 3 also have a SyncLock block protecting
the same resource and after executing some code in their blocks they both do
a Monitor.Pulse to hand of the locked resource back to thread 1.

While thread 1 has the resource locked both thread 2 and 3 are blocked at
the beginning of their SyncLock waiting for the resource to be released. The
question is: When thread 1 does a Monitor.Wait what determines the order of
which thread 2 or 3 is allowed to enter their SyncLock? And if thread 2 gets
the resource and then releases it with a Monitor.Pulse, will thread 3 then
get the resource or will thread 1 get it first?

Thanks
Perry
 
Since .Net Managed threads are backed by native operating system thread
objects, then the operating system decides which thread executes in what
order based on the time slots it has provided. The only way to influence the
order of execution is to muck with Thread Priorities, which is generally
considered a Bad Thing. When Thread1 calls Wait on the monitor object, all
it is doing is giving up the remainder of it's time slot, and telling the OS
it can switch to a different thread. The Monitor object adds items to a
queue whenever they call Wait(), so you can be pretty sure that the waiting
threads will be signaled in the order they originally executed, but you
really have no control over when a specific thread will reach that Wait()
statement.
 
The problem I am having is that both threads 2 and 3 are executing their
SyncLock block before returning the resource to thread 1. I was under the
assumption that as soon as either thread 2 or 3 executes its SyncLock block
(both containing a .Pulse statement) control would be given back to thread 1
as soon as either thread 2 or 3 is finished but not both. Since thread 2 and
3 where both waiting at the beginning of the SyncLock statement for the
resource to be released by thread 1, they seem to both execute before
relinquishing the resource. How can have either thread 2 or 3 (whichever
gets there first) execute but not both before the resource is returned to
thread 1.

Thanks
Perry
 
Could you also answer the more important last question I posed in my last
post?

"And if thread 2 gets the resource and then releases it with a
Monitor.Pulse, will thread 3 then get the resource or will thread 1 get it
first?"

Thanks
Perry
 
Okay, I think I understand more of your issue now. I'll see if I can explain
generally how Monitor Deals with Wait() and Pulse()/PulseAll() calls:

Whenever you call Monitor.Wait(), the thread that makes the call gets put
into a Wait Queue, and releases the lock on the monitored object. At this
point another thread may acquire the lock, and even call Wait() itself. Once
a thread calls Pulse() on the Monitor, the first thread in the Wait Queue
gets moved into the Lock Queue (if you use PulseAll() then all of the threads
in the Wait Queue get moved to the lock queue). Once the thread that called
Pulse() is done, the next item in the Lock Queue will get the lock.

I think the problem your running into comes from the fact that Thread 1 is
being put in the Lock Queue behind Thread 2 or Thread 3 (depending on which
got the lock on the object after Thread1 called Wait()).

I think, to directly answer your question below, that after Thread 2 calls
Pulse, then Thread 3 will get the object next, followed by Thread 1. At
least that is assuming the order of the threads is constant, which you can't
always rely on (see my original response).

Does that make sense?
 
Thank you for the detail description, that explains the unpredictable
behavior I am experiencing.

But, in order for my application to work properly, I absolutely need the
waiting thread (Thread 1) to be the next thread that aquires the lock after
either Thread 2 or 3 pulse. Thread 1 MUST be checked with after every and
any pulse from any other thread. How can I achieve this even if it has to be
done with a different mechanism?

Thanks
Perry
 
Well, my first suggestion might be to utilize a WaitHandle derived object
(Either a ManualResetEvent or AutoResetEvent most likely). WaitHandles
provide similar signalling to what is available with the Monitor class, but
the fact that you can manually reset an event means that other threads which
may call Wait on the handle will not block on that call until the handle is
reset.

For your description, it sounds like you may want to consider creating two
WaitHandle objects, one for Thread2 and one for Thread3, then in Thread1 call
the static WaitHandle.WaitAny() passing in an array with both of those
objects. Once Thread2 or Thread3 signals thier handle (using Set()) then
Thread1 will pick back up.

WaitHandle includes WaitOne, which waits for a single WaitHandle object to
be signaled, WaitAny, which waits for any in an array of WaitHandles to be
signaled, and WaitAll, which waits for all WaitHandles in an array to be
signaled. The instances include the Set/Reset methods, but also have a
SignalAndWait method, which allows you to signal one WaitHandle, and Wait on
another (as a single operation). It seems like some experimenting with these
might give you the some decent results.

