how to check if Mutex is Released

[pacerier]

i've got a Set and a Get in a property. I want to allow multiple threads to do Get anytime they wish to, however if any thread tries to do Set, all threads must not be able to Get/Set until the Set is complete.

what i tried is this (lock is a Mutex):

 Public Shared Property someproperty() As Boolean
        Get
            lock.WaitOne()
             lock.ReleaseMutex()
            Return VAR_propvalue
        End Get
        Set(ByVal value As Boolean)
            lock.WaitOne()
                      VAR_propvalue = value
       lock.ReleaseMutex()

        End Set
    End Property

the problem is that this is pretty inefficient because each request to Get will steal the Mutex and prevent other Gets until the stolen Mutex is Released.
Is there anyway to just check if a Mutex is released instead of using WaitOne because WaitOne checks and gains control of the Mutex which is not what i want

[gonzalioz]

I think 'Synclock' is what you are looking for. When you place synclock on a object, all other threads that want to access it will have to wait untill the synclock is gone.

    Public Shared Property someproperty() As Boolean
        Get
            Return VAR_propvalue
        End Get
        Set(ByVal value As Boolean)
            SyncLock VAR_propvalue
                VAR_propvalue = value
            End SyncLock
        End Set
    End Property

ps: If you want some more control, like skipping some code when a synclock is active. Try using Threading.Monitor.TryEnter. This will return true or false depending on if a object is locked.

[pacerier]

the synclock would do fine, however it doesn't allow me to Sync primitive types (boolean)

[NickThissen]

I've never used it before but I think you should simply declare a new Object and use that, something like this:

    Dim obj As New Object()

   Public Shared Property someproperty() As Boolean
        Get
            Return VAR_propvalue
        End Get
        Set(ByVal value As Boolean)
            SyncLock obj
                VAR_propvalue = value
            End SyncLock
        End Set
    End Property

Whether this is the right approach in a property, I've no idea.

[gonzalioz]

I've never used it before but I think you should simply declare a new Object and use that, something like this:

    Dim obj As New Object()

   Public Shared Property someproperty() As Boolean
        Get
            Return VAR_propvalue
        End Get
        Set(ByVal value As Boolean)
            SyncLock obj
                VAR_propvalue = value
            End SyncLock
        End Set
    End Property

Whether this is the right approach in a property, I've no idea.

That's ugly .

[NickThissen]

That's ugly .

That's the only way I've ever seen it used; with a new object purely for synclocking. Do you suggest another way?

[pacerier]

currently i'm doing something like this:

i wonder which is uglier:

 Get
           
                Do While VAR_busy_setting_permissions
                Loop
            Return VAR_value
        End Get

[chris128]

currently i'm doing something like this:

i wonder which is uglier:

Using a loop like that is definitely uglier if you ask me... and much more processor intensive. I would recommend not doing that.

I've never used it before but I think you should simply declare a new Object and use that, something like this:

Dim obj As New Object()

   Public Shared Property someproperty() As Boolean
        Get
            Return VAR_propvalue
        End Get
        Set(ByVal value As Boolean)
            SyncLock obj
                VAR_propvalue = value
            End SyncLock
        End Set
    End Property

Whether this is the right approach in a property, I've no idea.

Yeah that is how you should use it (that is how I have always used it and have never had any problems and that is how every example I have ever seen has used it), although you would normally put the Get in a SyncLock as well as otherwise another thread could read the property while it is still part way through being set.

Anyway, I dont think any kind of lock is really necessary for a boolean as there is no 'inconsistent' state for it to be in. Generally the purpose of sync locks or mutex etc is to prevent one thread from modifying something while another thread is using it, like with a list for example one thread could be looping through the list and then another thread could remove all of the items - so the list is then in an inconsistent state when the original thread comes to loop to the next item and finds that it no longer exists (and it will throw an exception because the collection has been modified). With a boolean though, you cant really get that problem as its just either True or False and nothing in between.

