Search Results

Search found 278 results on 12 pages for 'deadlock'.

Page 10/12 | < Previous Page | 6 7 8 9 10 11 12  | Next Page >

• pthreads: reader/writer locks, upgrading read lock to write lock

  - by ScaryAardvark

  I'm using read/write locks on Linux and I've found that trying to upgrade a read locked object to a write lock deadlocks. i.e. // acquire the read lock in thread 1. pthread_rwlock_rdlock( &lock ); // make a decision to upgrade the lock in threads 1. pthread_rwlock_wrlock( &lock ); // this deadlocks as already hold read lock. I've read the man page and it's quite specific. The calling thread may deadlock if at the time the call is made it holds the read-write lock (whether a read or write lock). What is the best way to upgrade a read lock to a write lock in these circumstances.. I don't want to introduce a race on the variable I'm protecting. Presumably I can create another mutex to encompass the releasing of the read lock and the acquiring of the write lock but then I don't really see the use of read/write locks. I might as well simply use a normal mutex. Thx

  Read the article

• sqlite3 'database is locked' won't go away with retries

  - by Azarias

  I have a sqlite3 database that is accessed by a few threads (3-4). I am aware of the general limitations of sqlite3 with regards to concurrency as stated http://www.sqlite.org/faq.html#q6 , but I am convinced that is not the problem. All of the threads both read and write from this database. Whenever I do a write, I have the following construct: try: Cursor.execute(q, params) Connection.commit() except sqlite3.IntegrityError: Notify except sqlite3.OperationalError: print sys.exc_info() print("DATABASE LOCKED; sleeping for 3 seconds and trying again") time.sleep(3) Retry On some runs, I won't even hit this block, but when I do, it never comes out of it (keeps retrying, but I keep getting the 'database is locked' error from exc_info. If I understand the reader/writer lock usage correctly, some amount of waiting should help with the contention. What this sounds like is deadlock, but I do not use any transactions in my code, and every SELECT or INSERT is simply a one off. Some threads, however, keep the same connection when they do their operation (which includes a mix of SELECTS and INSERTS and other modifiers). I would appericiate it if you could shade a light on this, and also ways around fixing it (besides using a different database engine.)

  Read the article

• Why there is no scoped locks for multiple mutexes in C++0x or Boost.Thread?

  - by Vicente Botet Escriba

  C++0x thread library or Boost.thread define non-member variadic template function that lock all lock avoiding dead lock. template <class L1, class L2, class... L3> void lock(L1&, L2&, L3&...); While this function avoid help to deadlock, the standard do not includes the associated scoped lock to write exception safe code. { std::lock(l1,l2); // do some thing // unlock li l2 exception safe } That means that we need to use other mechanism as try-catch block to make exception safe code or define our own scoped lock on multiple mutexes ourselves or even do that { std::lock(l1,l2); std::unique_lock lk1(l1, std::adopted); std::unique_lock lk2(l2, std::adopted); // do some thing // unlock li l2 on destruction of lk1 lk2 } Why the standard doesn't includes a scoped lock on multiple mutexes of the same type, as for example { std::array_unique_lock<std::mutex> lk(l1,l2); // do some thing // unlock l1 l2 on destruction of lk } or tuples of mutexes { std::tuple_unique_lock<std::mutex, std::recursive_mutex> lk(l1,l2); // do some thing // unlock l1 l2 on destruction of lk } Is there something wrong on the design?

  Read the article

• Is there any point in using a volatile long?

  - by Adamski

  I occasionally use a volatile instance variable in cases where I have two threads reading from / writing to it and don't want the overhead (or potential deadlock risk) of taking out a lock; for example a timer thread periodically updating an int ID that is exposed as a getter on some class: public class MyClass { private volatile int id; public MyClass() { ScheduledExecutorService execService = Executors.newScheduledThreadPool(1); execService.scheduleAtFixedRate(new Runnable() { public void run() { ++id; } }, 0L, 30L, TimeUnit.SECONDS); } public int getId() { return id; } } My question: Given that the JLS only guarantees that 32-bit reads will be atomic is there any point in ever using a volatile long? (i.e. 64-bit). Caveat: Please do not reply saying that using volatile over synchronized is a case of pre-optimisation; I am well aware of how / when to use synchronized but there are cases where volatile is preferable. For example, when defining a Spring bean for use in a single-threaded application I tend to favour volatile instance variables, as there is no guarantee that the Spring context will initialise each bean's properties in the main thread.

