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• Super simple high performance http server

  - by masylum

  I´m building a url shortener web application and I would like to know the best architecture to do it in order to provide a fast and reliable service. I would like to have two separate servicies in different machines. The first machine will have the application itself with a apache, nginx, whatever.. The second one will contain the database. The third one will be the one that will be responsible to handle the short url petitions. For the third machine I just need to accept one kind of http petition (GET www.domain.com/shorturl), but it have to do it really fast and it should be stable enough. Which server do you recommend me? Thank's in advance and sorry for my english

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• Is NFS capable of preserving order of operations?

  - by JustJeff

  I have a diskless host 'A', that has a directory NFS mounted on server 'B'. A process on A writes to two files F1 and F2 in that directory, and a process on B monitors these files for changes. Assume that B polls for changes faster than A is expected to make them. Process A seeks the head of the files, writes data, and flushes. Process B seeks the head of the files and does reads. Are there any guarantees about how the order of the changes performed by A will be detected at B? Specifically, if A alternately writes to one file, and then the other, is it reasonable to expect that B will notice alternating changes to F1 and F2? Or could B conceivably detect a series of changes on F1 and then a series on F2? I know there are a lot of assumptions embedded in the question. For instance, I am virtually certain that, even operating on just one file, if A performs 100 operations on the file, B may see a smaller number of changes that give the same result, due to NFS caching some of the actions on A before they are communicated to B. And of course there would be issues with concurrent file access even if NFS weren't involved and both the reading and the writing process were running on the same real file system. The reason I'm even putting the question up here is that it seems like most of the time, the setup described above does detect the changes at B in the same order they are made at A, but that occasionally some events come through in transposed order. So, is it worth trying to make this work? Is there some way to tune NFS to make it work, perhaps cache settings or something? Or is fine-grained behavior like this just too much expect from NFS?

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• What is your development checklist for Java low-latency application?

  - by user49767

  I would like to create comprehensive checklist for Java low latency application. Can you add your checklist here? Here is my list 1. Make your objects immutable 2. Try to reduce synchronized method 3. Locking order should be well documented, and handled carefully 4. Use profiler 5. Use Amdhal's law, and find the sequential execution path 6. Use Java 5 concurrency utilities, and locks 7. Avoid Thread priorities as they are platform dependent 8. JVM warmup can be used As per my definition, low-latency application is tuned for every Milli-seconds.

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• What is the fastest way to send 100,000 HTTP requests in Python?

  - by Igor G.

  Hello, I am opening a file which has 100,000 url's. I need to send an http request to each url and print the status code. I am using Python 2.6, and so far looked at the many confusing ways Python implements threading/concurrency. I have even looked at the python concurrence library, but cannot figure out how to write this program correctly. Has anyone come across a similar problem? I guess generally I need to know how to perform thousands of tasks in Python as fast as possible - I suppose that means 'concurrently'. Thank you, Igor

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• .NET or Windows Synchronization Primitives Performance Specifications

  - by ovanes

  Hello *, I am currently writing a scientific article, where I need to be very exact with citation. Can someone point me to either MSDN, MSDN article, some published article source or a book, where I can find performance comparison of Windows or .NET Synchronization primitives. I know that these are in the descending performance order: Interlocked API, Critical Section, .NET lock-statement, Monitor, Mutex, EventWaitHandle, Semaphore. Many Thanks, Ovanes P.S. I found a great book: Concurrent Programming on Windows by Joe Duffy. This book is written by one of the head concurrency developers for .NET Framework and is simply brilliant with lots of explanations, how things work or were implemented.

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• What is an "incompletely constructed object"?

  - by Joonas Pulakka

  Goetz's Java Concurrency in Practice, page 41, mentions how this reference can escape during construction. A "don't do this" example: public class ThisEscape { public ThisEscape(EventSource source) { source.registerListener( new EventListener() { public void onEvent(Event e) { doSomething(e); } }); } } Here this is "escaping" via the fact that doSomething(e) refers to the enclosing ThisEscape instance. The book states: Publishing an object from within its constructor can publish an incompletely constructed object. This is true even if the publication is the last statement in the constructor. If the this reference escapes during construction, the object is considered not properly constructed. I don't quite get this. If the publication is the last statement in the constructor, hasn't all the constructing work been done before that? How come is this not valid by then? Apparently there's some voodoo going on after that, but what?

