Develop Reference

Can this code cause a dead lock ?

using System;
using System.Threading;
namespace Threading
{
class Program
{
static void Main(string[] args)
{
Semaphore even = new Semaphore(1, 1);
Semaphore odd = new Semaphore(1, 1);
Thread evenThread = new Thread(() =>
{
for (int i = 1; i <= 100; i++)
{
even.WaitOne();
if(i % 2 == 0)
{
Console.WriteLine(i);
}
odd.Release();
}
});
Thread oddThread = new Thread(() =>
{
for(int i = 1; i <=100; i++)
{
odd.WaitOne();
if(i%2 != 0)
{
Console.WriteLine(i);
}
even.Release();
}
});
oddThread.Start();
evenThread.Start();
}
}
}
So I have written this code where one thread is producing Odd numbers and other is producing even numbers.
Using Semaphores I have made sure that they print numbers in orders and it works perfectly.
But I have a special situation in mind, for example each thread waits until the other thread releases its semaphore. So can there be a condition where both threads are waiting and no thread is making any progress and there is a deadlock situation ?

For deadlock to occur, two or more threads must be trying to acquire two or more resources, but do so in different orders. See e.g. Deadlock and Would you explain lock ordering?.
Your code does not involve more than one lock per thread† and so does not have the ability to deadlock.
It does have the ability to throw an exception. As noted in this comment, it is theoretically possible for one of the threads to get far enough ahead of the other thread that it attempts to release a semaphore lock that hasn't already been taken. For example, if evenThread is pre-empted (or simply doesn't get scheduled to start running) before it gets to its first call to even.WaitOne(), but oddThread gets to run, then oddThread can acquire the odd semaphore, handle the if statement, and then try to call even.Release() before evenThread has had a chance to acquire that semaphore.
This will result in a SemaphoreFullException being thrown by the call to Release().
This would be a more likely possibility on a single-CPU system, something that is very hard to find these days. :) But it's still theoretically possible for any CPU configuration.
† Actually, there's an implicit lock in the Console.WriteLine() call, which is thread-safe by design. But from your code's point of view, that's an atomic operation. It's not possible for your code to acquire that lock and then wait on another. So it doesn't have any relevance to your specific question.

Can a second thread enter the same critical section just because the first thread called Monitor.Wait using the same sync lock?

Please tell me if I am thinking it alright.
A different thread cannot enter the same critical section using
the same lock just because the first thread called Monitor.Wait, right? The Wait method only allows a different thread to acquire
the same monitor, i.e. the same synchronization lock but only for a different critical section and never for the same critical
section.
Is my understanding correct?
Because if the Wait method meant that anyone can now enter this
same critical section using this same lock, then that would defeat
the whole purpose of synchronization, right?
So, in the code below (written in notepad, so please forgive any
typos), ThreadProc2 can only use syncLock to enter the code in
ThreadProc2 and not in ThreadProc1 while the a previous thread
that held and subsequently relinquished the lock was executing
ThreadProc1, right?
Two or more threads can use the same synchronization lock to run
different pieces of code at the same time, right? Same question as
above, basically, but just confirming for the sake of symmetry with
point 3 below.
Two or more threads can use a different synchronization lock to
run the same piece of code, i.e. to enter the same critical section.
Boilerplate text to correct the formatting.
class Foo
{
private static object syncLock = new object();
public void ThreadProc1()
{
try
{
Monitor.Enter(syncLock);
Monitor.Wait(syncLock);
Thread.Sleep(1000);
}
finally
{
if (Monitor.IsLocked(syncLock))
{
Monitor.Exit(syncLock);
}
}
}
public void ThreadProc2()
{
bool acquired = false;
try
{
// Calling TryEnter instead of
// Enter just for the sake of variety
Monitor.TryEnter(syncLock, ref acquired);
if (acquired)
{
Thread.Sleep(200);
Monitor.Pulse(syncLock);
}
}
finally
{
if (acquired)
{
Monitor.Exit(syncLock);
}
}
}
}
Update
The following illustration confirms that #3 is correct although I don't think it will be a nice thing to do.
using System;
using System.Collections.Generic;
using System.Threading.Tasks;
namespace DifferentSyncLockSameCriticalSection
{
class Program
{
static void Main(string[] args)
{
var sathyaish = new Person { Name = "Sathyaish Chakravarthy" };
var superman = new Person { Name = "Superman" };
var tasks = new List<Task>();
// Must not lock on string so I am using
// an object of the Person class as a lock
tasks.Add(Task.Run( () => { Proc1(sathyaish); } ));
tasks.Add(Task.Run(() => { Proc1(superman); }));
Task.WhenAll(tasks);
Console.WriteLine("Press any key to exit.");
Console.ReadKey();
}
static void Proc1(object state)
{
// Although this would be a very bad practice
lock(state)
{
try
{
Console.WriteLine((state.ToString()).Length);
}
catch(Exception ex)
{
Console.WriteLine(ex.Message);
}
}
}
}
class Person
{
public string Name { get; set; }
public override string ToString()
{
return Name;
}
}
}

