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  • Launching a WPF Window in a Separate Thread, Part 1

    - by Reed
    Typically, I strongly recommend keeping the user interface within an application’s main thread, and using multiple threads to move the actual “work” into background threads.  However, there are rare times when creating a separate, dedicated thread for a Window can be beneficial.  This is even acknowledged in the MSDN samples, such as the Multiple Windows, Multiple Threads sample.  However, doing this correctly is difficult.  Even the referenced MSDN sample has major flaws, and will fail horribly in certain scenarios.  To ease this, I wrote a small class that alleviates some of the difficulties involved. The MSDN Multiple Windows, Multiple Threads Sample shows how to launch a new thread with a WPF Window, and will work in most cases.  The sample code (commented and slightly modified) works out to the following: // Create a thread Thread newWindowThread = new Thread(new ThreadStart( () => { // Create and show the Window Window1 tempWindow = new Window1(); tempWindow.Show(); // Start the Dispatcher Processing System.Windows.Threading.Dispatcher.Run(); })); // Set the apartment state newWindowThread.SetApartmentState(ApartmentState.STA); // Make the thread a background thread newWindowThread.IsBackground = true; // Start the thread newWindowThread.Start(); .csharpcode, .csharpcode pre { font-size: small; color: black; font-family: consolas, "Courier New", courier, monospace; background-color: #ffffff; /*white-space: pre;*/ } .csharpcode pre { margin: 0em; } .csharpcode .rem { color: #008000; } .csharpcode .kwrd { color: #0000ff; } .csharpcode .str { color: #006080; } .csharpcode .op { color: #0000c0; } .csharpcode .preproc { color: #cc6633; } .csharpcode .asp { background-color: #ffff00; } .csharpcode .html { color: #800000; } .csharpcode .attr { color: #ff0000; } .csharpcode .alt { background-color: #f4f4f4; width: 100%; margin: 0em; } .csharpcode .lnum { color: #606060; } This sample creates a thread, marks it as single threaded apartment state, and starts the Dispatcher on that thread. That is the minimum requirements to get a Window displaying and handling messages correctly, but, unfortunately, has some serious flaws. The first issue – the created thread will run continuously until the application shuts down, given the code in the sample.  The problem is that the ThreadStart delegate used ends with running the Dispatcher.  However, nothing ever stops the Dispatcher processing.  The thread was created as a Background thread, which prevents it from keeping the application alive, but the Dispatcher will continue to pump dispatcher frames until the application shuts down. In order to fix this, we need to call Dispatcher.InvokeShutdown after the Window is closed.  This would require modifying the above sample to subscribe to the Window’s Closed event, and, at that point, shutdown the Dispatcher: // Create a thread Thread newWindowThread = new Thread(new ThreadStart( () => { Window1 tempWindow = new Window1(); // When the window closes, shut down the dispatcher tempWindow.Closed += (s,e) => Dispatcher.CurrentDispatcher.BeginInvokeShutdown(DispatcherPriority.Background); tempWindow.Show(); // Start the Dispatcher Processing System.Windows.Threading.Dispatcher.Run(); })); // Setup and start thread as before This eliminates the first issue.  Now, when the Window is closed, the new thread’s Dispatcher will shut itself down, which in turn will cause the thread to complete. The above code will work correctly for most situations.  However, there is still a potential problem which could arise depending on the content of the Window1 class.  This is particularly nasty, as the code could easily work for most windows, but fail on others. The problem is, at the point where the Window is constructed, there is no active SynchronizationContext.  This is unlikely to be a problem in most cases, but is an absolute requirement if there is code within the constructor of Window1 which relies on a context being in place. While this sounds like an edge case, it’s fairly common.  For example, if a BackgroundWorker is started within the constructor, or a TaskScheduler is built using TaskScheduler.FromCurrentSynchronizationContext() with the expectation of synchronizing work to the UI thread, an exception will be raised at some point.  Both of these classes rely on the existence of a proper context being installed to SynchronizationContext.Current, which happens automatically, but not until Dispatcher.Run is called.  In the above case, SynchronizationContext.Current will return null during the Window’s construction, which can cause exceptions to occur or unexpected behavior. Luckily, this is fairly easy to correct.  We need to do three things, in order, prior to creating our Window: Create and initialize the Dispatcher for the new thread manually Create a synchronization context for the thread which uses the Dispatcher Install the synchronization context Creating the Dispatcher is quite simple – The Dispatcher.CurrentDispatcher property gets the current thread’s Dispatcher and “creates a new Dispatcher if one is not already associated with the thread.”  Once we have the correct Dispatcher, we can create a SynchronizationContext which uses the dispatcher by creating a DispatcherSynchronizationContext.  Finally, this synchronization context can be installed as the current thread’s context via SynchronizationContext.SetSynchronizationContext.  These three steps can easily be added to the above via a single line of code: // Create a thread Thread newWindowThread = new Thread(new ThreadStart( () => { // Create our context, and install it: SynchronizationContext.SetSynchronizationContext( new DispatcherSynchronizationContext( Dispatcher.CurrentDispatcher)); Window1 tempWindow = new Window1(); // When the window closes, shut down the dispatcher tempWindow.Closed += (s,e) => Dispatcher.CurrentDispatcher.BeginInvokeShutdown(DispatcherPriority.Background); tempWindow.Show(); // Start the Dispatcher Processing System.Windows.Threading.Dispatcher.Run(); })); // Setup and start thread as before This now forces the synchronization context to be in place before the Window is created and correctly shuts down the Dispatcher when the window closes. However, there are quite a few steps.  In my next post, I’ll show how to make this operation more reusable by creating a class with a far simpler API…

