I think i'm missing some basics in here and unable to fig out the problem..
Output of below program is not as expected. Can some one help me understand the issue here please.
using System;
using System.Threading;
public class Program
{
private static readonly object _lock = new object();
public static void Main()
{
for (var i = 0; i < 10; i++)
{
//Console.WriteLine("Start "+i);
System.Threading.Thread thread = new System.Threading.Thread(() => ExecuteInBackground(i));
thread.IsBackground = true;
thread.Start();
}
}
private static void ExecuteInBackground(Object obj)
{
lock (_lock)
{
Console.WriteLine("A "+obj);
test.ttt(obj);
}
}
}
public static class test
{
public static void ttt(object obj)
{
Console.WriteLine("B "+ obj);
}
}
I'm expecting to see 0 To 9 in output.. But actual output is as follows..
A 1
B 1
A 1
B 1
A 3
B 3
A 4
B 4
A 5
B 5
A 6
B 6
A 7
B 7
A 8
B 8
A 9
B 9
A 10
B 10
Any help is greatly appreciated.
Please feel free to play around with code in https://dotnetfiddle.net/nYfbMU
Thanks,
Reddy.
Change this:
for (var i = 0; i < 10; i++)
{
//Console.WriteLine("Start "+i);
System.Threading.Thread thread = new System.Threading.Thread(() => ExecuteInBackground(i));
to this:
for (var i = 0; i < 10; i++)
{
var temp = i;
//Console.WriteLine("Start "+i);
System.Threading.Thread thread = new System.Threading.Thread(() => ExecuteInBackground(temp));
This is a closure issue. See Why is it bad to use an iteration variable in a lambda expression
The reason the original code doesn't work as you expected, and why the temp variable does, is because () => ExecuteInBackground(i) is like saying "at some point in the future, I want this new thread to call the ExecuteInBackground method, passing in whatever value i has when that call is made". Since the loop variable goes into scope at the start of the loop, and out of scope after the loop is finished, the value of i changes between the time you call Thread, and when ExecuteInBackground executes. By using a temp variable inside the loop, that goes out of scope with every iteration of the loop, each thread's call to ExecuteInBackground is essentially getting a different variable with an unchanging value with each call, and the next incrementing of i doesn't mess things up.
This worked for me..
using System;
using System.Threading;
public class Program
{
private static readonly object _lock = new object();
public static void Main()
{
for (var i = 0; i <= 10; i++)
{
fn(i);
}
Console.ReadLine();
}
private static void fn(int i)
{
System.Threading.Thread thread = new System.Threading.Thread(() => ExecuteInBackground(i));
thread.IsBackground = true;
thread.Start();
}
private static void ExecuteInBackground(Object obj)
{
lock (_lock)
{
Thread.Sleep(500);
Console.WriteLine("A "+obj);
test.ttt(obj);
}
}
}
public static class test
{
//private static readonly object _lock = new object();
public static void ttt(object obj)
{
//lock(_lock)
Console.WriteLine("B "+ obj);
}
}
Related
I am starting with threads and wrote for the sake of learning the following simple program, which later would be used to calculate about 100,000 times a formula (it is a relatively simple one but which takes an iterated range of values).
The problem with it is that I expected every thread to execute in almost no time and thus the complete program to finish nearly immediately, but the fact is that everything runs too slow (about 10s)...
static readonly double TotalIterations = 1000;
public static Iterations ActualIterations = new Iterations();
public static void Main()
{
var par1 = "foo";
var par2 = "boo";
var par3 = 3;
for (int i = 0; i < TotalIterations; i++)
{
new Thread(() => new Calculations().Calculate(par1, par2, par3)).Start();
}
AwaitingThreads();
}
static void AwaitThreads()
{
Console.WriteLine("Awaiting threads to finished...");
while (true)
{
lock (ActualIterations)
{
if (ActualIterations.Progress() == TotalIterations) break;
}
Thread.Sleep(1 * 1000);
}
Console.WriteLine("All threads finished!");
}
public class Calculations {
public bool Calculate(string par1, string par2, int par3)
{
// ...