Having said that, you might just be using the Boolean as an example so I'll still give you my thoughts on the original question - I would agree that a SyncLock block is the best way to handle this, but just using SyncLock on its own you will still have the same issue of only one thread being allowed in either the Get or Set at any one time. So perhaps you could do something like this:

'Example:
    Private InSet As Boolean = False
    Private Lock As New Object

    Private _SomeString As String
    Public Property SomeString() As String
        Get
            If InSet Then
                SyncLock Lock
                    Return _SomeString
                End SyncLock
            Else
                Return _SomeString
            End If
        End Get
        Set(ByVal value As String)
            InSet = True
            SyncLock Lock
                _SomeString = value
            End SyncLock
            InSet = False
        End Set
    End Property

[pacerier]

btw does using a loop really take up alot of resources?

does SyncLock also make use of a loop internally?

[chris128]

try it yourself, create a new project and just stick this in the form load event handler and then check task manager when you launch the program:

Dim Something As Boolean = False
Do Until Something = True
Loop

Oh and good luck ever getting the form to actually appear on screen :P

As for SyncLock, no I do not think that uses a loop behind the scenes. It certainly does not cause the CPU usage to shoot up when a thread is waiting for a lock to be released so that it can enter the SyncLock section. Take a look at this page for more info: http://msdn.microsoft.com/en-us/libr...1t(VS.80).aspx

[gonzalioz]

When synclock is active, other threads will be stored into a 'waiting queue' or whatever you want to call it. Just doing nothing. When End Synclock is used the first thread in the queue will be be punched in the face to wake him up and he will continue where he left of.

So the threads that are waiting aren't asking the other thread that holds the lock. Are you done yet, are you done yet, constantly. They just say, hey I'll wait here in this queue. Give me a signal when your done .

[chris128]

When synclock is active, other threads will be stored into a 'waiting queue' or whatever you want to call it. Just doing nothing. When End Synclock is used the first thread in the queue will be be punched in the face to wake him up and he will continue where he left of.

So the threads that are waiting aren't asking the other thread that holds the lock. Are you done yet, are you done yet, constantly. They just say, hey I'll wait here in this queue. Give me a signal when your done .

You know I thought that, and I even typed pretty much the same thing in my last post... but then I edited it because I'm not so sure now. SyncLock is just a nice tidy way of using the Monitor class - this is taken from the MSDN description of the System.Threading.Monitor class:

The functionality provided by the Enter and Exit methods is identical to that provided by the C# lock statement (SyncLock in Visual Basic), except that lock and SyncLock wrap the Exit method in a try…finally block (Try…Finally in Visual Basic) to ensure that the monitor is released.

so I'm trying to find out exactly how the Monitor class works behind the scenes but not having much joy... I checked it out in Reflector but the Monitor.Enter method just shows as this:

<MethodImpl(MethodImplOptions.InternalCall)> _
Public Shared Sub Enter(ByVal obj As Object)

and that MethodImplOptions.InternalCall parameter basically means that this particular method is actually implemented in unmanaged code so I am going to download the source code for .NET 2.0 that Microsoft released and see if I can find anything about it in there. I guess this is straying away from the OP's question slightly now but I am curious about how Monitors work as there is no such object in the underlying Windows OS (there are Mutexes and Semaphores but no such thing as a Monitor as far as I know).

[chris128]

OK so I have been doing some digging and think I might finally be getting somewhere towards an informed answer to how SyncLock/Monitor.Enter works

As I mentioned in the last post, classes like Mutex and Semaphore are just wrappers around existing objects in the Windows OS, but a Monitor (and therefore the SyncLock function) does not exist in the Windows OS and is a pure .NET feature. I say 'pure' but the implementation of the Monitor's methods is actually written in unmanaged C++ code.