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  • Inside the Concurrent Collections: ConcurrentBag

    - by Simon Cooper
    Unlike the other concurrent collections, ConcurrentBag does not really have a non-concurrent analogy. As stated in the MSDN documentation, ConcurrentBag is optimised for the situation where the same thread is both producing and consuming items from the collection. We'll see how this is the case as we take a closer look. Again, I recommend you have ConcurrentBag open in a decompiler for reference. Thread Statics ConcurrentBag makes heavy use of thread statics - static variables marked with ThreadStaticAttribute. This is a special attribute that instructs the CLR to scope any values assigned to or read from the variable to the executing thread, not globally within the AppDomain. This means that if two different threads assign two different values to the same thread static variable, one value will not overwrite the other, and each thread will see the value they assigned to the variable, separately to any other thread. This is a very useful function that allows for ConcurrentBag's concurrency properties. You can think of a thread static variable: [ThreadStatic] private static int m_Value; as doing the same as: private static Dictionary<Thread, int> m_Values; where the executing thread's identity is used to automatically set and retrieve the corresponding value in the dictionary. In .NET 4, this usage of ThreadStaticAttribute is encapsulated in the ThreadLocal class. Lists of lists ConcurrentBag, at its core, operates as a linked list of linked lists: Each outer list node is an instance of ThreadLocalList, and each inner list node is an instance of Node. Each outer ThreadLocalList is owned by a particular thread, accessible through the thread local m_locals variable: private ThreadLocal<ThreadLocalList<T>> m_locals It is important to note that, although the m_locals variable is thread-local, that only applies to accesses through that variable. The objects referenced by the thread (each instance of the ThreadLocalList object) are normal heap objects that are not specific to any thread. Thinking back to the Dictionary analogy above, if each value stored in the dictionary could be accessed by other means, then any thread could access the value belonging to other threads using that mechanism. Only reads and writes to the variable defined as thread-local are re-routed by the CLR according to the executing thread's identity. So, although m_locals is defined as thread-local, the m_headList, m_nextList and m_tailList variables aren't. This means that any thread can access all the thread local lists in the collection by doing a linear search through the outer linked list defined by these variables. Adding items So, onto the collection operations. First, adding items. This one's pretty simple. If the current thread doesn't already own an instance of ThreadLocalList, then one is created (or, if there are lists owned by threads that have stopped, it takes control of one of those). Then the item is added to the head of that thread's list. That's it. Don't worry, it'll get more complicated when we account for the other operations on the list! Taking & Peeking items This is where it gets tricky. If the current thread's list has items in it, then it peeks or removes the head item (not the tail item) from the local list and returns that. However, if the local list is empty, it has to go and steal another item from another list, belonging to a different thread. It iterates through all the thread local lists in the collection using the m_headList and m_nextList variables until it finds one that has items in it, and it steals one item from that list. Up to this point, the two threads had been operating completely independently. To steal an item from another thread's list, the stealing thread has to do it in such a way as to not step on the owning thread's toes. Recall how adding and removing items both operate on the head of the thread's linked list? That gives us an easy way out - a thread trying to steal items from another thread can pop in round the back of another thread's list using the m_tail variable, and steal an item from the back without the owning thread knowing anything about it. The owning thread can carry on completely independently, unaware that one of its items has been nicked. However, this only works when there are at least 3 items in the list, as that guarantees there will be at least one node between the owning thread performing operations on the list head and the thread stealing items from the tail - there's no chance of the two threads operating on the same node at the same time and causing a race condition. If there's less than three items in the list, then there does need to be some synchronization between the two threads. In this case, the lock on the ThreadLocalList object is used to mediate access to a thread's list when there's the possibility of contention. Thread synchronization In ConcurrentBag, this is done using several mechanisms: Operations performed by the owner thread only take out the lock when there are less than three items in the collection. With three or greater items, there won't be any conflict with a stealing thread operating on the tail of the list. If a lock isn't taken out, the owning thread sets the list's m_currentOp variable to a non-zero value for the duration of the operation. This indicates to all other threads that there is a non-locked operation currently occuring on that list. The stealing thread always takes out the lock, to prevent two threads trying to steal from the same list at the same time. After taking out the lock, the stealing thread spinwaits until m_currentOp has been set to zero before actually performing the steal. This ensures there won't be a conflict with the owning thread when the number of items in the list is on the 2-3 item borderline. If any add or remove operations are started in the meantime, and the list is below 3 items, those operations try to take out the list's lock and are blocked until the stealing thread has finished. This allows a thread to steal an item from another thread's list without corrupting it. What about synchronization in the collection as a whole? Collection synchronization Any thread that operates on the collection's global structure (accessing anything outside the thread local lists) has to take out the collection's global lock - m_globalListsLock. This single lock is sufficient when adding a new thread local list, as the items inside each thread's list are unaffected. However, what about operations (such as Count or ToArray) that need to access every item in the collection? In order to ensure a consistent view, all operations on the collection are stopped while the count or ToArray is performed. This is done by freezing the bag at the start, performing the global operation, and unfreezing at the end: The global lock is taken out, to prevent structural alterations to the collection. m_needSync is set to true. This notifies all the threads that they need to take out their list's lock irregardless of what operation they're doing. All the list locks are taken out in order. This blocks all locking operations on the lists. The freezing thread waits for all current lockless operations to finish by spinwaiting on each m_currentOp field. The global operation can then be performed while the bag is frozen, but no other operations can take place at the same time, as all other threads are blocked on a list's lock. Then, once the global operation has finished, the locks are released, m_needSync is unset, and normal concurrent operation resumes. Concurrent principles That's the essence of how ConcurrentBag operates. Each thread operates independently on its own local list, except when they have to steal items from another list. When stealing, only the stealing thread is forced to take out the lock; the owning thread only has to when there is the possibility of contention. And a global lock controls accesses to the structure of the collection outside the thread lists. Operations affecting the entire collection take out all locks in the collection to freeze the contents at a single point in time. So, what principles can we extract here? Threads operate independently Thread-static variables and ThreadLocal makes this easy. Threads operate entirely concurrently on their own structures; only when they need to grab data from another thread is there any thread contention. Minimised lock-taking Even when two threads need to operate on the same data structures (one thread stealing from another), they do so in such a way such that the probability of actually blocking on a lock is minimised; the owning thread always operates on the head of the list, and the stealing thread always operates on the tail. Management of lockless operations Any operations that don't take out a lock still have a 'hook' to force them to lock when necessary. This allows all operations on the collection to be stopped temporarily while a global snapshot is taken. Hopefully, such operations will be short-lived and infrequent. That's all the concurrent collections covered. I hope you've found it as informative and interesting as I have. Next, I'll be taking a closer look at ThreadLocal, which I came across while analyzing ConcurrentBag. As you'll see, the operation of this class deserves a much closer look.