bool result = false;
lock (ActualIterations)
{
ActualIterations.Incr();
}
return result;
}
}
public class Iterations
{
int progress = 0;
public void Incr()
{
progress++;
}
public int Progress()
{
return progress;
}
}
I also tried using a ThreadPool like this, but there was no improvement...
static readonly double TotalIterations = 1000;
static string par1 = "foo";
static string par2 = "boo";
static int par3 = 3;
public static Iterations ActualIterations = new Iterations();
public static void Main()
{
ThreadPool.QueueUserWorkItem(MyThreadPool);
AwaitThreads();
}
static void AwaitThreads()
{
Console.WriteLine("Awaiting threads to finished...");
while (true)
{
lock (ActualIterations)
{
if (ActualIterations.Progress() == TotalIterations) break;
}
Thread.Sleep(1 * 1000);
}
Console.WriteLine("All threads finished!");
}
static void MyThreadPool(Object stateInfo)
{
for (int i = 0; i < TotalIterations; i++)
{
new Thread(() => new Calculations().Calculate(par1, par2, par3)).Start();
}
}
public class Calculations {
public bool Calculate(string par1, string par2, int par3)
{
// ...
bool result = false;
lock (ActualIterations)
{
ActualIterations.Incr();
}
return result;
}
}
public class Iterations
{
int progress = 0;
public void Incr()
{
progress++;
}
public int Progress()
{
return progress;
}
}
When I quit using threads in this example and use a static method, executing it sequentially in my for loop, the program finishes in 1s...
Can anybody enlighten me what I am doing wrong here with those threads?
The problem with it is that I expected every thread to execute in almost no time
Right. You're ignoring the fact that creating a new thread is a relatively expensive operation. Far, far more expensive than "acquiring a lock and incrementing an integer" which is the work you're doing in the thread.
To give a real world comparison, it's a little like ordering a new car, waiting it to be delivered, and then driving it 1km. That's going to be slower than just walking 1km.
Using the thread pool would be faster, but you're not using it correctly - you're launching one thread pool task which then creates all the other threads again.
I would encourage you to look at using Task<T> instead, which normally uses the thread pool under the hood, and is a generally more modern abstraction for this sort of work.
This is the way to proceed doing what you wanted to do:
class Program
{
static void Main(string[] args)
{
List<Task> tasks = new List<Task>();
for (int i = 0; i < 1000; i++)
{
tasks.Add(Task.Run(() =>
{
Console.WriteLine("Calculations " + DateTime.Now);
}));
}
Task.WaitAll(tasks.ToArray());
}
}
Tasks are actually optimized and programmer-friendly to use if you need to work with threads.
Another advice i want to give you is to create an Object just for locking purposes, example:
class Program
{
private static Object _locker = new Object();
static void Main(string[] args)
{
List<Task> tasks = new List<Task>();
for (int i = 0; i < 1000; i++)
{
tasks.Add(Task.Run(() =>
{
lock (_locker)
{
Console.WriteLine("Calculations " + DateTime.Now);
}
}));
}
Task.WaitAll(tasks.ToArray());
}
}
I see the problem in the AwaitThreads method.
It uses the same lock (ActualIterations) as working thread and it makes working threads to wait for shared resource additionally.
Also (as it was mentioned by #Euphoric) the thread working code you have shown is just about single increment and it uses the shared resource between all threads.
You have to change it in some another way and try to avoid shared resource usage in multi threaded environment.
For example, if you need to make some calculation on huge data array you have to feed each thread own data part to be processed and then wait for all tasks to be finished. There is Task concept and Task.WaitAll
Hello i am trying to pass the current index of a loop inside a thread lambda method and print it.The method will print only the last value of the index.
class Program {
public static EventWaitHandle handle = new EventWaitHandle(false, EventResetMode.AutoReset);
public static EventWaitHandle autohandle = new EventWaitHandle(false, EventResetMode.AutoReset);
static readonly int ThreadNum=3;
static void Main(string[] args) {
object lk = new object();
new Thread(() => {
while (true) {
var key = Console.ReadKey();
if(key.Key==ConsoleKey.A) {
handle.Set();
} else {
handle.Reset();
}
Thread.Sleep(3000);
}
}).Start();
for(int i=0;i<ThreadNum;i++) {
new Thread(() => {
int val = i;
Console.WriteLine($"Thread:{val} created");
while (true) {
handle.WaitOne();
Console.WriteLine($"From thread:{val}");
Thread.Sleep(1000);
}
}).Start();
}
Console.WriteLine("Hello World!");
}
}
Can someone explain to me why would i get only the last value of the index.I understand the index gets clojured (a class gets created which copies the index value) but when the first iteration enters the thread method it should clojure i=0 and keep it that way.