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• Scheduling Swingworker threads

  - by Simonw

  Hi, I have a 2 processes to perform in my swing application, one to fill a list, and one to do operations on each element on the list. I've just moved the 2 processes into Swingworker threads to stop the GUI locking up while the tasks are performed, and because I will need to do this set of operations to several lists, so concurrency wouldn't be a bad idea in the first place. However, when I just ran fillList.execute();doStuffToList.execute(); the doStuffToList thread to ran on the empty list (duh...). How do I tell the second process to wait until the first one is done? I suppose I could just nest the second process at the end of the first one, but i dunno, it seems like bad practice.

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• Is it safe to lock a static variable in a non-static class?

  - by Dario Solera

  I've got a class that manages a shared resource. Now, since access to the resource depends on many parameters, this class is instantiated and disposed several times during the normal execution of the program. The shared resource does not support concurrency, so some kind of locking is needed. The first thing that came into my mind is having a static instance in the class, and acquire locks on it, like this: // This thing is static! static readonly object MyLock = new object(); // This thing is NOT static! MyResource _resource = ...; public DoSomeWork() { lock(MyLock) { _resource.Access(); } } Does that make sense, or would you use another approach?

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• Can I get the stack traces of all threads in my c# app?

  - by Drew Shafer

  I'm debugging an apparent concurrency issue in a largish app that I hack on at work. The bug in question only manifests on certain lower-performance machines after running for many (12+) hours, and I have never reproduced it in the debugger. Because of this, my debugging tools are basically limited to analyzing log files. C# makes it easy to get the stack trace of the thread throwing the exception, but I'd like to additionally get the stack traces of every other thread currently executing in my AppDomain at the time the exception was thrown. Is this possible?

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• Can Ruby Fibers be Concurrent?

  - by Jesse J

  I'm trying to get some speed up in my program and I've been told that Ruby Fibers are faster than threads and can take advantage of multiple cores. I've looked around, but I just can't find how to actually run different fibers concurrently. With threads you can dO this: threads = [] threads << Thread.new {Do something} threads << Thread.new {Do something} threads.each {|thread| thread.join} I can't see how to do something like this with fibers. All I can find is yield and resume which seems like just a bunch of starting and stopping between the fibers. Is there a way to do true concurrency with fibers?

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• Recommendations for an in memory database vs thread safe data structures

  - by yx

  TLDR: What are the pros/cons of using an in-memory database vs locks and concurrent data structures? I am currently working on an application that has many (possibly remote) displays that collect live data from multiple data sources and renders them on screen in real time. One of the other developers have suggested the use of an in memory database instead of doing it the standard way our other systems behaves, which is to use concurrent hashmaps, queues, arrays, and other objects to store the graphical objects and handling them safely with locks if necessary. His argument is that the DB will lessen the need to worry about concurrency since it will handle read/write locks automatically, and also the DB will offer an easier way to structure the data into as many tables as we need instead of having create hashmaps of hashmaps of lists, etc and keeping track of it all. I do not have much DB experience myself so I am asking fellow SO users what experiences they have had and what are the pros & cons of inserting the DB into the system?

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• ASP.Net Architecture Specific to Shared/Static functions

  - by Maxim Gershkovich

  Hello All, Could someone please advise in the context of a ASP.Net application is a shared/static function common to all users? If for example you have a function Public shared function GetStockByID(StockID as Guid) as Stock Is that function common to all current users of your application? Or is the shared function only specific to the current user and shared in the context of ONLY that current user? So more specifically my question is this, besides database concurrency issues such as table locking do I need to concern myself with threading issues in shared functions in an ASP.Net application? In my head; let’s say my application namespace is MyTestApplicationNamespace. Everytime a new user connects to my site a new instance of the MyTestApplicationNamespace is created and therefore all shared functions are common to that instance and user but NOT common across multiple users. Is this correct?

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• Cocoa threading question.

  - by Steve918

  I would like to implement an observer pattern in Objective-C where the observer implements an interface similar to SKPaymentTransactionObserver and the observable class just extends my base observable. My observable class looks something like what is below. Notice I'm making copies of the observers before enumeration to avoid throwing an exception . I've tried adding an NSLock around add observers and notify observers, but I run into a deadlock. What would be the proper way to handle concurrency when observers are being added as notifications are being sent? @implementation Observable -(void)notifyObservers:(SEL)selector { @synchronized(self) { NSSet* observer_copy = [observers copy]; for (id observer in observer_copy) { if([observer respondsToSelector: selector]) { [observer performSelector: selector]; } } [observer_copy release]; } } -(void)notifyObservers:(SEL)selector withObject:(id)arg1 withObject:(id)arg2 { @synchronized(self) { NSSet* observer_copy = [observers copy]; for (id observer in observer_copy) { if([observer respondsToSelector: selector]) { [observer performSelector: selector withObject: arg1 withObject: arg2]; } } [observer_copy release]; } } -(void)addObserver:(id)observer { @synchronized(self) { [observers addObject: observer]; } } -(void)removeObserver:(id)observer { @synchronized(self) { [observers removeObject: observer]; } }