Anyway, basically any object that is constructed in .NET has a header in memory which contains a few fields and one of these fields is a SyncBlockIndex (thats SyncBlock not SyncLock). It looks like the .NET runtime keeps a load of these SyncBlock items in memory and basically each time you use SyncLock/Monitor.Enter on an object, the CLR associates one of these 'spare' SyncBlocks with the object you passed to SyncLock/Monitor.Enter. It does this by entering the SyncBlock's ID into the object's SyncBlockIndex field. Then when your code calls End SyncLock or Monitor.Exit then the locking object's SyncBlockIndex field is set back to a negative number to indicate that it has no SyncBlock associated with it.

So if another thread comes along and calls SyncLock/Monitor.Enter on this same object then it will see that the SyncBlockIndex field of the object is set and it will look at the SyncBlock that this field points to - if it sees that this SyncBlock is owned by another thread then it will suspend this thread and wait for the other thread to release the SyncBlock so that it can then take ownership.

So in short, I dont THINK using SyncLock incurs any kind of looping or intensive CPU usage, although I did read somewhere that it will check the SyncBlock ownership again a few times quickly before resorting to suspending the thread - presumably because sometimes the SyncBlock will have been released a few miliseconds later so it would be less efficient to suspend the thread and wait for the notification than it would be to just try again a few times first.


Well, I found it interesting even if no one else did

Oh and if anyone has anything to add to that or correct, please do!

Chris

[pacerier]

i think the code u provided above would not work:

 Get
            If InSet Then
                SyncLock Lock
                    Return _SomeString
                End SyncLock
            Else
                Return _SomeString
            End If
        End Get

because after you have Returned, everything else below it does not run. So the Lock never gets released. so what we have is this:

 Get
            If InSet Then
                SyncLock Lock
                End SyncLock
                Return _SomeString
            Else
                Return _SomeString
            End If
        End Get

there's 2 reasons why i do not wish to use SyncLock. number one is obviously we are created an object for just Synclocking, another and more Importantly is that there is a delay in SyncLock compared to using the Do While method.

During:

        SyncLock Lock
                End SyncLock

the thread steals priority and prevents other from reading. So if i had like 10 threads trying to do a Get, i want them to be able to occur simultaneously, in any order, because it would still be ThreadSafe (unless someone decides to do a Set). But using SyncLock, the 10 threads had to do it one by one, even if its fast, its still one by one, whereas Do While allows all of them to do Straight away without waiting for any

Do While allowed_to_do_a_get
Loop

Well of course 0 loops are taking place if allowed_to_do_a_get is set to True. I think the real problem is that alot of loops will take place if someone is Setting (which will set allowed_to_do_a_get to false)

Is there a better way to do things, a SyncLock that doesn't cause delay?




also,

if it sees that this SyncBlock is owned by another thread then it will suspend this thread and wait for the other thread to release the SyncBlock so that it can then take ownership.

so how does he actually know that the SyncBlock is released, doesn't he have to check it every now and then to see if SyncBlock is released? (which means a loop)

[dbasnett]

"although I did read somewhere that it will check the SyncBlock ownership again a few times quickly before resorting to suspending the thread - presumably because sometimes the SyncBlock will have been released a few miliseconds later so it would be less efficient to suspend the thread and wait for the notification than it would be to just try again a few times first."

I think what is fundamental to any synchronizing operation is the hardware's ability to do a read-alter-rewrite operation atomically.

[pacerier]

i've currently upgraded my Do While to slow down depending on user input.

'gap is an Integer specifying the wait time

Do While allowed_to_do_a_get
       If gap <> 0I then
                Threading.Thread.Sleep(gap)
       End If
Loop

[dbasnett]

I am just missing something. You have a property that can be true or false, and you are trying to make certain that only one process is able to set it, and you do not want it to be read while it is being set.

