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• What happens when I MPI_Send to a process that has finished?

  - by nieldw

  What happens when I MPI_Send to a process that has finished? I am learning MPI, and writing a small sugar distribution-simulation in C. When the factories stop producing, those processes end. When warehouses run empty, they end. Can I somehow tell if the shop's order to a warehouse did not succeed(because the warehouse process has ended) by looking at the return value of MPI_Send? The documentation doesn't mention a specific error code for this situation, but that no error is returned for success. Can I do: if (MPI_Send(...)) { ... /* destination has ended */ ... } And disregard the error code? Thanks

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• C++: is it safe to read an integer variable that's being concurrently modified without locking?

  - by Hongli

  Suppose that I have an integer variable in a class, and this variable may be concurrently modified by other threads. Writes are protected by a mutex. Do I need to protect reads too? I've heard that there are some hardware architectures on which, if one thread modifies a variable, and another thread reads it, then the read result will be garbage; in this case I do need to protect reads. I've never seen such architectures though. This question assumes that a single transaction only consists of updating a single integer variable so I'm not worried about the states of any other variables that might also be involved in a transaction.

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• Waiting for a subset of threads in a Java ThreadPool

  - by David Semeria

  Let's say I have a thread pool containing X items, and a given task employs Y of these items (where Y is much smaller than X). I want to wait for all of the threads of a given task (Y items) to finish, not the entire thread pool. If the thread pool's execute() method returned a reference to the employed thread I could simply join() to each of these Y threads, but it doesn't. Does anyone know of an elegant way to accomplish this? Thanks.

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• Limiting object allocation over multiple threads

  - by John

  I have an application which retrieves and caches the results of a clients query. The client then requests different chunks of data and the application sends the relevant results and removes them from the cache. A new requirement for this application is that there needs to be a run-time configurable maximum number of results which may be cached. I've taken the naive approach and implemented this by using a counter under a lock which is incremented every time a result is cached and decremented whenever a result is removed from the cache. Unfortunately, this has drastically reduced the applications performance when processing a large number of concurrent requests. I have tried both a critical section lock and spin-lock; the performance improves a bit with a spin-lock, but is still unacceptably slow. Is there a better way to solve this problem which may improve performance? Right now I have a thread pool that services requests and each request is tied to a Request object which stores that cached results for that particular request. Here is a simplified pseudo code version of my current implementation: void ResultCallback( Result result, Request *request ) { lock totalResultsCached lock cachedLimit if( totalResultsCached + 1 > cachedLimit ) { unlock cachedLimit unlock totalResultsCached //cancel the request return; } ++totalResultsCached; unlock cachedLimit unlock totalResultsCached request.add(result) } void SendResults( int resultsToSend, Request *request ) { while ( resultsToSend > 0 ) { send(request.remove()) lock totalResultsCached --totalResultsCached unlock totalResultsCached --resultsToSend; } }

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• Create table class as a singleton

  - by Mark

  I got a class that I use as a table. This class got an array of 16 row classes. These row classes all have 6 double variables. The values of these rows are set once and never change. Would it be a good practice to make this table a singleton? The advantage is that it cost less memory, but the table will be called from multiple threads so I have to synchronize my code which way cause a bit slower application. However lookups in this table are probably a very small portion of the total code that is executed. EDIT: This is my code, are there better ways to do this or is this a good practice? Removed synchronized keyword according to recommendations in this question. final class HalfTimeTable { private HalfTimeRow[] table = new HalfTimeRow[16]; private static final HalfTimeTable instance = new HalfTimeTable(); private HalfTimeTable() { if (instance != null) { throw new IllegalStateException("Already instantiated"); } table[0] = new HalfTimeRow(4.0, 1.2599, 0.5050, 1.5, 1.7435, 0.1911); table[1] = new HalfTimeRow(8.0, 1.0000, 0.6514, 3.0, 1.3838, 0.4295); //etc } @Override @Deprecated public Object clone() throws CloneNotSupportedException { throw new CloneNotSupportedException(); } public static HalfTimeTable getInstance() { return instance; } public HalfTimeRow getRow(int rownumber) { return table[rownumber]; } }

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• LinkedBlockingQueue limit ignored?