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• Concurrent cartesian product algorithm in Clojure

  - by jqno

  Is there a good algorithm to calculate the cartesian product of three seqs concurrently in Clojure? I'm working on a small hobby project in Clojure, mainly as a means to learn the language, and its concurrency features. In my project, I need to calculate the cartesian product of three seqs (and do something with the results). I found the cartesian-product function in clojure.contrib.combinatorics, which works pretty well. However, the calculation of the cartesian product turns out to be the bottleneck of the program. Therefore, I'd like to perform the calculation concurrently. Now, for the map function, there's a convenient pmap alternative that magically makes the thing concurrent. Which is cool :). Unfortunately, such a thing doesn't exist for cartesian-product. I've looked at the source code, but I can't find an easy way to make it concurrent myself. Also, I've tried to implement an algorithm myself using map, but I guess my algorithmic skills aren't what they used to be. I managed to come up with something ugly for two seqs, but three was definitely a bridge too far. So, does anyone know of an algorithm that's already concurrent, or one that I can parallelize myself?

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• Recursively adding threads to a Java thread pool

  - by Leith

  I am working on a tutorial for my Java concurrency course. The objective is to use thread pools to compute prime numbers in parallel. The design is based on the Sieve of Eratosthenes. It has an array of n bools, where n is the largest integer you are checking, and each element in the array represents one integer. True is prime, false is non prime, and the array is initially all true. A thread pool is used with a fixed number of threads (we are supposed to experiment with the number of threads in the pool and observe the performance). A thread is given a integer multiple to process. The thread then finds the first true element in the array that is not a multiple of thread's integer. The thread then creates a new thread on the thread pool which is given the found number. After a new thread is formed, the existing thread then continues to set all multiples of it's integer in the array to false. The main program thread starts the first thread with the integer '2', and then waits for all spawned threads to finish. It then spits out the prime numbers and the time taken to compute. The issue I have is that the more threads there are in the thread pool, the slower it takes with 1 thread being the fastest. It should be getting faster not slower! All the stuff on the internet about Java thread pools create n worker threads the main thread then wait for all threads to finish. The method I use is recursive as a worker can spawn more worker threads. I would like to know what is going wrong, and if Java thread pools can be used recursively.

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• How are you taking advantage of Multicore?

  - by tgamblin

  As someone in the world of HPC who came from the world of enterprise web development, I'm always curious to see how developers back in the "real world" are taking advantage of parallel computing. This is much more relevant now that all chips are going multicore, and it'll be even more relevant when there are thousands of cores on a chip instead of just a few. My questions are: How does this affect your software roadmap? I'm particularly interested in real stories about how multicore is affecting different software domains, so specify what kind of development you do in your answer (e.g. server side, client-side apps, scientific computing, etc). What are you doing with your existing code to take advantage of multicore machines, and what challenges have you faced? Are you using OpenMP, Erlang, Haskell, CUDA, TBB, UPC or something else? What do you plan to do as concurrency levels continue to increase, and how will you deal with hundreds or thousands of cores? If your domain doesn't easily benefit from parallel computation, then explaining why is interesting, too. Finally, I've framed this as a multicore question, but feel free to talk about other types of parallel computing. If you're porting part of your app to use MapReduce, or if MPI on large clusters is the paradigm for you, then definitely mention that, too. Update: If you do answer #5, mention whether you think things will change if there get to be more cores (100, 1000, etc) than you can feed with available memory bandwidth (seeing as how bandwidth is getting smaller and smaller per core). Can you still use the remaining cores for your application?

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• C# Multithreaded Domain Design

  - by Thijs Cramer

  Let's say i have a Domain Model that im trying to make compatible with multithreading. The prototype domain is a game domain that consists of Space, SpaceObject, and Location objects. A SpaceObject has the Move method and Asteroid and Ship extend this object with specific properties for the object (Ship has a name and Asteroid has a color) Let's say i want to make the Move method for each object run in a seperate thread. That would be stupid because with 10000 objects, i would have 10000 threads. What would be the best way to seperate the workload between cores/threads? I'm trying to learn the basics of concurrency, and building a small game to prototype a lot of concepts. What i've already done, is build a domain, and a threading model with a timer that launches events based on intervals. If the event occurs i want to update my entire model with the new locations of any SpaceObject. But i don't know how and when to launch new threads with workloads when the event occurs. Some people at work told me that u can't update your core domain multithreaded, because you have to synch everything. But in that case i can't run my game on a dual quadcore server, because it would only use 1 CPU for the hardest tasks. Anyone know what to do here?