Don't make this more complicated than it is:

        'to Set it
        SyncLock lock
            flag = True
        End SyncLock
        'or 
        Monitor.Enter(lock)
        flag = True
        Monitor.Exit(lock)

        'to Read it
        Dim retval As Boolean
        SyncLock lock
            retval = True
        End SyncLock
        'or 
        Monitor.Enter(lock)
        retval = True
        Monitor.Exit(lock)

[pacerier]

no its not just one property, its 2 (and it can be more), of course i do not wish it to be read while its being written. If some thread is writing it halfway, how could i allow it to be read? it can only be read until the write is over, unless there is no one writing, then any amount of threads can read it anyhow they want.

plus if 1 or more threads are reading, no threads should be allowed to do a Write, untill the read is complete, to ensure that the read is proper

[chris128]

i think the code u provided above would not work:

 Get
            If InSet Then
                SyncLock Lock
                    Return _SomeString
                End SyncLock
            Else
                Return _SomeString
            End If
        End Get

because after you have Returned, everything else below it does not run. So the Lock never gets released. so what we have is this:

 Get
            If InSet Then
                SyncLock Lock
                End SyncLock
                Return _SomeString
            Else
                Return _SomeString
            End If
        End Get

No that is not the case, when you come out of the scope of a SyncLock block the lock will be released - it doesnt have to actually hit the End SyncLock line. Just like if you use a Using statement, you dont have to hit the End Using line for the object to be disposed, its just when you come out of the scope of that statement. I assume this behaviour is achieved by the fact that internally the SyncLock uses a Try/Catch/Finally block and releases the lock in the Finally block. You can see the same behaviour yourself if you just do something like this inside a Function:

Try
    Dim something As String = "Some text"   
    Return something
Catch
    MessageBox.Show("Oh dear")
Finally
    MessageBox.Show("Merry Christmas") 'seasonal examples FTW
End Try

Now if you run that function you will see that you get your 'something' object returned but the Finally block is still reached so you should still see the Merry Christmas message box

[chris128]

Also, I have been assuming all along that you are just trying to make your code as efficient as is physically possible, but if the reason for you asking about this is because you are worried about this slowing your application down then I would say you have nothing to worry about unless you are making an extremely performance sensitive application. I have made plenty of applications that use several threads (some that use up to 20 at once) and have never had any performance problems with SyncLock at all and I'm sure most other developers would agree.

Also, a Property should never really be doing anything that takes a long time in its Set or Get methods, so your other threads should only ever be waiting a fraction of a second if there is another thread already inside either the Get or the Set.

[dbasnett]

As Chris pointed out, performance in the case you have provided, or even if there are several properties, should not be an issue.

You might also try

        Dim flag As Long
        'set
        Interlocked.CompareExchange(flag, 1, 0)
        'get
        Interlocked.Read(flag)
        'clear 
        Interlocked.CompareExchange(flag, 0, 1)

[pacerier]

if the reading thread had to steal priority as well, in other words, is it true that what a SyncLock can do, a Mutex can do as well?

[chris128]

if the reading thread had to steal priority as well, in other words, is it true that what a SyncLock can do, a Mutex can do as well?

You could make a Mutex and SyncLock both do the same thing yes, but its much easier, more maintainable, more readable, and the more ".NET way of doing things" to just use SyncLock if you ask me.

[dbasnett]

"You could make a Mutex and SyncLock both do the same thing yes, but its much easier, more maintainable, more readable, and the more ".NET way of doing things" to just use SyncLock if you ask me."


Certainly in this case I agree.

[chris128]

Also, a Mutex is slower than SyncLock according to this article: http://blogs.techrepublic.com.com/pr...elopment/?p=86

A word of warning: as the test result show, Mutex is an extremely slow system! You want to be very careful when using Mutex. According to MSDN Magazine (September, 2006), Mutex can take 9801.6 CPU cycles to acquire a lock on a CPU without contention. In comparison to 112 CPU cycles for a Win32 CRITICAL_SECTION (which SyncLock uses), it is easy to see why the decision to use Mutex must be careful weighed

However I do think they are not quite correct about SyncLock using the Win32 CRITICAL_SECTION structure (see post #13), but their tests still prove that SyncLock is much faster than Mutex.

EDIT: After reading quite a few MSDN articles and blog posts on the subject, I can confirm that SyncLock does not use the Win32 CRITICAL_SECTION as is claimed by that article mentioned above. They obviously do not understand that a SyncLock uses the Monitor class either as they conducted separate tests for SyncLock and Monitor, but like I said - their tests still show how much slower a Mutex is than SyncLock/Monitor.