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  • Thread Safety of C# List<T> for readers

    - by ILIA BROUDNO
    I am planning to create the list once in a static constructor and then have multiple instances of that class read it (and enumerate through it) concurrently without doing any locking. In this article http://msdn.microsoft.com/en-us/library/6sh2ey19.aspx MS describes the issue of thread safety as follows: Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe. A List can support multiple readers concurrently, as long as the collection is not modified. Enumerating through a collection is intrinsically not a thread-safe procedure. In the rare case where an enumeration contends with one or more write accesses, the only way to ensure thread safety is to lock the collection during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization. The "Enumerating through a collection is intrinsically not a thread-safe procedure." Statement is what worries me. Does this mean that it is thread safe for readers only scenario, but as long as you do not use enumeration? Or is it safe for my scenario?

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  • Boost BGL thread safety

    - by Budzoli
    Hi! I'd like multiple threads to use the dijkstra_shortest_paths and astar_search functions of the BGL, and then read the property maps of the result vertices and edges. I'm wondering wether I should use mutexes to ensure thread-safety. So here are my questions: 1., Are the dijkstra_shortest_paths and astar_search functions of the Boost.Graph thread safe? 2., If I only try to read the property maps of the graph from multiple threads, do I need to worry about thread safety?

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  • Java thread dump where main thread has no call stack? (jsvc)