  Read the article

• Thread mutex behaviour

  - by Alberteddu

  Hi there, I'm learning C. I'm writing an application with multiple threads; I know that when a variable is shared between two or more threads, it is better to lock/unlock using a mutex to avoid deadlock and inconsistency of variables. This is very clear when I want to change or view one variable. int i = 0; /** Global */ static pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER; /** Thread 1. */ pthread_mutex_lock(&mutex); i++; pthread_mutex_unlock(&mutex); /** Thread 2. */ pthread_mutex_lock(&mutex); i++; pthread_mutex_unlock(&mutex); This is correct, I think. The variable i, at the end of the executions, contains the integer 2. Anyway, there are some situations in which I don't know exactly where to put the two function calls. For example, suppose you have a function obtain(), which returns a global variable. I need to call that function from within the two threads. I have also two other threads that call the function set(), defined with a few arguments; this function will set the same global variable. The two functions are necessary when you need to do something before getting/setting the var. /** (0) */ /** Thread 1, or 2, or 3... */ if(obtain() == something) { if(obtain() == somethingElse) { // Do this, sometimes obtain() and sometimes set(random number) (1) } else { // Do that, just obtain(). (2) } } else { // Do this and do that (3) // If # of thread * 3 > 10, then set(3*10) For example. (4) } /** (5) */ Where I have to lock, and where I have to unlock? The situation can be, I think, even more complex. I will appreciate an exhaustive answer. Thank you in advance. —Alberto

  Read the article

• programs hangs during socket interaction

  - by herrturtur

  I have two programs, sendfile.py and recvfile.py that are supposed to interact to send a file across the network. They communicate over TCP sockets. The communication is supposed to go something like this: sender =====filename=====> receiver sender <===== 'ok' ======= receiver or sender <===== 'no' ======= receiver if ok: sender ====== file ======> receiver I've got The sender and receiver code is here: Sender: import sys from jmm_sockets import * if len(sys.argv) != 4: print "Usage:", sys.argv[0], "<host> <port> <filename>" sys.exit(1) s = getClientSocket(sys.argv[1], int(sys.argv[2])) try: f = open(sys.argv[3]) except IOError, msg: print "couldn't open file" sys.exit(1) # send filename s.send(sys.argv[3]) # receive 'ok' buffer = None response = str() while 1: buffer = s.recv(1) if buffer == '': break else: response = response + buffer if response == 'ok': print 'receiver acknowledged receipt of filename' # send file s.send(f.read()) elif response == 'no': print "receiver doesn't want the file" # cleanup f.close() s.close() Receiver: from jmm_sockets import * s = getServerSocket(None, 16001) conn, addr = s.accept() buffer = None filename = str() # receive filename while 1: buffer = conn.recv(1) if buffer == '': break else: filename = filename + buffer print "sender wants to send", filename, "is that ok?" user_choice = raw_input("ok/no: ") if user_choice == 'ok': # send ok conn.send('ok') #receive file data = str() while 1: buffer = conn.recv(1) if buffer=='': break else: data = data + buffer print data else: conn.send('no') conn.close() I'm sure I'm missing something here in the sorts of a deadlock, but don't know what it is.

  Read the article

• Help me find article on Multi-threading and Event Handling in Java

  - by JDR

  I once read an article on how to properly write event handlers for multi-threading in Java, but I can't for the life of me find it anymore. It described the pitfalls and potentials for deadlocks that can occur when firing events (not Swing events mind you, but general events like model update notifications). To clarify, the situation would be as such: // let's say this is code from an MVC model somewhere public void setSomeProperty(String myProperty){ if(!this.myProperty.equals(myProperty)){ this.myProperty = myProperty; fireMyPropertyChangedEvent(...); } } The article described how passing control to arbitrary external listener code was a potential cause for deadlock. I now find myself in a situation where I need to fire such events in a multithreaded environment and I would very much like to read the article again to see what it has to say before I continue. Does anyone know the article I'm referring to? I believe it came as a (fairly short) PDF. It started off with an initial naive implementation and incrementally pointed out flaws and improved upon it. It ended with a sort of final proper-way-to-fire-multithreaded-events. I've searched endlessly in my browse history and on google, but all I could find were endless amounts topics on Swing event dispatch threads. Thank you.