  - by tgguy

  I created a Java LinkedBlockingQueue like new LinkedBlockingQueue(1) to limit the size of the queue to 1. However, in my testing, this seems to be ignored and there is often several things in the queue at any given time. Why is this?

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• Queue implementation with blocked 'take()' but with eviction policy

  - by mjlee

  Is there an implementation with a blocking queue for take but bounded by a maximum size. However, when the maximum size is reached, instead of blocking, it will remove from the head. One usage is that if I have a very slow consumer, the system will not crash ( runs out of memory ) rather these message will be removed.

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• is this class thread safe?

  - by flash

  consider this class,with no instance variables and only methods which are non-synchronous can we infer from this info that this class in Thread-safe? public class test{ public void test1{ // do something } public void test2{ // do something } public void test3{ // do something } }

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• How to address thread-safety of service data used for maintaining static local variables in C++?

  - by sharptooth

  Consider the following scenario. We have a C++ function with a static local variable: void function() { static int variable = obtain(); //blahblablah } the function needs to be called from multiple threads concurrently, so we add a critical section to avoid concurrent access to the static local: void functionThreadSafe() { CriticalSectionLockClass lock( criticalSection ); static int variable = obtain(); //blahblablah } but will this be enough? I mean there's some magic that makes the variable being initialized no more than once. So there's some service data maintained by the runtime that indicates whether each static local has already been initialized. Will the critical section in the above code protect that service data as well? Is any extra protection required for this scenario?

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• Real World Examples of read-write in concurrent software

  - by Richard Fabian

  I'm looking for real world examples of needing read and write access to the same value in concurrent systems. In my opinion, many semaphores or locks are present because there's no known alternative (to the implementer,) but do you know of any patterns where mutexes seem to be a requirement? In a way I'm asking for candidates for the standard set of HARD problems for concurrent software in the real world.

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• What scenarios/settings will result in a query on SQL Server (2008) return stale data

  - by s1mm0t

  Most applications rarely need to display 100% accurate data. For example if this stack overflow question displays that there have been 0 views, when there have really been 10, it doesn't really matter. This is one way that the (perceived) performance of applications can be improved, by caching results and therefore sometimes not showing 100% accurate results. There are some cases where the data does need to be 100% accurate though. So if I run the query select * from Foo I want to be sure that the results are not stale. Now depending on how my database is set up, other activity on the database, use of transactions and isolation levels etc this query may or may not be a true reflection of the world. What scenario's and settings can people think of that will result in this query returning stale results or given that another connection is part way through a transaction that has updated this table, how can I guarantee that when the above query returns, the results will be accurate.

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• java threads don't see shared boolean changes

  - by andymur

  Here the code class Aux implements Runnable { private Boolean isOn = false; private String statusMessage; private final Object lock; public Aux(String message, Object lock) { this.lock = lock; this.statusMessage = message; } @Override public void run() { for (;;) { synchronized (lock) { if (isOn && "left".equals(this.statusMessage)) { isOn = false; System.out.println(statusMessage); } else if (!isOn && "right".equals(this.statusMessage)) { isOn = true; System.out.println(statusMessage); } if ("left".equals(this.statusMessage)) { System.out.println("left " + isOn); } } } } } public class Question { public static void main(String [] args) { Object lock = new Object(); new Thread(new Aux("left", lock)).start(); new Thread(new Aux("right", lock)).start(); } } In this code I expect to see: left, right, left right and so on, but when Thread with "left" message changes isOn to false, Thread with "right" message don't see it and I get ("right true" and "left false" console messages), left thread don't get isOn in true, but right Thread can't change it cause it always see old isOn value (true). When i add volatile modifier to isOn nothing changes, but if I change isOn to some class with boolean field and change this field then threads are see changes and it works fine Thanks in advance.