    - by dwhsix
    We have a java process running as a daemon (under jsvc). Every several days it just stops doing any work; output to the logfile stops (it is pretty verbose, on 5-minute intervals) and it consumes no CPU or IO. There are no exceptions logged in the logfile nor in syserr or sysout. The last log statement is just prior to a db commit being done, but there is no open connection on the db server (MySQL) and reviewing the code, there should always be additional log output after that, even if it had encountered an exception that was going to bubble up. The most curious thing I find is that in the thread dump (included below), there's no thread in our code at all, and the main thread seems to have no context whatsoever: "main" prio=10 tid=0x0000000000614000 nid=0x445d runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE As noted earlier, this is a daemon process running using jsvc, but I don't know if that has anything to do with it (I can restructure the code to also allow running it directly, to test). Any suggestions on what might be happening here? Thanks... dwh Full thread dump: Full thread dump Java HotSpot(TM) 64-Bit Server VM (14.2-b01 mixed mode): "MySQL Statement Cancellation Timer" daemon prio=10 tid=0x00002aaaf81b8800 nid=0x447b in Object.wait() [0x00002aaaf6a22000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x00002aaab5556d50> (a java.util.TaskQueue) at java.lang.Object.wait(Object.java:485) at java.util.TimerThread.mainLoop(Timer.java:483) - locked <0x00002aaab5556d50> (a java.util.TaskQueue) at java.util.TimerThread.run(Timer.java:462) "Low Memory Detector" daemon prio=10 tid=0x00000000006a4000 nid=0x4479 runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "CompilerThread1" daemon prio=10 tid=0x00000000006a1000 nid=0x4477 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "CompilerThread0" daemon prio=10 tid=0x000000000069d000 nid=0x4476 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Signal Dispatcher" daemon prio=10 tid=0x000000000069b000 nid=0x4465 waiting on condition [0x0000000000000000] java.lang.Thread.State: RUNNABLE "Finalizer" daemon prio=10 tid=0x0000000000678800 nid=0x4464 in Object.wait() [0x00002aaaf61d6000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x00002aaab54a1cb8> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:118) - locked <0x00002aaab54a1cb8> (a java.lang.ref.ReferenceQueue$Lock) at java.lang.ref.ReferenceQueue.remove(ReferenceQueue.java:134) at java.lang.ref.Finalizer$FinalizerThread.run(Finalizer.java:159) "Reference Handler" daemon prio=10 tid=0x0000000000676800 nid=0x4463 in Object.wait() [0x00002aaaf60d5000] java.lang.Thread.State: WAITING (on object monitor) at java.lang.Object.wait(Native Method) - waiting on <0x00002aaab54a1cf0> (a java.lang.ref.Reference$Lock) at java.lang.Object.wait(Object.java:485) at java.lang.ref.Reference$ReferenceHandler.run(Reference.java:116) - locked <0x00002aaab54a1cf0> (a java.lang.ref.Reference$Lock) "main" prio=10 tid=0x0000000000614000 nid=0x445d runnable [0x0000000000000000] java.lang.Thread.State: RUNNABLE "VM Thread" prio=10 tid=0x0000000000670000 nid=0x4462 runnable "GC task thread#0 (ParallelGC)" prio=10 tid=0x000000000061e000 nid=0x445e runnable "GC task thread#1 (ParallelGC)" prio=10 tid=0x0000000000620000 nid=0x445f runnable "GC task thread#2 (ParallelGC)" prio=10 tid=0x0000000000622000 nid=0x4460 runnable "GC task thread#3 (ParallelGC)" prio=10 tid=0x0000000000623800 nid=0x4461 runnable "VM Periodic Task Thread" prio=10 tid=0x00000000006a6800 nid=0x447a waiting on condition JNI global references: 797 Heap PSYoungGen total 162944K, used 48388K [0x00002aaadff40000, 0x00002aaaf2ab0000, 0x00002aaaf5490000) eden space 102784K, 47% used [0x00002aaadff40000,0x00002aaae2e81170,0x00002aaae63a0000) from space 60160K, 0% used [0x00002aaaeb850000,0x00002aaaeb850000,0x00002aaaef310000) to space 86720K, 0% used [0x00002aaae63a0000,0x00002aaae63a0000,0x00002aaaeb850000) PSOldGen total 699072K, used 699072K [0x00002aaab5490000, 0x00002aaadff40000, 0x00002aaadff40000) object space 699072K, 100% used [0x00002aaab5490000,0x00002aaadff40000,0x00002aaadff40000) PSPermGen total 21248K, used 9252K [0x00002aaab0090000, 0x00002aaab1550000, 0x00002aaab5490000) object space 21248K, 43% used [0x00002aaab0090000,0x00002aaab09993e8,0x00002aaab1550000)

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  • Los Angeles Department of Building & Safety Lowers Customer Service Costs with Oracle WebCenter

    - by Kellsey Ruppel
    Register Now for this Webcast. Los Angeles Department of Building & Safety Lowers Customer Service Costs with Oracle WebCenter Los Angeles Department of Building & Safety (LADBS) is one of the largest construction permitting departments in the country, serving over 350,000 walk-in and 530,000 phone customers, and issuing over 110,000 permits worth $3 Billion every year. LADBS needed a way to migrate walk-in and phone transactions to customer self-service, so they turned to Oracle WebCenter and teamed with Oracle Partner 3Di to deliver a customer self-service portal to lower their cost of customer service operation, while increasing customer satisfaction. Attend this Webcast to learn how Oracle WebCenter has allowed Los Angeles Department of Building & Safety to: Deliver a state of the art customer self-service portal Reduce traffic on high cost, low satisfaction customer service channels Integrate business workflows and legacy applications Register Now for this Webcast. REGISTER NOW Register now for this exclusive event. Wednesday, November 14, 2012 10 a.m. PT / 1 p.m. ET Presented by: Giovani DacumosDirector of Systems, Los Angeles Department of Building & Safety Jing ReyesApplications Development Group Manager, Los Angeles Department of Building & Safety Rajiv Desai CEO, 3Di Sheetal ParanjpyeProject Manager, 3Di Presented by: Copyright © 2012, Oracle. All rights reserved. Contact Us | Legal Notices and Terms of Use | Privacy Statement

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  • SIlverlight 4RC threading - can a new Thread return the UI Thread

    - by Darko Z
    Hi all, Let's say I have a situation in Silverlight where there is a background thread (guaranteed to NOT be the UI thread) doing some work and it needs to create a new thread. Something like this: //running in a background thread Thread t = new Thread(new ThreadStart(delegate{}); t.Start(); Lets also say that the UI thread at this particular time is just hanging around doing nothing. Keeping in mind that I am not that knowledgeable about the Silverlight threading model, is there any danger of the new Thread() call giving me the UI thread? The motivation or what I am trying to achieve is not important - I do not want modification to the existing code. I just want to know if there is a possibility of getting the UI thread back unexpectedly. Cheers

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  • Thread sleep and thread join.