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• Thread-safe initialization of function-local static const objects

  - by sbi

  This question made me question a practice I had been following for years. For thread-safe initialization of function-local static const objects I protect the actual construction of the object, but not the initialization of the function-local reference referring to it. Something like this: namspace { const some_type& create_const_thingy() { lock my_lock(some_mutex); static const some_type the_const_thingy; return the_const_thingy; } } void use_const_thingy() { static const some_type& the_const_thingy = create_const_thingy(); // use the_const_thingy } The idea is that locking takes time, and if the reference is overwritten by several threads, it won't matter. I'd be interested if this is safe enough in practice? safe according to The Rules? (I know, the current standard doesn't even know what "concurrency" is, but what about trampling over an already initialized reference? And do other standards, like POSIX, have something to say that's relevant to this?) For the inquiring minds: Many such function-local static const objects I used are maps which are initialized from const arrays upon first use and used for lookup. For example, I have a few XML parsers where tag name strings are mapped to enum values, so I could later switch over the tags enum values.

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• How to allow one thread to mutate an array property while another thread iterates on a copy of the a

  - by Steve918

  I would like to implement an observer pattern in Objective-C where the observer implements an interface similar to SKPaymentTransactionObserver and the observable class just extends my base observable. My observable class looks something like what is below. Notice I'm making copies of the observers before enumeration to avoid throwing an exception . I've tried adding an NSLock around add observers and notify observers, but I run into a deadlock. What would be the proper way to handle concurrency when observers are being added as notifications are being sent? @implementation Observable -(void)notifyObservers:(SEL)selector { @synchronized(self) { NSSet* observer_copy = [observers copy]; for (id observer in observer_copy) { if([observer respondsToSelector: selector]) { [observer performSelector: selector]; } } [observer_copy release]; } } -(void)notifyObservers:(SEL)selector withObject:(id)arg1 withObject:(id)arg2 { @synchronized(self) { NSSet* observer_copy = [observers copy]; for (id observer in observer_copy) { if([observer respondsToSelector: selector]) { [observer performSelector: selector withObject: arg1 withObject: arg2]; } } [observer_copy release]; } } -(void)addObserver:(id)observer { @synchronized(self) { [observers addObject: observer]; } } -(void)removeObserver:(id)observer { @synchronized(self) { [observers removeObject: observer]; } }

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• How can I increment a counter every N loops in JMeter?

  - by Dave Hunt

  I want to test concurrency, and reliably replicate an issue that JMeter brought to my attention. What I want to do is set a unique identifier (currently the time in milliseconds with a counter appended) and increment the counter between loops but not between threads. The idea being that the number of threads I have set up is the number of identical identifiers before incrementing and using another. If I had 3 threads with a loop count of 2 I want: 1. Unique ID: <current-time-in-millis>000000 2. Unique ID: <current-time-in-millis>000000 3. Unique ID: <current-time-in-millis>000000 4. Unique ID: <current-time-in-millis>000001 5. Unique ID: <current-time-in-millis>000001 6. Unique ID: <current-time-in-millis>000001 I've tried using Throughput Controllers to increment a counter, as well as several other things that seemed they should work but had no luck. This seems like something JMeter should be able to do. Is there any way to get the value of the loop count?

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• What is an efficient strategy for multiple threads posting jobs and waiting for response from a single thread?

  - by jakewins

  In java, what is an efficient solution to the following problem: I have multiple threads (10-20 or so) generating jobs ("Job Creators"), and a single thread capable of performing them ("The worker"). Once a job creator has posted a job, it should wait for the job to finish, yielding no result other than "it's done", before it keeps going. For sending the jobs to the worker thread, I think a ring buffer or similar standard fan-in setup would perhaps be a good approach? But for a Job Creator to find out that her job has been done, I'm not so sure.. The job creators could sleep, and the worker interrupt them when done.. Or each job creator could have an atomic boolean that it checks, and that the worker sets. I dunno, neither of those feel very nice. I'd like to do it with as few (none, if possible) locks as absolutely possible. So to be clear: What I'm looking for is speed, not necessarily simplicity. Does anyone have any suggestions? Links to reading about concurrency strategies would also be very welcome!