  Read the article

• How to send XML data through socket InputStream

  - by THeK3nger

  Hi, I'm trying to write a client-server application in Java with an XML-based protocol. But I have a great problem! See this part of client code: InputStream incoming = skt.getInputStream(); //I get Stream from Socket. OutputStream out = skt.getOutputStream(); [...] XMLSerializer serializer = new XMLSerializer(); //This create an XML document. tosend = WUTPClientWriter.createMessage100(projectid, cpuclock, cpunumber); serializer.setOutputByteStream(out); serializer.serialize(tosend); At this point server fall in deadlock. It wait for EOF but I can't send it because if I use out.close(); or skt.shutdownOutput(); I close Socket and I must keep alive this connection. I can't send '\0' becouse I get Parse Error in the server. How can I do? Can I "close" output stream without close socket? RESOLVED I've created new class XMLStreamOutput and XMLStreamInput with advanced Stream gesture.

  Read the article

• Do glue records in non-circular dns-lookups speed up domain resolution or not?

  - by Joe Hopfgartner

  Doing a lookup for my domain on http://www.intodns.com/ I noticed theese two messages: In Parent section: DNS Parent sent Glue The parent nameserver g.gtld-servers.net is not sending out GLUE for every nameservers listed, meaning he is sending out your nameservers host names without sending the A records of those nameservers. It's ok but you have to know that this will require an extra A lookup that can delay a little the connections to your site. This happens a lot if you have nameservers on different TLD (domain.com for example with nameserver ns.domain.org.) and in NS section: Glue for NS records INFO: GLUE was not sent when I asked your nameservers for your NS records.This is ok but you should know that in this case an extra A record lookup is required in order to get the IPs of your NS records. The nameservers without glue are: 109.230.225.96 84.201.40.52 You can fix this for example by adding A records to your nameservers for the zones listed above. I do perfectly understand that the primary objective of glue records is to resolve circular dependencies. The classic use case: my domain is example.com and I want to have the nameserver ns1.example.com. This will never work because i cannot know the ip of ns1.example.com if I don't fetch example.com and in order to do that I need to fetch it from ns1.example.com. To resolve this deadlock I add a glue record to ns1.example.com containing the ip adress of the nameserver, so this can work out. So this problem does not occour if the nameservers are in a different TLD than the domain i want to look up. But however to fetch the zone information from the nameservers I need to know their ip adress right? And in order to know that i need to fetch the zone the nameservers are in from their respective nameservers, right? (or rather my ISP needs to do that in the background) So an extra lookup that takes time? If I now have glue records, I know the IP adress right away without the need to look it up - so this should speed up the resolution of my domain, shouldnt it? However my DNS zone provider (tecserver.at) replied that this would make no sense because "we are not running ns1.ourdomain.com an ns1.ourdomain.com as authorative NS for ourdomain.com. This would be the only sense for glue records. Tecserver has a glue record because the NS for tecserver.at are ns1.tecserver.at and ns2.tecserver.at. Therefore a glue record is needed for resolution.