    - by Dhruv Gairola
    hi guys, if i put a thread to sleep in a loop, netbeans gives me a caution saying Invoking Thread.sleep in loop can cause performance problems. However, if i were to replace the sleep with join, no such caution is given. Both versions compile and work fine tho. My code is below (check the last few lines for "Thread.sleep() vs t.join()"). public class Test{ //Display a message, preceded by the name of the current thread static void threadMessage(String message) { String threadName = Thread.currentThread().getName(); System.out.format("%s: %s%n", threadName, message); } private static class MessageLoop implements Runnable { public void run() { String importantInfo[] = { "Mares eat oats", "Does eat oats", "Little lambs eat ivy", "A kid will eat ivy too" }; try { for (int i = 0; i < importantInfo.length; i++) { //Pause for 4 seconds Thread.sleep(4000); //Print a message threadMessage(importantInfo[i]); } } catch (InterruptedException e) { threadMessage("I wasn't done!"); } } } public static void main(String args[]) throws InterruptedException { //Delay, in milliseconds before we interrupt MessageLoop //thread (default one hour). long patience = 1000 * 60 * 60; //If command line argument present, gives patience in seconds. if (args.length > 0) { try { patience = Long.parseLong(args[0]) * 1000; } catch (NumberFormatException e) { System.err.println("Argument must be an integer."); System.exit(1); } } threadMessage("Starting MessageLoop thread"); long startTime = System.currentTimeMillis(); Thread t = new Thread(new MessageLoop()); t.start(); threadMessage("Waiting for MessageLoop thread to finish"); //loop until MessageLoop thread exits while (t.isAlive()) { threadMessage("Still waiting..."); //Wait maximum of 1 second for MessageLoop thread to //finish. /*******LOOK HERE**********************/ Thread.sleep(1000);//issues caution unlike t.join(1000) /**************************************/ if (((System.currentTimeMillis() - startTime) > patience) && t.isAlive()) { threadMessage("Tired of waiting!"); t.interrupt(); //Shouldn't be long now -- wait indefinitely t.join(); } } threadMessage("Finally!"); } } As i understand it, join waits for the other thread to complete, but in this case, arent both sleep and join doing the same thing? Then why does netbeans throw the caution?

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  • Thread-safety of read-only memory access

    - by Edmund
    I've implemented the Barnes-Hut gravity algorithm in C as follows: Build a tree of clustered stars. For each star, traverse the tree and apply the gravitational forces from each applicable node. Update the star velocities and positions. Stage 2 is the most expensive stage, and so is implemented in parallel by dividing the set of stars. E.g. with 1000 stars and 2 threads, I have one thread processing the first 500 stars and the second thread processing the second 500. In practice this works: it speeds the computation by about 30% with two threads on a two-core machine, compared to the non-threaded version. Additionally, it yields the same numerical results as the original non-threaded version. My concern is that the two threads are accessing the same resource (namely, the tree) simultaneously. I have not added any synchronisation to the thread workers, so it's likely they will attempt to read from the same location at some point. Although access to the tree is strictly read-only I am not 100% sure it's safe. It has worked when I've tested it but I know this is no guarantee of correctness! Questions Do I need to make a private copy of the tree for each thread? Even if it is safe, are there performance problems of accessing the same memory from multiple threads?

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  • python urllib2 thread safety

    - by jldupont
    Is urllib2 thread safe? I find that using urllib2.urlopen sometimes results in the thread issuing the call to stop functioning. Yes I have used the timeout option to no avail. Is there a thread safe HTTP GET functionality I could use? NOTE: I am not interested in using Twisted to solve this problem. I have used Twisted in the past and I love it but this time I need a simpler solution. NOTE2: I also tried httplib with the same result (blocking).

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  • python iterators and thread-safety

    - by Igor
    I have a class which is being operated on by two functions. One function creates a list of widgets and writes it into the class: def updateWidgets(self): widgets = self.generateWidgetList() self.widgets = widgets the other function deals with the widgets in some way: def workOnWidgets(self): for widget in self.widgets: self.workOnWidget(widget) each of these functions runs in it's own thread. the question is, what happens if the updateWidgets() thread executes while the workOnWidgets() thread is running? I am assuming that the iterator created as part of the for...in loop will keep some kind of reference to the old self.widgets object? So I will finish iterating over the old list... but I'd love to know for sure.

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  • Monitor.Wait, Pulse - When worker thread should conditionally behave as an actual worker thread

    - by Griever
    My particular scenario: - Main thread starts a worker thread. - Main thread needs to block itself until either worker thread is completed (yeah funny) or worker thread itself informs main thread to go on Alright, so what I did in main thread: wokerThread.Start(lockObj); lock(lockObj) Monitor.Wait(lockObj); Somewhere in worker thread: if(mainThreadShouldGoOn) lock(lockObj) Monitor.Pulse(lockObj); Also, at the end of worker thread: lock(lockObj) Monitor.Pulse(lockObj); So far, it's working perfect. But is it a good solution? Is there a better one?

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  • Thread safety with heap-allocated memory

    - by incrediman
    I was reading this: http://en.wikipedia.org/wiki/Thread_safety Is the following function thread-safe? void foo(int y){ int * x = new int[50]; /*...do some stuff with the allocated memory...*/ delete x; } In the article it says that to be thread-safe you can only use variables from the stack. Really? Why? Wouldn't subsequent calls of the above function allocate memory elsewhere?