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• Terminating a long-executing thread and then starting a new one in response to user changing parameters via UI in an applet

  - by user1817170

  I have an applet which creates music using the JFugue API and plays it for the user. It allows the user to input a music phrase which the piece will be based on, or lets them choose to have a phrase generated randomly. I had been using the following method (successfully) to simply stop and start the music, which runs in a thread using the Player class from JFugue. I generate the music using my classes and user input from the applet GUI...then... private playerThread pthread; private Thread threadPlyr; private Player player; (from variables declaration) public void startMusic(Pattern p) // pattern is a JFugue object which holds the generated music { if (pthread == null) { pthread = new playerThread(); } else { pthread = null; pthread = new playerThread(); } if (threadPlyr == null) { threadPlyr = new Thread(pthread); } else { threadPlyr = null; threadPlyr = new Thread(pthread); } pthread.setPattern(p); threadPlyr.start(); } class playerThread implements Runnable // plays midi using jfugue Player { private Pattern pt; public void setPattern(Pattern p) { pt = p; } @Override public void run() { try { player.play(pt); // takes a couple mins or more to execute resetGUI(); } catch (Exception exception) { } } } And the following to stop music when user presses the stop/start button while Player.isPlaying() is true: public void stopMusic() { threadPlyr.interrupt(); threadPlyr = null; pthread = null; player.stop(); } Now I want to implement a feature which will allow the user to change parameters while the music is playing, create an updated music pattern, and then play THAT pattern. Basically, the idea is to make it simulate "real time" adjustments to the generated music for the user. Well, I have been beating my head against the wall on this for a couple of weeks. I've read all the standard java documentation, researched, read, and searched forums, and I have tried many different ideas, none of which have succeeded. The problem I've run into with all approaches I've tried is that when I start the new thread with the new, updated musical pattern, all the old threads ALSO start, and there is a cacophony of unintelligible noise instead of my desired output. From what I've gathered, the issue seems to be that all the methods I've come across require that the thread is able to periodically check the value of a "flag" variable and then shut itself down from within its "run" block in response to that variable. However, since my thread makes a call that takes several minutes minimum to execute (playing the music), and I need to terminate it WHILE it is executing this, there is really no safe way to do so. So, I'm wondering if there is something I'm missing when it comes to threads, or if perhaps I can accomplish my goal using a totally different approach. Any ideas or guidance is greatly appreciated! Thank you!

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• Why do threads spontaneously awake from wait()?

  - by Zwei Steinen

  Hi, I was wondering why threads spontaneously awake from wait() in java. Is it a design decision? Is it a compromise?

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• definition of wait-free (referring to parallel programming)

  - by tecuhtli

  In Maurice Herlihy paper "Wait-free synchronization" he defines wait-free: "A wait-free implementation of a concurrent data object is one that guarantees that any process can complete any operation in a finite number of steps, regardless the execution speeds on the other processes." www.cs.brown.edu/~mph/Herlihy91/p124-herlihy.pdf Let's take one operation op from the universe. (1) Does the definition mean: "Every process completes a certain operation op in the same finite number n of steps."? (2) Or does it mean: "Every process completes a certain operation op in any finite number of steps. So that a process can complete op in k steps another process in j steps, where k != j."? Just by reading the definition i would understand meaning (2). However this makes no sense to me, since a process executing op in k steps and another time in k + m steps meets the definition, but m steps could be a waiting loop. If meaning (2) is right, can anybody explain to me, why this describes wait-free? In contrast to (2), meaning (1) would guarantee that op is executed in the same number of steps k. So there can't be any additional steps m that are necessary e.g. in a waiting loop. Which meaning is right and why? Thanks a lot, sebastian

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• What to do if one library is not multi-threaded ?

  - by LB

  Hi, I would like to multi-thread an application, however one library i'm using is not multi-thread capable (i don't know what's the right word ? synchronized ?). What are my options ? As far as i know there's nothing in between threads and processes (Runtime.exec) in java (no abstraction in the jvm to have something like an isolated "java process"). How would you deal with that ?