When a thread calls Monitor.Wait it is suspended and the lock released. This will allow another thread to acquire the lock, update some state, and then call Monitor.Pulse in order to communicate to other threads that something has happened. You must have acquired the lock in order to call Pulse. Before Monitor.Wait returns the framework will reacquire the lock for the thread that called Wait.
In order for two threads to communicate with each other they need to use the same synchronization primitive. In your example you've used a monitor, but you usually need to combine this with some kind of test that the Wait returned in response to a Pulse. This is because it is technically possible to Wait to return even if Pulse wasn't called (although this doesn't happen in practice).
It's also worth remembering that a call to Pulse isn't "sticky", so if nobody is waiting on the monitor then Pulse does nothing and a subsequent call to Wait will miss the fact that Pulse was called. This is another reason why you tend to record the fact that something has been done before calling Pulse (see the example below).
It's perfectly valid for two different threads to use the same lock to run different bits of code - in fact this is the typical use-case. For example, one thread acquires the lock to write some data and another thread acquires the lock to read the data. However, it's important to realize that they don't run at the same time. The act of acquiring the lock prevents another thread from acquiring the same lock, so any thread attempting to acquire the lock when it is already locked will block until the other thread releases the lock.
In point 3 you ask:
Two or more threads can use a different synchronization lock to run
the same piece of code, i.e. to enter the same critical section.
However, if two threads are using different locks then they are not entering the same critical section. The critical section is denoted by the lock that protects it - if they're different locks then they are different sections that just happen to access some common data within the section. You should avoid doing this as it can lead to some difficult to debug data race conditions.
Your code is a bit over-complicated for what you're trying to accomplish. For example, let's say we've got 2 threads, and one will signal when there is data available for another to process:
class Foo
{
private readonly object syncLock = new object();
private bool dataAvailable = false;
public void ThreadProc1()
{
lock(syncLock)
{
while(!dataAvailable)
{
// Release the lock and suspend
Monitor.Wait(syncLock);
}
// Now process the data
}
}
public void ThreadProc2()
{
LoadData();
lock(syncLock)
{
dataAvailable = true;
Monitor.Pulse(syncLock);
}
}
private void LoadData()
{
// Gets some data
}
}
}

Parallel A* search in C# - Thread sync

I have two threads which use two different functions. First one to search from start to end and the second one to search from end to start.
Now I'm using Thread.Sleep(10) for synchronisation, but it takes too much time, and testing is not possible in such condition.
Any idea how can I sync two threads with different functions?