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  • final fields and thread-safety

    - by pcjuzer
    Should it be all fields, including super-fields, of a purposively immutable java class 'final' in order to be thread-safe or is it enough to have no modifier methods? Suppose I have a POJO with non-final fields where all fields are type of some immutable class. This POJO has getters-setters, and a constructor wich sets some initial value. If I extend this POJO with knocking out modifier methods, thus making it immutable, will extension class be thread-safe?

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  • C++ thread safety - exchange data between worker and controller

    - by peterchen
    I still feel a bit unsafe about the topic and hope you folks can help me - For passing data (configuration or results) between a worker thread polling something and a controlling thread interested in the most recent data, I've ended up using more or less the following pattern repeatedly: Mutex m; tData * stage; // temporary, accessed concurrently // send data, gives up ownership, receives old stage if any tData * Send(tData * newData) { ScopedLock lock(m); swap(newData, stage); return newData; } // receiving thread fetches latest data here tData * Fetch(tData * prev) { ScopedLock lock(m); if (stage != 0) { // ... release prev prev = stage; stage = 0; } return prev; // now current } Note: This is not supposed to be a full producer-consumer queue, only the msot recent data is relevant. Also, I've skimmed ressource management somewhat here. When necessary I'm using two such stages: one to send config changes to the worker, and for sending back results. Now, my questions assuming that ScopedLock implements a full memory barrier: do stage and/or workerData need to be volatile? is volatile necessary for tData members? can I use smart pointers instead of the raw pointers - say boost::shared_ptr? Anything else that can go wrong? I am basically trying to avoid "volatile infection" spreading into tData, and minimize lock contention (a lock free implementation seems possible, too). However, I'm not sure if this is the easiest solution. ScopedLock acts as a full memory barrier. Since all this is more or less platform dependent, let's say Visual C++ x86 or x64, though differences/notes for other platforms are welcome, too. (a prelimenary "thanks but" for recommending libraries such as Intel TBB - I am trying to understand the platform issues here)

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  • Achieving Thread-Safety

    - by Smasher
    Question How can I make sure my application is thread-safe? Are their any common practices, testing methods, things to avoid, things to look for? Background I'm currently developing a server application that performs a number of background tasks in different threads and communicates with clients using Indy (using another bunch of automatically generated threads for the communication). Since the application should be highly availabe, a program crash is a very bad thing and I want to make sure that the application is thread-safe. No matter what, from time to time I discover a piece of code that throws an exception that never occured before and in most cases I realize that it is some kind of synchronization bug, where I forgot to synchronize my objects properly. Hence my question concerning best practices, testing of thread-safety and things like that. mghie: Thanks for the answer! I should perhaps be a little bit more precise. Just to be clear, I know about the principles of multithreading, I use synchronization (monitors) throughout my program and I know how to differentiate threading problems from other implementation problems. But nevertheless, I keep forgetting to add proper synchronization from time to time. Just to give an example, I used the RTL sort function in my code. Looked something like FKeyList.Sort (CompareKeysFunc); Turns out, that I had to synchronize FKeyList while sorting. It just don't came to my mind when initially writing that simple line of code. It's these thins I wanna talk about. What are the places where one easily forgets to add synchronization code? How do YOU make sure that you added sync code in all important places?

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  • Static dictionary in .Net Thread safety

    - by Emmanuel
    Reading msdn documentation for dictionaries it says : "Public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe." Those this mean that with a dictionary such as this : static object syncObject = new object(); static Dictionary<string,MyObject> mydictionary= new Dictionary<string, MyObject>(); Is doing something like the code below unnecessary? lock (syncObject) { context = new TDataContext(); mydictionary.Add("key", myObject); }

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  • XNA Xbox 360 Content Manager Thread freezing Draw Thread