I think that the behaviour you are seeing is because the loop is iterating before the local variable 'val' is assigned. So by the time the statement
int val = i;
is executed for the first time, the loop has already iterated 3 times, so you get 'val' set to the last value of i.
When I run it, I get variable behaviour, due to the relative speed at which threads are created.
To get the behaviour I think you want, you need to capture the count of the loop iteration locally, like this.
class Program
{
public static EventWaitHandle handle = new EventWaitHandle(false, EventResetMode.AutoReset);
public static EventWaitHandle autohandle = new EventWaitHandle(false, EventResetMode.AutoReset);
static readonly int ThreadNum = 3;
static void Main(string[] args)
{
object lk = new object();
new Thread(() => {
while (true)
{
var key = Console.ReadKey();
if (key.Key == ConsoleKey.A)
{
handle.Set();
}
else
{
handle.Reset();
}
Thread.Sleep(3000);
}
}).Start();
for (int i = 0; i < ThreadNum; i++)
{
int temp = i;
new Thread(() => ThreadMethod(temp)).Start();
}
Console.WriteLine("Hello World!");
}
private static void ThreadMethod(object obj)
{
int val = (int)obj;
Console.WriteLine($"Thread:{val} created");
while (true)
{
handle.WaitOne();
Console.WriteLine($"From thread:{val}");
Thread.Sleep(1000);
}
}
}
I create an example about thread,
I know that use lock could avoid thread suspending at critical section, but I have two questions.
1.Why my program get stuck if I use Thread.Sleep?
In this example, I add sleep to two thread.
Because I wish the console output more slowly, so I can easily see if there's anything wrong.
But if I use Thread.Sleep() then this program will get stuck!
2.What situation should I use Thread.Sleep?
Thanks for your kind response, have a nice day.
class MyThreadExample
{
private static int count1 = 0;
private static int count2 = 0;
Thread t1;
Thread t2;
public MyThreadExample() {
t1 = new Thread(new ThreadStart(increment));
t2 = new Thread(new ThreadStart(checkequal));
}
public static void Main() {
MyThreadExample mt = new MyThreadExample();
mt.t1.Start();
mt.t2.Start();
}
void increment()
{
lock (this)
{
while (true)
{
count1++; count2++;
//Thread.Sleep(0); stuck when use Sleep!
}
}
}
void checkequal()
{
lock (this)
{
while (true)
{
if (count1 == count2)
Console.WriteLine("Synchronize");
else
Console.WriteLine("unSynchronize");
// Thread.Sleep(0);
}
}
}
}
Please take a look at these following codes. Never use lock(this), instead use lock(syncObj) because you have better control over it. Lock only the critical section (ex.: only variable) and dont lock the whole while loop. In method Main, add something to wait at the end "Console.Read()", otherwise, your application is dead. This one works with or without Thread.Sleep. In your code above, your thread will enter "Increment" or "Checkequal" and the lock will never release. Thats why, it works only on Increment or Checkequal and never both.
internal class MyThreadExample
{
private static int m_Count1;
private static int m_Count2;
private readonly object m_SyncObj = new object();
private readonly Thread m_T1;
private readonly Thread m_T2;
public MyThreadExample()
{
m_T1 = new Thread(Increment) {IsBackground = true};
m_T2 = new Thread(Checkequal) {IsBackground = true};
}
public static void Main()
{
var mt = new MyThreadExample();
mt.m_T1.Start();
mt.m_T2.Start();
Console.Read();
}
private void Increment()
{
while (true)
{
lock (m_SyncObj)
{
m_Count1++;
m_Count2++;
}
Thread.Sleep(1000); //stuck when use Sleep!