It depends slightly on what you want to do.
If you have two threads and you just want to exit one when the other reaches "success" (or n threads and you want to exit them all when one reaches "success" first) you just need to periodically check for success on each thread.
Use Interlocked to do this without locks, or some other mechanism (see below)
Use cancellable Task objects
If you need to do your search in phases, where each thread does something and then waits for the other to catch up, you need a different approach.
Use Barrier
Given that you are doing an A*-search you likely need a combination of all two/three anyway:
Barrier to coordinate the steps and update the open set between steps
Success signalling to work out when to exit threads if another thread succeeded
Task objects with CancellationToken to allow callers to cancel the search.
Another answer suggested Semaphore - this is not really suitable for your needs (see comments below).
Barrier can be used for searches such as this by:
enter step 0 of the algorithm
n threads split the current level into equal parts and work on each half, when each completes then it signals and waits for the other thread
when all threads are ready, proceed to the next step and repeat the search
Simple check for exit - Interlocked
The first part is checking for success. If you want to stay "lockless", you can use Interlocked to do this, the general pattern is:
// global success indicator
private const int NotDone = 0;
private const int AllDone = 1;
private int _allDone = NotDone;
private GeneralSearchFunction(bool directionForward) {
bool iFoundIt = false;
... do some search operations that won't take much time
if (iFoundIt) {
// set _allDone to AllDone!
Interlocked.Exchange(ref _allDone, AllDone);
return;
}
... do more work
// after one or a few iterations, if this thread is still going
// see if another thread has set _allDone to AllDone
if (Interlocked.CompareExchange(ref _allDone, NotDone, NotDone) == AllDone) {
return; // if they did, then exit
}
... loop to the top and carry on working
}
// main thread:
Thread t1 = new Thread(() => GeneralSearchFunction(true));
Thread t2 = new Thread(() => GeneralSearchFunction(false));
t1.Start(); t2.Start(); // start both
t1.Join(); t2.Join();
// when this gets to here, one of them will have succeeded
This is the general pattern for any kind of success or cancellation token:
do some work
if you succeed, set a signal every other thread checks periodically
if you haven't yet succeeded then in the middle of that work, either every iteration, or every few iterations, check to see if this thread should exit
So an implementation would look like:
class Program
{
// global success indicator
private const int NotDone = 0;
private const int AllDone = 1;
private static int _allDone = NotDone;
private static int _forwardsCount = 0; // counters to simulate a "find"
private static int _backwardsCount = 0; // counters to simulate a "find"
static void Main(string[] args) {
var searchItem = "foo";
Thread t1 = new Thread(() => DoSearchWithBarrier(SearchForwards, searchItem));
Thread t2 = new Thread(() => DoSearchWithBarrier(SearchBackwards, searchItem));
t1.Start(); t2.Start();
t1.Join(); t2.Join();
Console.WriteLine("all done");
}
private static void DoSearchWithBarrier(Func<string, bool> searchMethod, string searchItem) {
while (!searchMethod(searchItem)) {
// after one or a few iterations, if this thread is still going
// see if another thread has set _allDone to AllDone
if (Interlocked.CompareExchange(ref _allDone, NotDone, NotDone) == AllDone) {
return; // if they did, then exit
}
}
Interlocked.Exchange(ref _allDone, AllDone);
}
public static bool SearchForwards(string item) {
// return true if we "found it", false if not
return (Interlocked.Increment(ref _forwardsCount) == 10);
}
public static bool SearchBackwards(string item) {
// return true if we "found it", false if not
return (Interlocked.Increment(ref _backwardsCount) == 20); // make this less than 10 to find it backwards first
}
}
Using Tasks to the same end
Of course, this wouldn't be .NET 4.5 without using Task:
class Program
{
private static int _forwardsCount = 0; // counters to simulate a "find"
private static int _backwardsCount = 0; // counters to simulate a "find"
static void Main(string[] args) {
var searchItem = "foo";
var tokenSource = new CancellationTokenSource();
var allDone = tokenSource.Token;
Task t1 = Task.Factory.StartNew(() => DoSearchWithBarrier(SearchForwards, searchItem, tokenSource, allDone), allDone);
Task t2 = Task.Factory.StartNew(() => DoSearchWithBarrier(SearchBackwards, searchItem, tokenSource, allDone), allDone);
Task.WaitAll(new[] {t2, t2});
Console.WriteLine("all done");
}
private static void DoSearchWithBarrier(Func<string, bool> searchMethod, string searchItem, CancellationTokenSource tokenSource, CancellationToken allDone) {
while (!searchMethod(searchItem)) {
if (allDone.IsCancellationRequested) {
return;
}
}
tokenSource.Cancel();
}
...
}
However, now you have used the CancellationToken for the wrong things - really this should be kept for the caller of the search to cancel the search, so you should use CancellationToken to check for a requested cancellation (only the caller needs tokenSource then), and a different success synchronisation (such as the Interlocked sample above) to exit.
Phase/step synchronisation
This gets harder for many reasons, but there is a simple approach. Using Barrier (new to .NET 4) in conjunction with an exit signal you can:
Perform the assigned thread's work for the current step, and then wait for the other thread to catch up before doing another iteration
Exit both threads when one succeeds