    - by Alikar
    I currently have a game that takes in large images, easily bigger than 1MB, to serve as backgrounds. I know exactly when this transition is supposed to take place, so I made a loader class to handle loading these large images in the background, but when I load the images it still freezes the main thread where the drawing takes place. Since this code runs on the 360 I move the thread to the 4th hardware thread, but that doesn't seem to help. Below is the class I am using. Any thoughts as to why my new content manager which should be in its own thread is interrupting the draw in my main thread would be appreciated. namespace FileSystem { /// <summary> /// This is used to reference how many objects reference this texture. /// Everytime someone references a texture we increase the iNumberOfReferences. /// When a class calls remove on a specific texture we check to see if anything /// else is referencing the class, if it is we don't remove it. If there isn't /// anything referencing the texture its safe to dispose of. /// </summary> class TextureContainer { public uint uiNumberOfReferences = 0; public Texture2D texture; } /// <summary> /// This class loads all the files from the Content. /// </summary> static class FileManager { static Microsoft.Xna.Framework.Content.ContentManager Content; static EventWaitHandle wh = new AutoResetEvent(false); static Dictionary<string, TextureContainer> Texture2DResourceDictionary; static List<Texture2D> TexturesToDispose; static List<String> TexturesToLoad; static int iProcessor = 4; private static object threadMutex = new object(); private static object Texture2DMutex = new object(); private static object loadingMutex = new object(); private static bool bLoadingTextures = false; /// <summary> /// Returns if we are loading textures or not. /// </summary> public static bool LoadingTexture { get { lock (loadingMutex) { return bLoadingTextures; } } } /// <summary> /// Since this is an static class. This is the constructor for the file loadeder. This is the version /// for the Xbox 360. /// </summary> /// <param name="_Content"></param> public static void Initalize(IServiceProvider serviceProvider, string rootDirectory, int _iProcessor ) { Content = new Microsoft.Xna.Framework.Content.ContentManager(serviceProvider, rootDirectory); Texture2DResourceDictionary = new Dictionary<string, TextureContainer>(); TexturesToDispose = new List<Texture2D>(); iProcessor = _iProcessor; CreateThread(); } /// <summary> /// Since this is an static class. This is the constructor for the file loadeder. /// </summary> /// <param name="_Content"></param> public static void Initalize(IServiceProvider serviceProvider, string rootDirectory) { Content = new Microsoft.Xna.Framework.Content.ContentManager(serviceProvider, rootDirectory); Texture2DResourceDictionary = new Dictionary<string, TextureContainer>(); TexturesToDispose = new List<Texture2D>(); CreateThread(); } /// <summary> /// Creates the thread incase we wanted to set up some parameters /// Outside of the constructor. /// </summary> static public void CreateThread() { Thread t = new Thread(new ThreadStart(StartThread)); t.Start(); } // This is the function that we thread. static public void StartThread() { //BBSThreadClass BBSTC = (BBSThreadClass)_oData; FileManager.Execute(); } /// <summary> /// This thread shouldn't be called by the outside world. /// It allows the File Manager to loop. /// </summary> static private void Execute() { // Make sure our thread is on the correct processor on the XBox 360. #if WINDOWS #else Thread.CurrentThread.SetProcessorAffinity(new int[] { iProcessor }); Thread.CurrentThread.IsBackground = true; #endif // This loop will load textures into ram for us away from the main thread. while (true) { wh.WaitOne(); // Locking down our data while we process it. lock (threadMutex) { lock (loadingMutex) { bLoadingTextures = true; } bool bContainsKey = false; for (int con = 0; con < TexturesToLoad.Count; con++) { // If we have already loaded the texture into memory reference // the one in the dictionary. lock (Texture2DMutex) { bContainsKey = Texture2DResourceDictionary.ContainsKey(TexturesToLoad[con]); } if (bContainsKey) { // Do nothing } // Otherwise load it into the dictionary and then reference the // copy in the dictionary else { TextureContainer TC = new TextureContainer(); TC.uiNumberOfReferences = 1; // We start out with 1 referece. // Loading the texture into memory. try { TC.texture = Content.Load<Texture2D>(TexturesToLoad[con]); // This is passed into the dictionary, thus there is only one copy of // the texture in memory. // There is an issue with Sprite Batch and disposing textures. // This will have to wait until its figured out. lock (Texture2DMutex) { bContainsKey = Texture2DResourceDictionary.ContainsKey(TexturesToLoad[con]); Texture2DResourceDictionary.Add(TexturesToLoad[con], TC); } // We don't have the find the reference to the container since we // already have it. } // Occasionally our texture will already by loaded by another thread while // this thread is operating. This mainly happens on the first level. catch (Exception e) { // If this happens we don't worry about it since this thread only loads // texture data and if its already there we don't need to load it. } } Thread.Sleep(100); } } lock (loadingMutex) { bLoadingTextures = false; } } } static public void LoadTextureList(List<string> _textureList) { // Ensuring that we can't creating threading problems. lock (threadMutex) { TexturesToLoad = _textureList; } wh.Set(); } /// <summary> /// This loads a 2D texture which represents a 2D grid of Texels. /// </summary> /// <param name="_textureName">The name of the picture you wish to load.</param> /// <returns>Holds the image data.</returns> public static Texture2D LoadTexture2D( string _textureName ) { TextureContainer temp; lock (Texture2DMutex) { bool bContainsKey = false; // If we have already loaded the texture into memory reference // the one in the dictionary. lock (Texture2DMutex) { bContainsKey = Texture2DResourceDictionary.ContainsKey(_textureName); if (bContainsKey) { temp = Texture2DResourceDictionary[_textureName]; temp.uiNumberOfReferences++; // Incrementing the number of references } // Otherwise load it into the dictionary and then reference the // copy in the dictionary else { TextureContainer TC = new TextureContainer(); TC.uiNumberOfReferences = 1; // We start out with 1 referece. // Loading the texture into memory. try { TC.texture = Content.Load<Texture2D>(_textureName); // This is passed into the dictionary, thus there is only one copy of // the texture in memory. } // Occasionally our texture will already by loaded by another thread while // this thread is operating. This mainly happens on the first level. catch(Exception e) { temp = Texture2DResourceDictionary[_textureName]; temp.uiNumberOfReferences++; // Incrementing the number of references } // There is an issue with Sprite Batch and disposing textures. // This will have to wait until its figured out. Texture2DResourceDictionary.Add(_textureName, TC); // We don't have the find the reference to the container since we // already have it. temp = TC; } } } // Return a reference to the texture return temp.texture; } /// <summary> /// Go through our dictionary and remove any references to the /// texture passed in. /// </summary> /// <param name="texture">Texture to remove from texture dictionary.</param> public static void RemoveTexture2D(Texture2D texture) { foreach (KeyValuePair<string, TextureContainer> pair in Texture2DResourceDictionary) { // Do our references match? if (pair.Value.texture == texture) { // Only one object or less holds a reference to the // texture. Logically it should be safe to remove. if (pair.Value.uiNumberOfReferences <= 1) { // Grabing referenc to texture TexturesToDispose.Add(pair.Value.texture); // We are about to release the memory of the texture, // thus we make sure no one else can call this member // in the dictionary. Texture2DResourceDictionary.Remove(pair.Key); // Once we have removed the texture we don't want to create an exception. // So we will stop looking in the list since it has changed. break; } // More than one Object has a reference to this texture. // So we will not be removing it from memory and instead // simply marking down the number of references by 1. else { pair.Value.uiNumberOfReferences--; } } } } /*public static void DisposeTextures() { int Count = TexturesToDispose.Count; // If there are any textures to dispose of. if (Count > 0) { for (int con = 0; con < TexturesToDispose.Count; con++) { // =!THIS REMOVES THE TEXTURE FROM MEMORY!= // This is not like a normal dispose. This will actually // remove the object from memory. Texture2D is inherited // from GraphicsResource which removes it self from // memory on dispose. Very nice for game efficency, // but "dangerous" in managed land. Texture2D Temp = TexturesToDispose[con]; Temp.Dispose(); } // Remove textures we've already disposed of. TexturesToDispose.Clear(); } }*/ /// <summary> /// This loads a 2D texture which represnets a font. /// </summary> /// <param name="_textureName">The name of the font you wish to load.</param> /// <returns>Holds the font data.</returns> public static SpriteFont LoadFont( string _fontName ) { SpriteFont temp = Content.Load<SpriteFont>( _fontName ); return temp; } /// <summary> /// This loads an XML document. /// </summary> /// <param name="_textureName">The name of the XML document you wish to load.</param> /// <returns>Holds the XML data.</returns> public static XmlDocument LoadXML( string _fileName ) { XmlDocument temp = Content.Load<XmlDocument>( _fileName ); return temp; } /// <summary> /// This loads a sound file. /// </summary> /// <param name="_fileName"></param> /// <returns></returns> public static SoundEffect LoadSound( string _fileName ) { SoundEffect temp = Content.Load<SoundEffect>(_fileName); return temp; } } }