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• Why does java.util.concurrent.ArrayBlockingQueue use 'while' loops instead of 'if' around calls to

  - by theFunkyEngineer

  I have been playing with my own version of this, using 'if', and all seems to be working fine. Of course this will break down horribly if signalAll() is used instead of signal(), but if only one thread at a time is notified, how can this go wrong? Their code here - check out the put() and take() methods; a simpler and more-to-the-point implementation can be seen at the top of the JavaDoc for Condition. Relevant portion of my implementation below. public Object get() { lock.lock(); try { if( items.size() < 1 ) hasItems.await(); Object poppedValue = items.getLast(); items.removeLast(); hasSpace.signal(); return poppedValue; } catch (InterruptedException e) { e.printStackTrace(); return null; } finally { lock.unlock(); } } public void put(Object item) { lock.lock(); try { if( items.size() >= capacity ) hasSpace.await(); items.addFirst(item); hasItems.signal(); return; } catch (InterruptedException e) { e.printStackTrace(); } finally { lock.unlock(); } } P.S. I know that generally, particularly in lib classes like this, one should let the exceptions percolate up.

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• Do these methods have same output?

  - by devrimbaris

  protected synchronized boolean isTimeoutOccured(Duration timeoutDuration) { DateTime now = new DateTime(); if (timeoutOccured == false) { if (new Duration(requestTime.getMillis(), now.getMillis()).compareTo(timeoutDuration) > 0) { timeoutOccured = true; } } return timeoutOccured; } protected boolean isTimeoutOccured2(Duration timeoutDuration) { return atomicTimeOut.compareAndSet(false, new Duration(requestTime.getMillis(), new DateTime().getMillis()).compareTo(timeoutDuration) > 0); }

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• How to bring coordination between file system and database?

  - by Lock up

  I am working on a online file management project. We are storing references on the database (sql server) and files data on the on file system. We are facing a problem of coordination between file system and database while we are uploading a file and also in case of deleting a file. First we create a reference in the database or store files on file system. The problem is that if I create a reference in the database first and then store a file on file system, but while storing files on the file system any type of error occur, then the reference for that file is created in the database but no file data exist on the file system. Please give me some solution how to deal with such situation. I am badly in need of it. This case happens also while we deleting a file?

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• how to bring coordination btween file system and databse?

  - by Lock up

  i am working on a online file management project.In which we are storing references on the database(sql server) and files data on the on file system;.In which we are facing a problem of coordination between file system and database while we are uploading a file and also in case of deleting a file that first we create a reference in the data base or store files on file system;;the problem is that if create a reference in the database first and then storing a file on file system.bur while storing files on the file system any type of error occur.then reference for that file is created in the database but no file data on the file system;; please give me som esolution how to deal with such situation;;i am badly in need of it;; This case is also while we deleting a file?

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• Why is my concurrency capacity so low for my web app on a LAMP EC2 instance?