There are many different approaches for thread sync, depending on exactly what you want to achieve. Some are:
Barrier: This is probably the most suitable if you are intending for both your forwards and backwards searches to run at the same time. It also screams out your intent, i.e. "all threads can't go on until they everyone reaches a barrier"
ManualResetEvent - when one thread releases a signal, all others can proceed until it is set again. AutoResetEvent is similar, except it only allows one thread to proceed before blocking again.
Interlocked - in combination with SpinWait this is a viable lockless solution
Semaphore - possible to use, but not really suited for your scenario
I have only provided a full sample for Barrier here as it seems the most suitable in your case. Barrier is one of the most performant, second only to ManualResetEventSlim (ref. albahari), but using ManualResetEvent will need more complex code.
Other techniques to look at, if none of the above work for you are Monitor.Wait and Monitor.Pulse (now you're using locking) and Task Continuations. The latter is more used for passing data from one async operation to another, but it could be used for your scenario. And, as with the samples at the top of the answer, you are more likely to combine Task with Barrier than use one instead of the other. Task Continuations could be used to do the post-step revision of the open set in the A*-search, but you can just as easily use Barrier for that anyway.
This code, using Barrier works. In essence, DoSearchWithBarrier is the only bit doing the synchronisation - all the rest is setup and teardown code.
class Program {
...
private static int _forwardsCount = 0; // counters to simulate a "find"
private static int _backwardsCount = 0; // counters to simulate a "find"
static void Main(string[] args) {
Barrier barrier = new Barrier(numThreads,
b => Console.WriteLine("Completed search iteration {0}", b.CurrentPhaseNumber));
var searchItem = "foo";
Thread t1 = new Thread(() => DoSearchWithBarrier(SearchForwards, searchItem, barrier));
Thread t2 = new Thread(() => DoSearchWithBarrier(SearchBackwards, searchItem, barrier));
t1.Start(); Console.WriteLine("Started t1");
t2.Start(); Console.WriteLine("Started t2");
t1.Join(); Console.WriteLine("t1 done");
t2.Join(); Console.WriteLine("t2 done");
Console.WriteLine("all done");
}
private static void DoSearchWithBarrier(Func<string, bool> searchMethod, string searchItem, Barrier barrier) {
while (!searchMethod(searchItem)) {
// while we haven't found it, wait for the other thread to catch up
barrier.SignalAndWait(); // check for the other thread AFTER the barrier
if (Interlocked.CompareExchange(ref _allDone, NotDone, NotDone) == AllDone) {
return;
}
}
// set success signal on this thread BEFORE the barrier
Interlocked.Exchange(ref _allDone, AllDone);
// wait for the other thread, and then exit (and it will too)
barrier.SignalAndWait();
}
...
}
There are two things going on here:
Barrier is used to synchronise the two threads so they can't do their next step until the other has caught up
The exit signal uses Interlocked, as I first described.
Implementing this for A* searches is very similar to the above sample. Once all threads reach the barrier and therefore continue you could use a ManualResetEvent or a simple lock to then let one (and only one) revise the open set.
A note on Semaphore
This is probably not what you want as it's most often used when you have a limited pool of resources, with more resource users requiring access than you have resources.
Think of the PlayStation with CoD on it in the corner of the work canteen - 4 controllers, 20 people waiting (WaitOne) to use it, as soon as your character dies you Release the controller and someone else takes your place. No particular FIFO/LIFO ordering is enforced, and in fact Release can be called by the bouncer you employ to prevent the inevitable fights (i.e. thread identity is not enforced).
Simple check for exit - other approaches
Use of lock for simple success indication
You can achieve the same with locking. Both Interlocked and lock ensure you don't see any memory cache issues with reading a common variable between threads:
private readonly object _syncAllDone = new object();
...
if (iFoundIt) {
lock (_syncAllDone) { _allDone = AllDone };
return;
}
...
// see if another thread has set _allDone to AllDone
lock (_syncAllDone) {
if (_allDone == AllDone) {
return; // if they did, then exit
}
}
The disadvantage of this is that locking may well be slower, but you need to test for your situation. The advantage is that if you are using lock anyway to do other things such as writing out results from your thread, you don't have any extra overhead.
Use of ManualResetEvent for simple success indication
This is not really the intended use of reset events, but it can work. (If using .NET 4 or later, use ManualResetEventSlim instead of ManualResetEvent):
private ManualResetEvent _mreAllDone = new ManualResetEvent(true); // will not block a thread
...
if (iFoundIt) {
_mreAllDone.Reset(); // stop other threads proceeding
return;
}
...
// see if another thread has reset _mreAllDone by testing with a 0 timeout
if (!_mreAllDone.WaitOne(0)) {
return; // if they did, then exit
}
Phase synchronisation - other approaches
All of the other approaches get a lot more complex, as you have to do two-way continuation checks to prevent race conditions and permanently blocked threads. I don't recommend them, so I won't provide a sample here (it would be long and complicated).
References:
Interlocked
ManualResetEvent
MSDN - ManualResetEvent and ManualResetEventSlim
Barrier
MSDN - Continuation Tasks
MSDN - Task Cancellation
Semaphore