  Read the article

• Coldfusion 8 Application Crashes Under Heavy Load

  - by KM01

  Hello, We have a CF8 app that runs for 20-25 minutes before crashing under heavy load ~ 1200 users. This load is generated by our load testing tool: 1200 users ramped up in 5 mins (approx behavior of our users), running for an hour. We have this app on Solaris 10, Apache 2, JRun 4 and Oracle 10g. Java version is 1.6. During the initial load tests, the thread dumps pointed to monitor deadlocks that pointed to sessions. "jrpp-173": waiting to lock monitor 0x019fdc60 (object 0x6b893530, a java.util.Hashtable), which is held by "scheduler-1" "scheduler-1": waiting to lock monitor 0x026c3ce0 (object 0x6abe2f20, a coldfusion.monitor.memory.SessionMemoryMonitor$TopMemoryUsedSessions), which is held by "jrpp-167" "jrpp-167": waiting to lock monitor 0x019fdc60 (object 0x6b893530, a java.util.Hashtable), which is held by "scheduler-1" We increased the number of sessions relative to the number of CPUs (48 simultaneous threads against 32 CPUs), and the deadlock went away. While varying the simultaneous threads helped a little bit in terms of response time, the CF server still tanked in 20-25 minutes during all of these tests. We ran more thread dumps, and saw a thread locking a monitor, for e.g.: "jrpp-475" prio=3 tid=0x02230800 nid=0x2c5 runnable [0x4397d000] java.lang.Thread.State: RUNNABLE at java.util.HashMap.getEntry(HashMap.java:347) at java.util.HashMap.containsKey(HashMap.java:335) at java.util.HashSet.contains(HashSet.java:184) at coldfusion.monitor.memory.MemoryTracker.onAddObject(MemoryTracker.java:124) at coldfusion.monitor.memory.MemoryTrackerProxy.onReplaceValue(MemoryTrackerProxy.java:598) at coldfusion.monitor.memory.MemoryTrackerProxy.onPut(MemoryTrackerProxy.java:510) at coldfusion.util.CaseInsensitiveMap.put(CaseInsensitiveMap.java:250) at coldfusion.util.FastHashtable.put(FastHashtable.java:43) - locked <0x6f7e1a78> (a coldfusion.runtime.Struct) at coldfusion.runtime.CfJspPage._arrayset(CfJspPage.java:1027) at coldfusion.runtime.CfJspPage._arraySetAt(CfJspPage.java:2117) at cfvalidation2ecfc1052964961$funcSETUSERAUDITDATA.runFunction(/app/docs/apply/cfcs/validation.cfc:377) As you see in the last line above there were several references CFMs and CFCs, and the lines have "cflock" tags, which were scoped to the "application." We (the dev team) then changed them to be scoped to a "name". After more load tests, there is no locking going on and there no deadlocks, but now the application tanks in 7-10 minutes. We've gotten system, network and DB reports from the respective admins, and they are not being taxed; even watched the server stats with server monitor, top, prstat, ran sar reports, etc. So we believe it is an issue with the CF server or maybe the JVM. I am running out of ideas as to what else we can try. Disclaimer: I am not a CF developer or Admin. I am just running the load test, analyzing the reports, threads etc, and sharing the results with the dev and admin teams, and trying the next change, and so on. So far no dice. Has anyone run into something similar? How did you go about diagnosing and troubleshooting? All thoughts and pointers welcome. Thank you for your time! KM

  Read the article

• SQL SERVER – Beginning of SQL Server Architecture – Terminology – Guest Post

  - by pinaldave

  SQL Server Architecture is a very deep subject. Covering it in a single post is an almost impossible task. However, this subject is very popular topic among beginners and advanced users.  I have requested my friend Anil Kumar who is expert in SQL Domain to help me write  a simple post about Beginning SQL Server Architecture. As stated earlier this subject is very deep subject and in this first article series he has covered basic terminologies. In future article he will explore the subject further down. Anil Kumar Yadav is Trainer, SQL Domain, Koenig Solutions. Koenig is a premier IT training firm that provides several IT certifications, such as Oracle 11g, Server+, RHCA, SQL Server Training, Prince2 Foundation etc. In this Article we will discuss about MS SQL Server architecture. The major components of SQL Server are: Relational Engine Storage Engine SQL OS Now we will discuss and understand each one of them. 1) Relational Engine: Also called as the query processor, Relational Engine includes the components of SQL Server that determine what your query exactly needs to do and the best way to do it. It manages the execution of queries as it requests data from the storage engine and processes the results returned. Different Tasks of Relational Engine: Query Processing Memory Management Thread and Task Management Buffer Management Distributed Query Processing 2) Storage Engine: Storage Engine is responsible for storage and retrieval of the data on to the storage system (Disk, SAN etc.). to understand more, let’s focus on the following diagram. When we talk about any database in SQL server, there are 2 types of files that are created at the disk level – Data file and Log file. Data file physically stores the data in data pages. Log files that are also known as write ahead logs, are used for storing transactions performed on the database. Let’s understand data file and log file in more details: Data File: Data File stores data in the form of Data Page (8KB) and these data pages are logically organized in extents. Extents: Extents are logical units in the database. They are a combination of 8 data pages i.e. 64 KB forms an extent. Extents can be of two types, Mixed and Uniform. Mixed extents hold different types of pages like index, System, Object data etc. On the other hand, Uniform extents are dedicated to only one type. Pages: As we should know what type of data pages can be stored in SQL Server, below mentioned are some of them: Data Page: It holds the data entered by the user but not the data which is of type text, ntext, nvarchar(max), varchar(max), varbinary(max), image and xml data. Index: It stores the index entries. Text/Image: It stores LOB ( Large Object data) like text, ntext, varchar(max), nvarchar(max),  varbinary(max), image and xml data. GAM & SGAM (Global Allocation Map & Shared Global Allocation Map): They are used for saving information related to the allocation of extents. PFS (Page Free Space): Information related to page allocation and unused space available on pages. IAM (Index Allocation Map): Information pertaining to extents that are used by a table or index per allocation unit. BCM (Bulk Changed Map): Keeps information about the extents changed in a Bulk Operation. DCM (Differential Change Map): This is the information of extents that have modified since the last BACKUP DATABASE statement as per allocation unit. Log File: It also known as write ahead log. It stores modification to the database (DML and DDL). Sufficient information is logged to be able to: Roll back transactions if requested Recover the database in case of failure Write Ahead Logging is used to create log entries Transaction logs are written in chronological order in a circular way Truncation policy for logs is based on the recovery model SQL OS: This lies between the host machine (Windows OS) and SQL Server. All the activities performed on database engine are taken care of by SQL OS. It is a highly configurable operating system with powerful API (application programming interface), enabling automatic locality and advanced parallelism. SQL OS provides various operating system services, such as memory management deals with buffer pool, log buffer and deadlock detection using the blocking and locking structure. Other services include exception handling, hosting for external components like Common Language Runtime, CLR etc. I guess this brief article gives you an idea about the various terminologies used related to SQL Server Architecture. In future articles we will explore them further. Guest Author  The author of the article is Anil Kumar Yadav is Trainer, SQL Domain, Koenig Solutions. Koenig is a premier IT training firm that provides several IT certifications, such as Oracle 11g, Server+, RHCA, SQL Server Training, Prince2 Foundation etc. Reference: Pinal Dave (http://blog.sqlauthority.com) Filed under: PostADay, SQL, SQL Authority, SQL Query, SQL Security, SQL Server, SQL Tips and Tricks, SQL Training, T SQL, Technology