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  • Difference between Thread.Sleep(0) and Thread.Yield()

    - by Xose Lluis
    As Java has had Sleep and Yield from long ago, I've found answers for that platform, but not for .Net .Net 4 includes the new Thread.Yield() static method. Previously the common way to hand over the CPU to other process was Thread.Sleep(0). Apart from Thread.Yield() returning a boolean, are there other performance, OS internals differences? For example, I'm not sure if Thread.Sleep(0) checks if other thread is ready to run before changing the current Thread to waiting state... if that's not the case, when no other threads are ready, Thread.Sleep(0) would seem rather worse that Thread.Yield().

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  • [Java] Type safety: Unchecked cast from Object

    - by Matthew
    Hi, I try to cast an object to my Action class, but it results in a warning: Type safety: Unchecked cast from Object to Action<ClientInterface> Action<ClientInterface> action = null; try { Object o = c.newInstance(); if (o instanceof Action<?>) { action = (Action<ClientInterface>) o; } else { // TODO 2 Auto-generated catch block throw new InstantiationException(); } [...] Thank you for any help

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  • Singletons, thread safety and structuremap

    - by Ben
    Hi, Currently I have the following class: public class PluginManager { private static bool s_initialized; private static object s_lock = new object(); public static void Initialize() { if (!s_initialized) { lock (s_lock) { if (!s_initialized) { // initialize s_initialized = true; } } } } } The important thing here is that Initialize() should only be executed once whilst the application is running. I thought that I would refactor this into a singleton class since this would be more thread safe?: public sealed class PluginService { static PluginService() { } private static PluginService _instance = new PluginService(); public static PluginService Instance { get { return _instance; } } private bool s_initialized; public void Initialize() { if (!s_initialized) { // initialize s_initialized = true; } } } Question one, is it still necessary to have the lock here (I have removed it) since we will only ever be working on the same instance? Finally, I want to use DI and structure map to initialize my servcices so I have refactored as below: public interface IPluginService { void Initialize(); } public class NewPluginService : IPluginService { private bool s_initialized; public void Initialize() { if (!s_initialized) { // initialize s_initialized = true; } } } And in my registry: ForRequestedType<IPluginService>() .TheDefaultIsConcreteType<NewPluginService>().AsSingletons(); This works as expected (singleton returning true in the following code): var instance1 = ObjectFactory.GetInstance<IPluginService>(); var instance2 = ObjectFactory.GetInstance<IPluginService>(); bool singleton = (instance1 == instance2); So my next question, is the structure map solution as thread safe as the singleton class (second example). The only downside is that this would still allow NewPluginService to be instantiated directly (if not using structure map). Many thanks, Ben

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  • .net thread safety

    - by george9170
    Why is locking a type considered very bad? For example, lock(typeof(DateTime)) I understand that static methods of any class in .net is considered thread safe, and that instance members are not. So it isn't necessary to lock DateTime while you are using it. The book I am reading doesn't explain why it is bad it just says it is. Any explanation will be great.

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