thread.Join()
is possibly what your after. This will make your current thread block until the other thread ends.
It's possible to Join multiple threads there by syncing all of them to one point.
List<Thread> threads = new List<Thread>();
threads.Add(new Thread(new ThreadStart(<Actual method here>)));
threads.Add(new Thread(new ThreadStart(<Another method here>)));
threads.Add(new Thread(new ThreadStart(<Another method here>)));
foreach(Thread thread in threads)
{
thread.Start();
}
//All your threads are now running
foreach(Thread thread in threads)
{
thread.Join();
}
//You wont get here until all those threads have finished

In some cases You can use AutoResetEvent to wait some result from thread.
You can use Task's for start/stop/wait result of some workers.
You can use Producer/Consumer pattern with BlockingCollection in case your functions eat some data and returns collection of something.

C# : Monitor - Wait,Pulse,PulseAll

I am having hard time in understanding Wait(), Pulse(), PulseAll(). Will all of them avoid deadlock? I would appreciate if you explain how to use them?

Short version:
lock(obj) {...}
is short-hand for Monitor.Enter / Monitor.Exit (with exception handling etc). If nobody else has the lock, you can get it (and run your code) - otherwise your thread is blocked until the lock is aquired (by another thread releasing it).
Deadlock typically happens when either A: two threads lock things in different orders:
thread 1: lock(objA) { lock (objB) { ... } }
thread 2: lock(objB) { lock (objA) { ... } }
(here, if they each acquire the first lock, neither can ever get the second, since neither thread can exit to release their lock)
This scenario can be minimised by always locking in the same order; and you can recover (to a degree) by using Monitor.TryEnter (instead of Monitor.Enter/lock) and specifying a timeout.
or B: you can block yourself with things like winforms when thread-switching while holding a lock:
lock(obj) { // on worker
this.Invoke((MethodInvoker) delegate { // switch to UI
lock(obj) { // oopsiee!
...
}
});
}
The deadlock appears obvious above, but it isn't so obvious when you have spaghetti code; possible answers: don't thread-switch while holding locks, or use BeginInvoke so that you can at least exit the lock (letting the UI play).
Wait/Pulse/PulseAll are different; they are for signalling. I use this in this answer to signal so that:
Dequeue: if you try to dequeue data when the queue is empty, it waits for another thread to add data, which wakes up the blocked thread
Enqueue: if you try and enqueue data when the queue is full, it waits for another thread to remove data, which wakes up the blocked thread
Pulse only wakes up one thread - but I'm not brainy enough to prove that the next thread is always the one I want, so I tend to use PulseAll, and simply re-verify the conditions before continuing; as an example:
while (queue.Count >= maxSize)
{
Monitor.Wait(queue);
}
With this approach, I can safely add other meanings of Pulse, without my existing code assuming that "I woke up, therefore there is data" - which is handy when (in the same example) I later needed to add a Close() method.

Simple recipe for use of Monitor.Wait and Monitor.Pulse. It consists of a worker, a boss, and a phone they use to communicate:
object phone = new object();
A "Worker" thread:
lock(phone) // Sort of "Turn the phone on while at work"
{
while(true)
{
Monitor.Wait(phone); // Wait for a signal from the boss
DoWork();
Monitor.PulseAll(phone); // Signal boss we are done
}
}
A "Boss" thread:
PrepareWork();
lock(phone) // Grab the phone when I have something ready for the worker
{
Monitor.PulseAll(phone); // Signal worker there is work to do
Monitor.Wait(phone); // Wait for the work to be done
}
More complex examples follow...
A "Worker with something else to do":
lock(phone)
{
while(true)
{
if(Monitor.Wait(phone,1000)) // Wait for one second at most
{
DoWork();
Monitor.PulseAll(phone); // Signal boss we are done
}
else
DoSomethingElse();
}
}
An "Impatient Boss":
PrepareWork();
lock(phone)
{
Monitor.PulseAll(phone); // Signal worker there is work to do
if(Monitor.Wait(phone,1000)) // Wait for one second at most
Console.Writeline("Good work!");
}

No, they don't protect you from deadlocks. They are just more flexible tools for thread synchronization. Here is a very good explanation how to use them and very important pattern of usage - without this pattern you will break all the things:
http://www.albahari.com/threading/part4.aspx

Something that total threw me here is that Pulse just gives a "heads up" to a thread in a Wait. The Waiting thread will not continue until the thread that did the Pulse gives up the lock and the waiting thread successfully wins it.
lock(phone) // Grab the phone
{
Monitor.PulseAll(phone); // Signal worker
Monitor.Wait(phone); // ****** The lock on phone has been given up! ******
}
or
lock(phone) // Grab the phone when I have something ready for the worker
{
Monitor.PulseAll(phone); // Signal worker there is work to do
DoMoreWork();
} // ****** The lock on phone has been given up! ******
In both cases it's not until "the lock on phone has been given up" that another thread can get it.
There might be other threads waiting for that lock from Monitor.Wait(phone) or lock(phone). Only the one that wins the lock will get to continue.

They are tools for synchronizing and signaling between threads. As such they do nothing to prevent deadlocks, but if used correctly they can be used to synchronize and communicate between threads.
Unfortunately most of the work needed to write correct multithreaded code is currently the developers' responsibility in C# (and many other languages). Take a look at how F#, Haskell and Clojure handles this for an entirely different approach.