}
}
private void Checkequal()
{
while (true)
{
lock (m_SyncObj)
{
Console.WriteLine(m_Count1 == m_Count2 ? "Synchronize" : "unSynchronize");
}
Thread.Sleep(1000);
}
}
}
Thread is a little bit old style. If you are a beginner of .NET and using .NET 4.5 or above, then use Task. Much better. All new multithreading in .NET are based on Task, like async await:
public static void Main()
{
var mt = new MyThreadExample();
Task.Run(() => { mt.Increment(); });
Task.Run(() => { mt.Checkequal(); });
Console.Read();
}
I have a main task that is spawning threads to do some work. When the work is completed it will write to the console.
My problem is that some of the threads that are created later will finish faster than those created earlier. However I need the writing to the console to be done in the same exact sequence as the thread was created.
So if a thread had completed its task, while some earlier threads had not, it has to wait till those earlier threads complete too.
public class DoRead
{
public DoRead()
{
}
private void StartReading()
{
int i = 1;
while (i < 10000)
{
Runner r = new Runner(i, "Work" + i.ToString());
r.StartThread();
i += 1;
}
}
}
internal class Runner : System.IDisposable
{
int _count;
string _work = "";
public Runner(int Count, string Work)
{
_count = Count;
_work = Work;
}
public void StartThread()
{
ThreadPool.QueueUserWorkItem(new WaitCallback(runThreadInPool), this);
}
public static void runThreadInPool(object obj)
{
((Runner)obj).run();
}
public void run()
{
try
{
Random r = new Random();
int num = r.Next(1000, 2000);
DateTime end = DateTime.Now.AddMilliseconds(num);
while (end > DateTime.Now)
{
}
Console.WriteLine(_count.ToString() + " : Done!");
}
catch
{
}
finally
{
_work = null;
}
}
public void Dispose()
{
this._work = null;
}
}
There may be a simpler way to do this than I used, (I'm used to .Net 4.0).
using System;
using System.Collections.Generic;
using System.Text;
using System.Threading;
namespace ConsoleApplication5
{
class Program
{
public static readonly int numOfTasks = 100;
public static int numTasksLeft = numOfTasks;
public static readonly object TaskDecrementLock = new object();
static void Main(string[] args)
{
DoRead dr = new DoRead();
dr.StartReading();
int tmpNumTasks = numTasksLeft;
while ( tmpNumTasks > 0 )
{
Thread.Sleep(1000);
tmpNumTasks = numTasksLeft;
}
List<string> strings = new List<string>();
lock( DoRead.locker )
{
for (int i = 1; i <= Program.numOfTasks; i++)
{
strings.Add( DoRead.dicto[i] );
}
}
foreach (string s in strings)
{
Console.WriteLine(s);
}
Console.ReadLine();
}
public class DoRead
{
public static readonly object locker = new object();
public static Dictionary<int, string> dicto = new Dictionary<int, string>();
public DoRead()
{
}
public void StartReading()
{
int i = 1;
while (i <= Program.numOfTasks )
{
Runner r = new Runner(i, "Work" + i.ToString());
r.StartThread();
i += 1;
}
}
}
internal class Runner : System.IDisposable
{
int _count;
string _work = "";
public Runner(int Count, string Work)
{
_count = Count;
_work = Work;
}
public void StartThread()
{
ThreadPool.QueueUserWorkItem(new WaitCallback(runThreadInPool), this);
}
public static void runThreadInPool(object obj)
{
Runner theRunner = ((Runner)obj);
string theString = theRunner.run();
lock (DoRead.locker)
{
DoRead.dicto.Add( theRunner._count, theString);
}
lock (Program.TaskDecrementLock)
{
Program.numTasksLeft--;
}
}
public string run()
{
try
{
Random r = new Random();
int num = r.Next(1000, 2000);
Thread.Sleep(num);
string theString = _count.ToString() + " : Done!";
return theString;
}
catch
{
}
finally
{
_work = null;
}
return "";
}
public void Dispose()
{
this._work = null;
}
}
}
}
Basically, I store the string you want printed from each task into a dictionary where the index is the task#. (I use a lock to make accessing the dictionary safe).