Looking at your original post again, it looks like this example from Juval
Lowy's "Programming .Net Components" 2nd ed. may be helpful. He calls this a
"Rendezvous Helper", and describes it as a way to allow two threads to do
work independently, and then wait for one another to to complete before
moving on... For what it is worth here is the code (hope C# is okay):

public class Rendezvous
{
AutoResetEvent m_First = new AutoResetEvent(true);
AutoResetEvent m_Event1 = new AutoResetEvent(false);
AutoResetEvent m_Event2 = new AutoResetEvent(false);

public void Wait( )
{
bool first = m_First.WaitOne(TimeSpan.Zero,false);
if(first)
{
WaitHandle.SignalAndWait(m_Event1,m_Event2);
}
else
{
WaitHandle.SignalAndWait(m_Event2,m_Event1);
}
}
public void Reset( )
{
m_First.Set( );
}
}

Here's a usage sample:

public class RendezvousDemo
{
Rendezvous m_Rendezvous = new Rendezvous( );

public void ThreadMethod1( )
{
//Do some work, then
m_Rendezvous.Wait( );
//Continue executing
}
public void ThreadMethod2( )
{
//Do some work, then
m_Rendezvous.Wait( );
//Continue executing

}
}
RendezvousDemo demo = new RendezvousDemo( );
Thread thread1 = new Thread(demo.ThreadMethod1);
thread1.Start( );

Thread thread2 = new Thread(demo.ThreadMethod2);
thread2.Start( );

Basically what this boils down to is that thread1 and thread2 are started,
and whichever one finishes first is blocked until the other is finished, at
which point they both continue.

I'm not sure if the example is too abstract for your needs, but hopefully it
gives you enough to tinker with.
 
I had the same thought as you so I tried the AutoResetEvent. This approach
has a strange side effect though. The WaitHandle, I'am guessing, seems to
prevent the SerialPort.DataReceived event from fireing so thread 2 and above
are not being called causing the WaitOne to always expire on the timout.
When using the Monitor.Wait function instead the event is raised again. Can
this be explained? Here a snipit that shows the basics:

'global
Private _ARE1 As New AutoResetEvent(False)
Private _ARE2 As New AutoResetEvent(False)

'---------------------- transmit tread 1 ----------------------
SyncLock _objTxRxSync
_objSerialPort.Write(objTxPacket.Bytes, 0, objCurrTxPacket.Bytes.Length)
_objTxRxSync.AssertedPacket = objTxPacket
_objTxRxSync.MatchNeeded = TxRxSync.MatchType.Echo
If _ARE1.WaitOne(PACKET_ECHO_TIMEOUT, True) Then
If objTxPacket.NeedsPacketResponse Then
_objTxRxSync.MatchNeeded = TxRxSync.MatchType.Response
_ARE2.WaitOne(PACKET_RESPONSE_TIMEOUT, True)
End If
End If
End SyncLock

'------------------receive threads 2,3,n.... ---------------------
SyncLock _objTxRxSync
If _objTxRxSync.MatchNeeded = TxRxSync.MatchType.Echo AndAlso
_objTxRxSync.AssertedPacket.Equals(_objRxPacket) Then
RaiseCallBackEvent(_objTxRxSync.AssertedPacket)
_ARE1.Set()
Exit Sub
End If
End SyncLock

'do some work that does not need syncronization for non echo packets only
'..............

SyncLock _objTxRxSync
If _objTxRxSync.MatchNeeded = TxRxSync.MatchType.Response AndAlso
_objTxRxSync.AssertedPacket.ValidPacketResponse(objRxPacket) Then
_ARE2.Set()
End If
End SyncLock

RaiseCallBackEvent(objRxPacket)



Thanks
Perry
 
I think the problem is that you are calling the Wait() mehod on your wait
handle within your SynLock block. The one thing Monitor.Wait does that you
don't get with WaitHandles is that it releases the Lock on the Object you
call Wait on.

I would say break it out into two SynLock groups in Thread 1, one locking
the resources just prior to calling WaitHandle.Wait(), and the other after
the Wait returns.

You'll need to limit the scope of the SynLock in Thread's 2 & 3 as well.

I'm afraid, though, that you may run into a similar problem as the one you
were experiencing with the Monitor object. You may want to consider using
the Semiphore class to allow 2 classes to access your _objTxRxSync object at
a time. You would probably also need to use either a WaitHandle, or a global
(and locked) variable to make sure that Threads 2 & 3 don't do their thing
until Thread 1 is ready. Adding the Semiphore will give you the ability to
make sure only two threads will be working with the resources at a time,
though.

Does this make any sense?

Casey
 
I think the problem is that you are calling the Wait() mehod on your wait
handle within your SynLock block. The one thing Monitor.Wait does that
you
don't get with WaitHandles is that it releases the Lock on the Object you
call Wait on.
Click to expand...

I thought by setting the last boolean parameter in the WaitOne method to
True will allow the exit of the locking context. The documentation seems to
imply this.
_ARE1.WaitOne(PACKET_ECHO_TIMEOUT, True)

I would say break it out into two SynLock groups in Thread 1, one locking
the resources just prior to calling WaitHandle.Wait(), and the other after
the Wait returns.
Click to expand...