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• Sequential access to asynchronous sockets

  - by Lars A. Brekken

  I have a server that has several clients C1...Cn to each of which there is a TCP connection established. There are less than 10,000 clients. The message protocol is request/response based, where the server sends a request to a client and then the client sends a response. The server has several threads, T1...Tm, and each of these may send requests to any of the clients. I want to make sure that only one of these threads can send a request to a specific client at any one time, while the other threads wanting to send a request to the same client will have to wait. I do not want to block threads from sending requests to different clients at the same time. E.g. If T1 is sending a request to C3, another thread T2 should not be able to send anything to C3 until T1 has received its response. I was thinking of using a simple lock statement on the socket: lock (c3Socket) { // Send request to C3 // Get response from C3 } I am using asynchronous sockets, so I may have to use Monitor instead: Monitor.Enter(c3Socket); // Before calling .BeginReceive() And Monitor.Exit(c3Socket); // In .EndReceive I am worried about stuff going wrong and not letting go of the monitor and therefore blocking all access to a client. I'm thinking that my heartbeat thread could use Monitor.TryEnter() with a timeout and throw out sockets that it cannot get the monitor for. Would it make sense for me to make the Begin and End calls synchronous in order to be able to use the lock() statement? I know that I would be sacrificing concurrency for simplicity in this case, but it may be worth it. Am I overlooking anything here? Any input appreciated.

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• Message sent to deallocated instance which has never been released

  - by Jakub

  Hello, I started dealing with NSOperations and (as usual with concurrency) I'm observing strange behaviour. In my class I've got an instance variable: NSMutableArray *postResultsArray; when one button in the UI is pressed I initialize the array: postResultsArray = [NSMutableArray array]; and setup the operations (together with dependencies). In the operations I create a custom object and try to add to the array: PostResult *result = [[PostResult alloc] initWithServiceName:@"Sth" andResult:someResult]; [self.postResultsArray addObject:result]; and while adding I get: -[CFArray retain]: message sent to deallocated instance 0x3b40c30 which is strange as I don't release the array anywhere in my code (I did, but when the problem started to appear I commented all the release operations to be sure that they are not the case). I also used to have @synchronized section like below: PostResult *result = [[PostResult alloc] initWithServiceName:@"Sth" andResult:someResult]; @synchronized (self.postResultsArray) { [self.postResultsArray addObject:result]; } but the problem was the same (however, the error was for the synchronized operation). Any ideas what I may be doing wrong?

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• Locking database edit by key name

  - by Will Glass

  I need to prevent simultaneous edits to a database field. Users are executing a push operation on a structured data field, so I want to sequence the operations, not simply ignore one edit and take the second. Essentially I want to do synchronized(key name) { push value onto the database field } and set up the synchronized item so that only one operation on "key name" will occur at a time. (note: I'm simplifying, it's not always a simple push). A crude way to do this would be a global synchronization, but that bottlenecks the entire app. All I need to do is sequence two simultaneous writes with the same key, which is rare but annoying occurrence. This is a web-based java app, written with Spring (and using JPA/MySQL). The operation is triggered by a user web service call. (the root cause is when a user sends two simultaneous http requests with the same key). I've glanced through the Doug Lea/Josh Bloch/et al Concurrency in Action, but don't see an obvious solution. Still, this seems simple enough I feel there must be an elegant way to do this.

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• Mysterious combination

  - by pstone

  I decided to learn concurrency and wanted to find out in how many ways instructions from two different processes could overlap. The code for both processes is just a 10 iteration loop with 3 instructions performed in each iteration. I figured out the problem consisted of leaving X instructions fixed at a point and then fit the other X instructions from the other process between the spaces taking into account that they must be ordered (instruction 4 of process B must always come before instruction 20). I wrote a program to count this number, looking at the results I found out that the solution is n Combination k, where k is the number of instructions executed throughout the whole loop of one process, so for 10 iterations it would be 30, and n is k*2 (2 processes). In other words, n number of objects with n/2 fixed and having to fit n/2 among the spaces without the latter n/2 losing their order. Ok problem solved. No, not really. I have no idea why this is, I understand that the definition of a combination is, in how many ways can you take k elements from a group of n such that all the groups are different but the order in which you take the elements doesn't matter. In this case we have n elements and we are actually taking them all, because all the instructions are executed ( n C n). If one explains it by saying that there are 2k blue (A) and red (B) objects in a bag and you take k objects from the bag, you are still only taking k instructions when 2k instructions are actually executed. Can you please shed some light into this? Thanks in advance.

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