Unfortunately, none of Wait(), Pulse() or PulseAll() have the magical property which you are wishing for - which is that by using this API you will automatically avoid deadlock.
Consider the following code
object incomingMessages = new object(); //signal object
LoopOnMessages()
{
lock(incomingMessages)
{
Monitor.Wait(incomingMessages);
}
if (canGrabMessage()) handleMessage();
// loop
}
ReceiveMessagesAndSignalWaiters()
{
awaitMessages();
copyMessagesToReadyArea();
lock(incomingMessages) {
Monitor.PulseAll(incomingMessages); //or Monitor.Pulse
}
awaitReadyAreaHasFreeSpace();
}
This code will deadlock! Maybe not today, maybe not tomorrow. Most likely when your code is placed under stress because suddenly it has become popular or important, and you are being called to fix an urgent issue.
Why?
Eventually the following will happen:
All consumer threads are doing some work
Messages arrive, the ready area can't hold any more messages, and PulseAll() is called.
No consumer gets woken up, because none are waiting
All consumer threads call Wait() [DEADLOCK]
This particular example assumes that producer thread is never going to call PulseAll() again because it has no more space to put messages in. But there are many, many broken variations on this code possible. People will try to make it more robust by changing a line such as making Monitor.Wait(); into
if (!canGrabMessage()) Monitor.Wait(incomingMessages);
Unfortunately, that still isn't enough to fix it. To fix it you also need to change the locking scope where Monitor.PulseAll() is called:
LoopOnMessages()
{
lock(incomingMessages)
{
if (!canGrabMessage()) Monitor.Wait(incomingMessages);
}
if (canGrabMessage()) handleMessage();
// loop
}
ReceiveMessagesAndSignalWaiters()
{
awaitMessagesArrive();
lock(incomingMessages)
{
copyMessagesToReadyArea();
Monitor.PulseAll(incomingMessages); //or Monitor.Pulse
}
awaitReadyAreaHasFreeSpace();
}
The key point is that in the fixed code, the locks restrict the possible sequences of events:
A consumer threads does its work and loops
That thread acquires the lock
And thanks to locking it is now true that either:
a. Messages haven't yet arrived in the ready area, and it releases the lock by calling Wait() BEFORE the message receiver thread can acquire the lock and copy more messages into the ready area, or
b. Messages have already arrived in the ready area and it receives the messages INSTEAD OF calling Wait(). (And while it is making this decision it is impossible for the message receiver thread to e.g. acquire the lock and copy more messages into the ready area.)
As a result the problem of the original code now never occurs:
3. When PulseEvent() is called No consumer gets woken up, because none are waiting
Now observe that in this code you have to get the locking scope exactly right. (If, indeed I got it right!)
And also, since you must use the lock (or Monitor.Enter() etc.) in order to use Monitor.PulseAll() or Monitor.Wait() in a deadlock-free fashion, you still have to worry about possibility of other deadlocks which happen because of that locking.
Bottom line: these APIs are also easy to screw up and deadlock with, i.e. quite dangerous

This is a simple example of monitor use :
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
namespace ConsoleApp4
{
class Program
{
public static int[] X = new int[30];
static readonly object _object = new object();
public static int count=0;
public static void PutNumbers(int numbersS, int numbersE)
{
for (int i = numbersS; i < numbersE; i++)
{
Monitor.Enter(_object);
try
{
if(count<30)
{
X[count] = i;
count++;
Console.WriteLine("Punt in " + count + "nd: "+i);
Monitor.Pulse(_object);
}
else
{
Monitor.Wait(_object);
}
}
finally
{
Monitor.Exit(_object);
}
}
}
public static void RemoveNumbers(int numbersS)
{
for (int i = 0; i < numbersS; i++)
{
Monitor.Enter(_object);
try
{
if (count > 0)
{
X[count] = 0;
int x = count;
count--;
Console.WriteLine("Removed " + x + " element");
Monitor.Pulse(_object);
}
else
{
Monitor.Wait(_object);
}
}
finally
{
Monitor.Exit(_object);
}
}
}
static void Main(string[] args)
{
Thread W1 = new Thread(() => PutNumbers(10,50));
Thread W2 = new Thread(() => PutNumbers(1, 10));
Thread R1 = new Thread(() => RemoveNumbers(30));
Thread R2 = new Thread(() => RemoveNumbers(20));
W1.Start();
R1.Start();
W2.Start();
R2.Start();
W1.Join();
R1.Join();
W2.Join();
R2.Join();
}
}
}

How to effectively log asynchronously?

I am using Enterprise Library 4 on one of my projects for logging (and other purposes). I've noticed that there is some cost to the logging that I am doing that I can mitigate by doing the logging on a separate thread.
The way I am doing this now is that I create a LogEntry object and then I call BeginInvoke on a delegate that calls Logger.Write.
new Action<LogEntry>(Logger.Write).BeginInvoke(le, null, null);
What I'd really like to do is add the log message to a queue and then have a single thread pulling LogEntry instances off the queue and performing the log operation. The benefit of this would be that logging is not interfering with the executing operation and not every logging operation results in a job getting thrown on the thread pool.
How can I create a shared queue that supports many writers and one reader in a thread safe way? Some examples of a queue implementation that is designed to support many writers (without causing synchronization/blocking) and a single reader would be really appreciated.
Recommendation regarding alternative approaches would also be appreciated, I am not interested in changing logging frameworks though.