If I break the SyncLock into two groups then the memebers of the objTxPacket
can be changed by other threads in between the SyncLocks. Also don't know
how to synclock the second WaitOne because it is in a conditional statement
that get its value from the locked object. It the object is not locked at
that point then there could be problems.

SyncLock _objTxRxSync
_objSerialPort.Write(objTxPacket.Bytes, 0, objCurrTxPacket.Bytes.Length)
_objTxRxSync.AssertedPacket = objTxPacket
_objTxRxSync.MatchNeeded = TxRxSync.MatchType.Echo
If _ARE1.WaitOne(PACKET_ECHO_TIMEOUT, True) Then
If objTxPacket.NeedsPacketResponse Then
_objTxRxSync.MatchNeeded = TxRxSync.MatchType.Response
_ARE2.WaitOne(PACKET_RESPONSE_TIMEOUT, True)
End If
End If
End SyncLock

You'll need to limit the scope of the SynLock in Thread's 2 & 3 as well.
Click to expand...

How can I limit it any smaller that it already is?
I'm afraid, though, that you may run into a similar problem as the one you
were experiencing with the Monitor object. You may want to consider using
the Semiphore class to allow 2 classes to access your _objTxRxSync object
at
a time. You would probably also need to use either a WaitHandle, or a
global
(and locked) variable to make sure that Threads 2 & 3 don't do their thing
until Thread 1 is ready. Adding the Semiphore will give you the ability
to
make sure only two threads will be working with the resources at a time,
though.
Does this make any sense?
Click to expand...

Not clear how to do this.

Perry
 
I figured out a solution that is surprisingly simple.
Apparently if you do a Thread.Sleep(0), at the beginning of each thread 2 or
3 iteration (before the first SyncLock on those threads), the framework
assumes for a brief period that there are no more tasks to follow and gives
a time slice back to the waiting thread that was pulsed, rather than taking
on another lock request from thread 2 or 3.

The Sleep() seems to behave similar to DoEvents() in the UI context.
With this approach I can keep using the Monitor.Wait/Pulse which buys me the
context unlocking.

Thank you for all your help. It definitely got ideas steering in my head by
understanding what is really happening.

Perry
 
That is true....The Sleep() call basically tells the OS that the thread is
done with it's time slot for now. I can say that officially, using Sleep for
thread synchronization is considered a "Bad Thing" since there is no way to
know for sure that the threads are getting started in a particular
order....In reality, though, if your creating the thread objects yourself (as
opposed to using the thread pool, which is shared for all apps on the
machine), you typically don't run into problems.

Also, just for your knowelege, the bool property on the
WaitOne()/WaitAny()/WaitAll() methods of the WaitHandle indicates whether you
want to exit from a synchronization domain, which is different than the
manual synchronization you get using Monitor/Synlock/WaitHandles/Whatever.
Synchronization Domains are virtual spaces set up within the AppDomain that
allow for coarse grained locking. To use it you must have an object that is
derived from ContextBoundObject, and then use the SynchronizationAttribute to
tell the Framework how to set up the Synchronization domain. Any call coming
into a Synchronization domain from outside will effectvly be wrapped in a
Synlock block by the framework. It can be easier to manage (assuming you are
okay with having access to objects or object graphs synchronized), but most
folks aren't confortable with the overhead of using a ContextBoundObject.

So now that I've filled my useless knowlege quota for the day.....

I'm glad you've got a solution working...Good luck

Casey
 
Thanks again for the great insights.

Perry

Casey Kramer said:
That is true....The Sleep() call basically tells the OS that the thread is
done with it's time slot for now. I can say that officially, using Sleep
for
thread synchronization is considered a "Bad Thing" since there is no way
to
know for sure that the threads are getting started in a particular
order....In reality, though, if your creating the thread objects yourself
(as
opposed to using the thread pool, which is shared for all apps on the
machine), you typically don't run into problems.

Also, just for your knowelege, the bool property on the
WaitOne()/WaitAny()/WaitAll() methods of the WaitHandle indicates whether
you
want to exit from a synchronization domain, which is different than the
manual synchronization you get using Monitor/Synlock/WaitHandles/Whatever.
Synchronization Domains are virtual spaces set up within the AppDomain
that
allow for coarse grained locking. To use it you must have an object that
is
derived from ContextBoundObject, and then use the SynchronizationAttribute
to
tell the Framework how to set up the Synchronization domain. Any call
coming
into a Synchronization domain from outside will effectvly be wrapped in a
Synlock block by the framework. It can be easier to manage (assuming you
are
okay with having access to objects or object graphs synchronized), but
most
folks aren't confortable with the overhead of using a ContextBoundObject.

So now that I've filled my useless knowlege quota for the day.....

I'm glad you've got a solution working...Good luck

Casey
Click to expand...
 
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