Threading / random number problem

[Mark Rae]

Hi,

My R&D department has asked me to look at threading in a Web Service written
in C#, so I came up with the following code:

using System;
using System.Componen tModel;
using System.Threadin g;
using System.Web.Serv ices;

namespace CWSThreading
{
public class CThreading : System.Web.Serv ices.WebService
{
private double mdThread1;
private double mdThread2;

public CThreading()
{
//CODEGEN: This call is required by the ASP.NET Web Services Designer
InitializeCompo nent();
}

[WebMethod]
public string TestThreading()
{
string strOutput = "";

Thread thread1 = new Thread(new ThreadStart(Ran domNumber1));
thread1.Start() ;
strOutput += "Result from 1st thread: " + mdThread1.ToStr ing() + "\n";

Thread thread2 = new Thread(new ThreadStart(Ran domNumber2));
thread2.Start() ;
strOutput += "Result from 2nd thread: " + mdThread2.ToStr ing() + "\n";

return strOutput;
}

protected void RandomNumber1()
{
System.Random rndNumber = new Random((int)Dat eTime.Now.Ticks );
mdThread1 = rndNumber.NextD ouble();
}

protected void RandomNumber2()
{
System.Random rndNumber = new Random((int)Dat eTime.Now.Ticks );
mdThread2 = rndNumber.NextD ouble();
}
}
}

This is being called by a Windows app to which a web reference has been
added pointing to the above web service. A simple form with a button runs
the following code:

using TestWSThreading .wsThreading;

private void cmdTestWSThread ing_Click(objec t sender, System.EventArg s e)
{
wsThreading.CTh reading wsTestThreading = new CThreading();
string strOutput = wsTestThreading .TestThreading( );
MessageBox.Show (strOutput);
wsTestThreading = null;
}
If I set a breakpoint anywhere in the above code which causes me to step
through it, it always returns different random numbers e.g.:

Result from 1st thread: 0.5133791689357 62
Result from 2nd thread: 0.2027999298660 08

However, if I remove the breakpoint so that the web service runs "normally",
I always get the following result:

Result from 1st thread: 0
Result from 2nd thread: 0

Any assistance gratefully received.

Best regards,

Mark Rae

[Sami Vaaraniemi]

Comments inline:

"Mark Rae" <ma**@markrae.c o.uk> wrote in message
news:%2******** ********@TK2MSF TNGP10.phx.gbl. ..

Hi,

My R&D department has asked me to look at threading in a Web Service written in C#, so I came up with the following code:

using System;
using System.Componen tModel;
using System.Threadin g;
using System.Web.Serv ices;

namespace CWSThreading
{
public class CThreading : System.Web.Serv ices.WebService
{
private double mdThread1;
private double mdThread2;

public CThreading()
{
//CODEGEN: This call is required by the ASP.NET Web Services Designer
InitializeCompo nent();
}

[WebMethod]
public string TestThreading()
{
string strOutput = "";

Thread thread1 = new Thread(new ThreadStart(Ran domNumber1));
thread1.Start() ;
strOutput += "Result from 1st thread: " + mdThread1.ToStr ing() + "\n";

Thread thread2 = new Thread(new ThreadStart(Ran domNumber2));
thread2.Start() ;
strOutput += "Result from 2nd thread: " + mdThread2.ToStr ing() + "\n";

return strOutput;
}
The main issue here as far as I see is that you start the threads and then
immediately read the variables that the threads are supposed to assign
values. There is no guarantee that the threads execute before you read the
variables. To fix this, you should wait for the threads to complete before
you access the variables:

// start both threads
Thread thread1 = new Thread(new ThreadStart(Ran domNumber1));
thread1.Start() ;
Thread thread2 = new Thread(new ThreadStart(Ran domNumber2));
thread2.Start() ;

// wait for thread1 to complete, read its results
thread1.Join();
strOutput += "Result from 1st thread: " + mdThread1.ToStr ing() + "\n";

// wait for thread2 to complete, read its results
thread2.Join();
strOutput += "Result from 2nd thread: " + mdThread2.ToStr ing() + "\n";

If I set a breakpoint anywhere in the above code which causes me to step
through it, it always returns different random numbers e.g.:

Result from 1st thread: 0.5133791689357 62
Result from 2nd thread: 0.2027999298660 08

However, if I remove the breakpoint so that the web service runs "normally", I always get the following result:

Result from 1st thread: 0
Result from 2nd thread: 0

Any assistance gratefully received.