  Read the article

• Deciding On Features For Open Source

  - by Robz / Fervent Coder

  Open source feature selection is subjective. An interesting question was posed to me recently at a presentation - “How do you decide what features to include in the [open source] projects you manage?” Is It Objective? I’d like to say that it’s really objective and that we vote on features and look at what carries the most interest of the populace. Actually no I wouldn’t. I don’t think I would enjoy working on open source (OSS) as much if it someone else decided on what features I should include. It already works that way at work. I don’t want to come home from work and work on things that others decide for me unless they are paying me for those features. So how do I decide on features for our open source projects? I think there are at least three paths to feature selection and they are not necessarily mutually exclusive. Feature Selection IS the Set of Features For the Domain Your product, in whatever domain it is in, needs to have the basic set of features that make it answer the needs of that domain. That is different for every product, but if you take for example a build tool, at the very least it needs to be able to compile source. And these basic needed features are not always objective either. Two people could completely disagree what makes for a required feature to meet a domain need for a product. Even one person may disagree with himself/herself about what features are needed based on different timeframes. So that leads us down to subjective. Feature Selection IS An Answer To Competition Some features go in because the competition adds a feature that may draw others away from your product offering. With OSS, there are all free alternatives, so if your competition adds a killer feature and you don’t, there isn’t much other than learning (how to use the other product) to move your customers off to the competition. If you want to keep your customers, you need to be ready to answer the questions of adding the features your competition has added.  Sometimes it’s about adding a feature that your competition charges for, but you add it for free. That draws people to the free alternative – so sometimes that adds a motivation to select a feature. Sometimes it’s because you want those features in your product, either to learn how you can answer the question of how to do something and/or because you have a need for that feature and you want it in your product. That also leads us down the road to subjective. Feature Selection IS Subjective I decide on features based on what I want to see in the product I am working on. Things I am interested in or have the biggest need for usually get picked first, with things that do not interest me either coming later or not at all. Most people get interested in an area of OSS because it solves a need for them and/or they find it interesting. If one of these two things is not happening and they are not being paid, it’s likely that person will move on to something else they find interesting or just stop OSS altogether. OSS feature selection is just that – subjective. If it wasn’t, it wouldn’t be opinionated and it wouldn’t have a personality about it. Most people like certain OSS because they like where the product is going or the personalities behind the product. For me, I want my products to be easy to use and solve an important problem. If it takes you more than 5-10 minutes to learn how to use my product, I know you are probably going somewhere else. So I pick features that make the product easy to use and learn, and those are not always the simplest features to work on. I work for conventions and make the product opinionated, because I think that is what makes using a product easier, if it already works with little setup. And I like to provide the ability for power users to get in and change the conventions to suit their needs. So those are required features for me above and beyond the domain features. I like to think I do a pretty good job at this. Usually when I present on something I’ve created, I like seeing people’s eyes light up when they see how simple it is to set up a powerful product like UppercuT. Patches And/Or Donations But remember before you say I’m a bad person or won’t use my product, I’ll always accept patches or I might like the feature that you suggest. If you like using the products I provide and they solve a problem for you the two biggest compliments you can provide are either a patch or a donation.  If you think the product is great, but if it could do this one other thing, it would be awesome(!), then consider contacting me and providing a patch, or consider contacting me with a donation and a request to put the feature in. And alternatively, if it’s a big feature, you could hire me to work on the product to make it even better. What If There Are Multiple Committers? In the question of multiple committers, I choose that someone always makes the ultimate decision to select whether a feature should be part of a product or not. But for other OSS project maybe this is not the case. If there is not an ultimate decision maker, then there is the possibility of either adding every feature suggested or having a deadlock on two conflicting features.   So let me pose this question. If you work on Open Source, how do you decide on what features to put in your open source projects? How do you decide what doesn’t belong? What do you do when there are conflicting features?