Also see this graph from an MSDN article discussing how much processor time each option uses (remember that a Monitor is what SyncLock uses so its the first item in the graph that we are comparing with the last item in the graph):

[dbasnett]

    12/25/09  09:04:28.433    CPU - 1.794GHz.   Stopwatch(HR) Freq. is 3,579,545
    x86 Family 6 Model 13 Stepping 6    Address width - 32   Memory - 2.0GB
    
    One - Monitor
    Two - SyncLock
    Number of Test(NT) = 7    Iterations Per NT(IL) = 1,048,576
    Times given in ticks.  1 ms. = 10,000 ticks;  1,000 ms. = 1 second.
    
    Ticks / Loop     Fastest? One
    0.0040        	0.3428        	0.5056        	-0.1628
    NT Base IL    	One IL        	Two IL        	 Difference (One - Two)
    1   4,238         	351,193       	529,387       	-178,194
    2   4,210         	352,212       	529,212       	-177,000
    3   4,211         	351,027       	533,413       	-182,386
    4   4,211         	407,650       	529,245       	-121,595
    5   4,208         	350,983       	528,916       	-177,933
    6   4,232         	352,004       	529,359       	-177,355
    7   4,208         	350,819       	531,909       	-181,090
    
     Average
    4,216         	359,412       	530,205       	-170,793    (-17.079 ms./IL)
    
    12/25/09  09:04:30.217   RunMode = CTRL-F5   End Dewayne Relative Speed Test
    Source Code Follows
        Dim Lock As New Object
        Dim x As Integer
        Private Function TestCase1() As Boolean 'One
            'Test One Code Follows
            Monitor.Enter(Lock)
            x = 1
            Monitor.Exit(Lock)
            'End Test One Code
            Return True
        End Function
        '
        Private Function TestCase2() As Boolean 'Two
            'Test Two Code Follows
            SyncLock Lock
                x = 1
            End SyncLock
            'End Test Two Code
            Return True
        End Function
        '
        Private Sub _init()
            'Code needed to get the Test to run.  NOT part of the timing.
            'Only called once!!!
        End Sub

[pacerier]

No that is not the case, when you come out of the scope of a SyncLock block the lock will be released - it doesnt have to actually hit the End SyncLock line. Just like if you use a Using statement, you dont have to hit the End Using line for the object to be disposed, its just when you come out of the scope of that statement. I assume this behaviour is achieved by the fact that internally the SyncLock uses a Try/Catch/Finally block and releases the lock in the Finally block. You can see the same behaviour yourself if you just do something like this inside a Function:

what if i had something like this:

1:                              SyncLock Lock
                                    GoTo 1
                                End SyncLock

does the thread leaves synclock when it hits GoTo 1 or does he enters the lock 2 times, den 3, den 4 ..

[chris128]

I'm pretty sure it would release the lock when it jumps out of the SyncLock block. Although I think if you had a method call in there like the code below for example, then the lock would not be released until after the method had been executed and you actually reached the End SyncLock statement:

SyncLock Something
       MyMethod()
End SyncLock

Sub MyMethod()
    'Do some stuff...
    Dim something As String = "Hello"
    'I think the lock is still held at this point
End Sub

[dbasnett]

try it, then you will know:

        Dim lock As New Object
        Dim i As Integer = 0
1:      SyncLock lock
            i += 1
            If i < 10 Then GoTo 1
        End SyncLock
        Debug.WriteLine("made it")

I am not advocating the use of GoTo BTW.

[chris128]

try it, then you will know:

        Dim lock As New Object
        Dim i As Integer = 0
1:      SyncLock lock
            i += 1
            If i < 10 Then GoTo 1
        End SyncLock
        Debug.WriteLine("made it")

I am not advocating the use of GoTo BTW.

That doesnt prove anything about when the lock is released though...