The results are highly unpredictable unless you do some sort of
synchronization . You're getting 0s because the main thread reads the
variables before the background threads have a chance to execute. Use
Thread.Join to synchronize the threads.

Sami
www.capehill.net

[Mark Rae]

"Sami Vaaraniemi" <sa************ ***@jippii.fi> wrote in message
news:c2******** **@phys-news1.kolumbus. fi...

Comments inline:
Thanks for the response.
The main issue here as far as I see is that you start the threads and then
immediately read the variables that the threads are supposed to assign
values. There is no guarantee that the threads execute before you read the
variables. To fix this, you should wait for the threads to complete before
you access the variables:

// start both threads
Thread thread1 = new Thread(new ThreadStart(Ran domNumber1));
thread1.Start() ;
Thread thread2 = new Thread(new ThreadStart(Ran domNumber2));
thread2.Start() ;

// wait for thread1 to complete, read its results
thread1.Join();
strOutput += "Result from 1st thread: " + mdThread1.ToStr ing() + "\n";

// wait for thread2 to complete, read its results
thread2.Join();
strOutput += "Result from 2nd thread: " + mdThread2.ToStr ing() + "\n";
I did as you suggested. Now, I don't get zeroes any more, but now the two
threads always return the same value e.g.

Result from 1st thread: 0.5337773345102 45
Result from 2nd thread: 0.5337773345102 45
The results are highly unpredictable unless you do some sort of
synchronization . You're getting 0s because the main thread reads the
variables before the background threads have a chance to execute. Use
Thread.Join to synchronize the threads.

I'm obviously missing something else... :-)

Mark

[Steve McLellan]

Random's being seeded from the current time, and both your threads are
essentially executing at the same time (give or take) so it's possible
they're getting seeded with the same number. It does seem a bit strange
(since you'd think that Tick would never return the same value even if the
execution times are very similar). That would be my best guess.

Steve

"Mark Rae" <ma**@markrae.c o.uk> wrote in message
news:Oa******** ******@TK2MSFTN GP11.phx.gbl...

"Sami Vaaraniemi" <sa************ ***@jippii.fi> wrote in message
news:c2******** **@phys-news1.kolumbus. fi...

Comments inline:

Thanks for the response.

The main issue here as far as I see is that you start the threads and then immediately read the variables that the threads are supposed to assign
values. There is no guarantee that the threads execute before you read the variables. To fix this, you should wait for the threads to complete before you access the variables:

// start both threads
Thread thread1 = new Thread(new ThreadStart(Ran domNumber1));
thread1.Start() ;
Thread thread2 = new Thread(new ThreadStart(Ran domNumber2));
thread2.Start() ;

// wait for thread1 to complete, read its results
thread1.Join();
strOutput += "Result from 1st thread: " + mdThread1.ToStr ing() + "\n";

// wait for thread2 to complete, read its results
thread2.Join();
strOutput += "Result from 2nd thread: " + mdThread2.ToStr ing() + "\n";

I did as you suggested. Now, I don't get zeroes any more, but now the two
threads always return the same value e.g.

Result from 1st thread: 0.5337773345102 45
Result from 2nd thread: 0.5337773345102 45

The results are highly unpredictable unless you do some sort of
synchronization . You're getting 0s because the main thread reads the
variables before the background threads have a chance to execute. Use
Thread.Join to synchronize the threads.

I'm obviously missing something else... :-)

Mark

[Sami Vaaraniemi]

"Mark Rae" <ma**@markrae.c o.uk> wrote in message
news:Oa******** ******@TK2MSFTN GP11.phx.gbl...

I did as you suggested. Now, I don't get zeroes any more, but now the two
threads always return the same value e.g.

Result from 1st thread: 0.5337773345102 45
Result from 2nd thread: 0.5337773345102 45

The results are highly unpredictable unless you do some sort of
synchronization . You're getting 0s because the main thread reads the
variables before the background threads have a chance to execute. Use
Thread.Join to synchronize the threads.