  - by AMF

  I come from a web developer background and have been humming along building my PHP app, using the CakePHP framework. The problem arose when I began the ab (Apache Bench) testing on the Amazon EC2 instance in which the app resides. I'm getting pretty horrendous average page load times, even though I'm running a c1.medium instance (2 cores, 2GB RAM), and I think I'm doing everything right. I would run: ab -n 200 -c 20 http://localhost/heavy-but-view-cached-page.php Here are the results: Concurrency Level: 20 Time taken for tests: 48.197 seconds Complete requests: 200 Failed requests: 0 Write errors: 0 Total transferred: 392111200 bytes HTML transferred: 392047600 bytes Requests per second: 4.15 [#/sec] (mean) Time per request: 4819.723 [ms] (mean) Time per request: 240.986 [ms] (mean, across all concurrent requests) Transfer rate: 7944.88 [Kbytes/sec] received While the ab test is running, I run VMStat, which shows that Swap stays at 0, CPU is constantly at 80-100% (although I'm not sure I can trust this on a VM), RAM utilization ramps up to about 1.6G (leaving 400M free). Load goes up to about 8 and site slows to a crawl. Here's what I think I'm doing right on the code side: In Chrome browser uncached pages typically load in 800-1000ms, and cached pages load in 300-500ms. Not stunning, but not terrible either. Thanks to view caching, there might be at most one DB query per page-load to write session data. So we can rule out a DB bottleneck. I have APC on. I am using Memcached to serve the view cache and other site caches. xhprof code profiler shows that cached pages take up 10MB-40MB in memory and 100ms - 1000ms in wall time. Pages that would be the worst offenders would look something like this in xhprof: Total Incl. Wall Time (microsec): 330,143 microsecs Total Incl. CPU (microsecs): 320,019 microsecs Total Incl. MemUse (bytes): 36,786,192 bytes Total Incl. PeakMemUse (bytes): 46,667,008 bytes Number of Function Calls: 5,195 My Apache config: KeepAlive On MaxKeepAliveRequests 100 KeepAliveTimeout 3 <IfModule mpm_prefork_module> StartServers 5 MinSpareServers 5 MaxSpareServers 10 MaxClients 120 MaxRequestsPerChild 1000 </IfModule> Is there something wrong with the server? Some gotcha with the EC2? Or is it my code? Some obvious setting I should look into? Too many DNS lookups? What am I missing? I really want to get to 1,000 concurrency capacity, but at this rate, it ain't gonna happen.

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• How can we handle concurrency errors in LINQ to SQL?

  - by masoud ramezani

  How can we handle concurrency errors in LINQ to SQL?

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• How to solve concurrency problems in ASP.NET Windows-Workflow and ActiveRecord/NHibernate?

  - by Famous Nerd

  I have found that ActiveRecord uses the Session-Scope object within the ASP.NET application and that if the web-site is read-write we can have a tug-o-war between the Workflow's own Data-Access SessionScope and that of the ASP.NET site. I would really like to have the WindowsWorkflow Runtime use the same object session as the web-site however, they have different lifetimes. Sometimes, a web-request may save a very simple piece of data which would execute quickly however, if the web-site kicks off a workflow process.. how can that workflow make data-modifications while still allowing the Appliaction_EndRequest to dispose the ASP.NET SessionScope ... it's like ownership of the SessionScope should be shared between the workflow runtime and the ASP.NET website. Manual Workflow Scheduler may be the Savior... if a workflow is synchronous and merely uses CallExternalMethod to interact with the Host then we could constrain all the data-access to the host.. then the sessionScope can exist once. This however, won't solve the problem of a delay activity... if this delay fires, we could need to update data... in this case we'd need an isolated Session Scope and concurrency may arise. This however, differs from SharePoint workflows where it seems that the SharePoint workflow can save data from the web and the workflow and that concurrency is handled through other means. Can anyone offer any suggestions on how to allow the workflow to manage data and play nice with ASP.NET web sites?

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• Please suggest good book/website to for Threads and Concurrency?

  - by learner

  I have gone through Head First Java and some other sites but I couldn't find complete stuff related to Threads and additional concurrency packages at one place. Please suggest a book/website which covers complete Threads with more details like Synchronize and locking of objects More detailed about volatile Visibility issues in Threads java.util.concurrent package java.util.concurrent.atomic package

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• how to handle db concurrency in client-server application in C#?

  - by RAJ K

  I am developing an application in C# WPF which will have Client-Server architecture (Client will do products sales billing). I am novice in this area and I asked this question to start my development process. Click here to view question. So, ultimately I have selected MySQl, WCf & WPF. Now I have one silly question. Do i need to handle DB concurrency explicitly in my application (like 3 clients inserting data same time) or MySQl will handle this without any conflict? To accomplish my project i thought, I will create a service in WCf which will do DB queries from client application. Do you have any suggestion to improve my application performance.

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