I wrote this code a while back, feel free to use it.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
namespace MediaBrowser.Library.Logging {
public abstract class ThreadedLogger : LoggerBase {
Queue<Action> queue = new Queue<Action>();
AutoResetEvent hasNewItems = new AutoResetEvent(false);
volatile bool waiting = false;
public ThreadedLogger() : base() {
Thread loggingThread = new Thread(new ThreadStart(ProcessQueue));
loggingThread.IsBackground = true;
loggingThread.Start();
}
void ProcessQueue() {
while (true) {
waiting = true;
hasNewItems.WaitOne(10000,true);
waiting = false;
Queue<Action> queueCopy;
lock (queue) {
queueCopy = new Queue<Action>(queue);
queue.Clear();
}
foreach (var log in queueCopy) {
log();
}
}
}
public override void LogMessage(LogRow row) {
lock (queue) {
queue.Enqueue(() => AsyncLogMessage(row));
}
hasNewItems.Set();
}
protected abstract void AsyncLogMessage(LogRow row);
public override void Flush() {
while (!waiting) {
Thread.Sleep(1);
}
}
}
}
Some advantages:
It keeps the background logger alive, so it does not need to spin up and spin down threads.
It uses a single thread to service the queue, which means there will never be a situation where 100 threads are servicing the queue.
It copies the queues to ensure the queue is not blocked while the log operation is performed
It uses an AutoResetEvent to ensure the bg thread is in a wait state
It is, IMHO, very easy to follow
Here is a slightly improved version, keep in mind I performed very little testing on it, but it does address a few minor issues.
public abstract class ThreadedLogger : IDisposable {
Queue<Action> queue = new Queue<Action>();
ManualResetEvent hasNewItems = new ManualResetEvent(false);
ManualResetEvent terminate = new ManualResetEvent(false);
ManualResetEvent waiting = new ManualResetEvent(false);
Thread loggingThread;
public ThreadedLogger() {
loggingThread = new Thread(new ThreadStart(ProcessQueue));
loggingThread.IsBackground = true;
// this is performed from a bg thread, to ensure the queue is serviced from a single thread
loggingThread.Start();
}
void ProcessQueue() {
while (true) {
waiting.Set();
int i = ManualResetEvent.WaitAny(new WaitHandle[] { hasNewItems, terminate });
// terminate was signaled
if (i == 1) return;
hasNewItems.Reset();
waiting.Reset();
Queue<Action> queueCopy;
lock (queue) {
queueCopy = new Queue<Action>(queue);
queue.Clear();
}
foreach (var log in queueCopy) {
log();
}
}
}
public void LogMessage(LogRow row) {
lock (queue) {
queue.Enqueue(() => AsyncLogMessage(row));
}
hasNewItems.Set();
}
protected abstract void AsyncLogMessage(LogRow row);
public void Flush() {
waiting.WaitOne();
}
public void Dispose() {
terminate.Set();
loggingThread.Join();
}
}
Advantages over the original:
It's disposable, so you can get rid of the async logger
The flush semantics are improved
It will respond slightly better to a burst followed by silence

Yes, you need a producer/consumer queue. I have one example of this in my threading tutorial - if you look my "deadlocks / monitor methods" page you'll find the code in the second half.
There are plenty of other examples online, of course - and .NET 4.0 will ship with one in the framework too (rather more fully featured than mine!). In .NET 4.0 you'd probably wrap a ConcurrentQueue<T> in a BlockingCollection<T>.
The version on that page is non-generic (it was written a long time ago) but you'd probably want to make it generic - it would be trivial to do.
You would call Produce from each "normal" thread, and Consume from one thread, just looping round and logging whatever it consumes. It's probably easiest just to make the consumer thread a background thread, so you don't need to worry about "stopping" the queue when your app exits. That does mean there's a remote possibility of missing the final log entry though (if it's half way through writing it when the app exits) - or even more if you're producing faster than it can consume/log.