Next, so that the main program waits until all the background threads are done, I used another locked access to a NumTasksLeft variable.
I added stuff into the callback for the Runner.
It is bad practice to use busy loops, so I changed it to a Thread.Sleep( num ) statement.
Just change numOfTasks to 10000 to match your example.
I pull the return strings out of the dictionary in order, and then print it to the screen.
I'm sure you could refactor this to move or otherwise deal with the global variables, but this works.
Also, you might have noticed I didn't use the lock in the command
tmpNumTasks = numTasksLeft;
That's threadsafe, since numTasksLeft is an int which is read atomically on 32-bit computers and higher.
I don't know much on C#, but the whole idea of multi-threading is that you have multiple thread executing independently and you can never know which one will finish earlier (and you shouldn't expect earlier thread to end earlier).
One workaround is, instead writing out the finish message in the processing thread, have the processing thread setup a flag somewhere (probably a list with no of elements = no of thread spawned), and have a separate thread print out the finish message base on the flags in that list, and report up to the position that previous flag is consecutively "finished".
Honestly I don't feel that reasonable for you to print finish message like this anyway. I think changing the design is way better to have such meaningless "feature".
Typically, such requirements are met with an incrementing sequence number, much as you have already done.
Usually, the output from the processing threads is fed through a filter object that contains a list, (or dictionary), of all out-of-order result objects, 'holding them back' until all results with a lower seqeuence-number have come in. Again, similar to what you have already done.
What is not necessary is any kind of sleep() loop. The work threads themselves can operate the filter object, (which would beed a lock), or the work threads can producer-consumer-queue the results to an 'output thread' that operates the out-of-order filter.
This scheme works fine with pooled work threads, ie. those without continual create/terminate/destroy overhead.
I have a sample application and wonder if anyone can shine a light on this. When I put a breakpoint on the for loop an step into to the code in the debugger, why does it switch from one thread to another? it does so when running the application at runtime...Please see code below:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace TPLSample
{
class Program
{
static void Main(string[] args)
{
Management m = new Management();
Task a = new Task(() => m.Operation1());
a.Start();
Task b = new Task(() => m.Operation2());
b.Start();
Console.ReadLine();
}
}
public class Management
{
A a = null;
B b = null;
public void Operation1()
{
a = new A();
}
public void Operation2()
{
b = new B();
}
}
public class A
{ Client a = new Client();
public A()
{
while (true)
{
a.Test("Im AAAAA");
}
}
}
public class B
{
Client a = new Client();
public B()
{
while (true)
{
a.Test("Im BBBBB");
}
}
}
public class Client
{
Object ibj = new Object();
public void Test(string item)
{
lock (ibj)
{
for (int i = 0; i < 200000; i++)
{
Console.WriteLine(item);
}
}
}
}
}
The result for this is a mixture of As and Bs. Isnt the lock suppose to block the thread so that the result will end up in sequence? The same is happening in an application I am writing except that the WHILE loops run forever (each task needs to poll continuosly). Please note that I am starting two tasks because I dont want one WHILE loop to prevent the other from running if the WHILE loop is to run forever. How do I make them go in sequence to that function in CLIENT class?
This happens because if you trace your object creation tree carefully you will notice that each thread is using a different lock object, thus rendering your lock statement useless.
Your object creation happens like this:
new A() -> new Client() -> new Object()
/
new Management()
\
new B() -> new Client() -> new Object()
You are using the last two objects on the right as locks, and you can clearly see that they are different objects.
Try to make the lock static:
public class Client
{
static Object ibj = new Object();
...
or otherwise rethink your hierarchy to pass the same lock to both Tasks.
Each object must have access to the same object to make the lock work. You can do this by making the ibj object static, or passing it into the Client class.
public class Client
{
static Object ibj = new Object();
public void Test(string item)
{
lock (ibj)
{
for (int i = 0; i < 200000; i++)
{
Console.WriteLine(item);
}
}
}
}