I'm obviously missing something else... :-)

The reason for the same number being returned is that the .NET Random number
generator is a so-called pseudo-random generator. Given the same seed it
always generates the same sequence of "random" numbers. So the numbers are
not really random (whatever that means), they are just evenly distributed.
You are getting the same random number because the threads happen to
initialize the generator with the same seed value.

To ensure unique seeds for the random number generator, you could use e.g.,
the thread ID (AppDomain.GetC urrentThreadId or Thread.GetHashC ode()) as the
seed. You could also combine DateTime.Now.Ti cks with the thread ID somehow.

Sami
www.capehill.net

[Morten Wennevik]

Considering that

DateTime.Ticks:
"The value of this property is the number of 100-nanosecond intervals that
have elapsed since 12:00 A.M., January 1, 0001."

and 3GHz computers have clock cycles of ~3 nanoseconds it's quite possible
to get a random initiated with the same seed if you do it close enough.

--
Using M2, Opera's revolutionary e-mail client: http://www.opera.com/m2/

[Mark Rae]

"Steve McLellan" <sj*@fixerlabs. com.NOSPAM> wrote in message
news:uj******** *****@tk2msftng p13.phx.gbl...

Random's being seeded from the current time, and both your threads are
essentially executing at the same time (give or take) so it's possible
they're getting seeded with the same number. It does seem a bit strange
(since you'd think that Tick would never return the same value even if the
execution times are very similar). That would be my best guess.

Thanks for the response. Yes, the reason I was using the code

System.Random rndNumber = new Random((int)Dat eTime.Now.Ticks );

was in the hope that it would always generate a different number...

[Mark Rae]

"Sami Vaaraniemi" <sa************ ***@jippii.fi> wrote in message
news:c2******** **@phys-news1.kolumbus. fi...

Thanks for the response.

The reason for the same number being returned is that the .NET Random number generator is a so-called pseudo-random generator. Given the same seed it
always generates the same sequence of "random" numbers. So the numbers are
not really random (whatever that means), they are just evenly distributed.
You are getting the same random number because the threads happen to
initialize the generator with the same seed value.
I understand.
To ensure unique seeds for the random number generator, you could use e.g., the thread ID (AppDomain.GetC urrentThreadId or Thread.GetHashC ode()) as the seed. You could also combine DateTime.Now.Ti cks with the thread ID

somehow.

Yes - I changed part of the code to:

thread1.Join();
strOutput += "Result from 1st thread: " + Convert.ToStrin g((mdThread1 /
thread1.GetHash Code()) * 1000) + "\n";
thread2.Join();
strOutput += "Result from 2nd thread: " + Convert.ToStrin g((mdThread2 /
thread2.GetHash Code()) * 1000) + "\n";

and it now produces different (albeit very similar!) numbers every time.

Job done - thanks for your help.

Mark

[Mark Rae]

"Morten Wennevik" <Mo************ @hotmail.com> wrote in message
news:op******** ******@msnews.m icrosoft.com...

Considering that

DateTime.Ticks:
"The value of this property is the number of 100-nanosecond intervals that
have elapsed since 12:00 A.M., January 1, 0001."

and 3GHz computers have clock cycles of ~3 nanoseconds it's quite possible
to get a random initiated with the same seed if you do it close enough.

You're right - I hadn't considered that! It's been a while since I
encountered a problem because my development machine was too fast... :-)

[Jon Skeet [C# MVP]]

Mark Rae <ma**@markrae.c o.uk> wrote:

Yes - I changed part of the code to:

thread1.Join();
strOutput += "Result from 1st thread: " + Convert.ToStrin g((mdThread1 /
thread1.GetHash Code()) * 1000) + "\n";
thread2.Join();
strOutput += "Result from 2nd thread: " + Convert.ToStrin g((mdThread2 /
thread2.GetHash Code()) * 1000) + "\n";

and it now produces different (albeit very similar!) numbers every time.

That's not changed the seed at all - that's just divided the *result*
by the hash code.

Instead, it would be better to have *one* random number generator, and
serialize access to it. You only need to have one seed then, and using
the current time for that is reasonable.

--
Jon Skeet - <sk***@pobox.co m>
http://www.pobox.com/~skeet
If replying to the group, please do not mail me too