Here is what I came up with... also see Sam Saffron's answer. This answer is community wiki in case there are any problems that people see in the code and want to update.
/// <summary>
/// A singleton queue that manages writing log entries to the different logging sources (Enterprise Library Logging) off the executing thread.
/// This queue ensures that log entries are written in the order that they were executed and that logging is only utilizing one thread (backgroundworker) at any given time.
/// </summary>
public class AsyncLoggerQueue
{
//create singleton instance of logger queue
public static AsyncLoggerQueue Current = new AsyncLoggerQueue();
private static readonly object logEntryQueueLock = new object();
private Queue<LogEntry> _LogEntryQueue = new Queue<LogEntry>();
private BackgroundWorker _Logger = new BackgroundWorker();
private AsyncLoggerQueue()
{
//configure background worker
_Logger.WorkerSupportsCancellation = false;
_Logger.DoWork += new DoWorkEventHandler(_Logger_DoWork);
}
public void Enqueue(LogEntry le)
{
//lock during write
lock (logEntryQueueLock)
{
_LogEntryQueue.Enqueue(le);
//while locked check to see if the BW is running, if not start it
if (!_Logger.IsBusy)
_Logger.RunWorkerAsync();
}
}
private void _Logger_DoWork(object sender, DoWorkEventArgs e)
{
while (true)
{
LogEntry le = null;
bool skipEmptyCheck = false;
lock (logEntryQueueLock)
{
if (_LogEntryQueue.Count <= 0) //if queue is empty than BW is done
return;
else if (_LogEntryQueue.Count > 1) //if greater than 1 we can skip checking to see if anything has been enqueued during the logging operation
skipEmptyCheck = true;
//dequeue the LogEntry that will be written to the log
le = _LogEntryQueue.Dequeue();
}
//pass LogEntry to Enterprise Library
Logger.Write(le);
if (skipEmptyCheck) //if LogEntryQueue.Count was > 1 before we wrote the last LogEntry we know to continue without double checking
{
lock (logEntryQueueLock)
{
if (_LogEntryQueue.Count <= 0) //if queue is still empty than BW is done
return;
}
}
}
}
}

I suggest to start with measuring actual performance impact of logging on the overall system (i.e. by running profiler) and optionally switching to something faster like log4net (I've personally migrated to it from EntLib logging a long time ago).
If this does not work, you can try using this simple method from .NET Framework:
ThreadPool.QueueUserWorkItem
Queues a method for execution. The method executes when a thread pool thread becomes available.
MSDN Details
If this does not work either then you can resort to something like John Skeet has offered and actually code the async logging framework yourself.

In response to Sam Safrons post, I wanted to call flush and make sure everything was really finished writting. In my case, I am writing to a database in the queue thread and all my log events were getting queued up but sometimes the application stopped before everything was finished writing which is not acceptable in my situation. I changed several chunks of your code but the main thing I wanted to share was the flush:
public static void FlushLogs()
{
bool queueHasValues = true;
while (queueHasValues)
{
//wait for the current iteration to complete
m_waitingThreadEvent.WaitOne();
lock (m_loggerQueueSync)
{
queueHasValues = m_loggerQueue.Count > 0;
}
}
//force MEL to flush all its listeners
foreach (MEL.LogSource logSource in MEL.Logger.Writer.TraceSources.Values)
{
foreach (TraceListener listener in logSource.Listeners)
{
listener.Flush();
}
}
}
I hope that saves someone some frustration. It is especially apparent in parallel processes logging lots of data.
Thanks for sharing your solution, it set me into a good direction!
--Johnny S

I wanted to say that my previous post was kind of useless. You can simply set AutoFlush to true and you will not have to loop through all the listeners. However, I still had crazy problem with parallel threads trying to flush the logger. I had to create another boolean that was set to true during the copying of the queue and executing the LogEntry writes and then in the flush routine I had to check that boolean to make sure something was not already in the queue and the nothing was getting processed before returning.
Now multiple threads in parallel can hit this thing and when I call flush I know it is really flushed.
public static void FlushLogs()
{
int queueCount;
bool isProcessingLogs;
while (true)
{
//wait for the current iteration to complete
m_waitingThreadEvent.WaitOne();
//check to see if we are currently processing logs
lock (m_isProcessingLogsSync)
{
isProcessingLogs = m_isProcessingLogs;
}
//check to see if more events were added while the logger was processing the last batch
lock (m_loggerQueueSync)
{
queueCount = m_loggerQueue.Count;
}
if (queueCount == 0 && !isProcessingLogs)
break;
//since something is in the queue, reset the signal so we will not keep looping
Thread.Sleep(400);
}
}

Just an update:
Using enteprise library 5.0 with .NET 4.0 it can easily be done by:
static public void LogMessageAsync(LogEntry logEntry)
{
Task.Factory.StartNew(() => LogMessage(logEntry));
}
See:
http://randypaulo.wordpress.com/2011/07/28/c-enterprise-library-asynchronous-logging/

An extra level of indirection may help here.
Your first async method call can put messages onto a synchonized Queue and set an event -- so the locks are happening in the thread-pool, not on your worker threads -- and then have yet another thread pulling messages off the queue when the event is raised.

If you log something on a separate thread, the message may not be written if the application crashes, which makes it rather useless.
The reason goes why you should always flush after every written entry.

If what you have in mind is a SHARED queue, then I think you are going to have to synchronize the writes to it, the pushes and the pops.
But, I still think it's worth aiming at the shared queue design. In comparison to the IO of logging and probably in comparison to the other work your app is doing, the brief amount of blocking for the pushes and the pops will probably not be significant.