  Read the article

• C# async and actors

  - by Alex.Davies

  If you read my last post about async, you might be wondering what drove me to write such odd code in the first place. The short answer is that .NET Demon is written using NAct Actors. Actors are an old idea, which I believe deserve a renaissance under C# 5. The idea is to isolate each stateful object so that only one thread has access to its state at any point in time. That much should be familiar, it's equivalent to traditional lock-based synchronization. The different part is that actors pass "messages" to each other rather than calling a method and waiting for it to return. By doing that, each thread can only ever be holding one lock. This completely eliminates deadlocks, my least favourite concurrency problem. Most people who use actors take this quite literally, and there are plenty of frameworks which help you to create message classes and loops which can receive the messages, inspect what type of message they are, and process them accordingly. But I write C# for a reason. Do I really have to choose between using actors and everything I love about object orientation in C#? Type safety Interfaces Inheritance Generics As it turns out, no. You don't need to choose between messages and method calls. A method call makes a perfectly good message, as long as you don't wait for it to return. This is where asynchonous methods come in. I have used NAct for a while to wrap my objects in a proxy layer. As long as I followed the rule that methods must always return void, NAct queued up the call for later, and immediately released my thread. When I needed to get information out of other actors, I could use EventHandlers and callbacks (continuation passing style, for any CS geeks reading), and NAct would call me back in my isolated thread without blocking the actor that raised the event. Using callbacks looks horrible though. To remind you: m_BuildControl.FilterEnabledForBuilding(    projects,    enabledProjects = m_OutOfDateProjectFinder.FilterNeedsBuilding(        enabledProjects,             newDirtyProjects =             {                 ....... Which is why I'm really happy that NAct now supports async methods. Now, methods are allowed to return Task rather than just void. I can await those methods, and C# 5 will turn the rest of my method into a continuation for me. NAct will run the other method in the other actor's context, but will make sure that when my method resumes, we're back in my context. Neither actor was ever blocked waiting for the other one. Apart from when they were actually busy doing something, they were responsive to concurrent messages from other sources. To be fair, you could use async methods with lock statements to achieve exactly the same thing, but it's ugly. Here's a realistic example of an object that has a queue of data that gets passed to another object to be processed: class QueueProcessor {    private readonly ItemProcessor m_ItemProcessor = ...     private readonly object m_Sync = new object();    private Queue<object> m_DataQueue = ...    private List<object> m_Results = ...     public async Task ProcessOne() {         object data = null;         lock (m_Sync)         {             data = m_DataQueue.Dequeue();         }         var processedData = await m_ItemProcessor.ProcessData(data); lock (m_Sync)         {             m_Results.Add(processedData);         }     } } We needed to write two lock blocks, one to get the data to process, one to store the result. The worrying part is how easily we could have forgotten one of the locks. Compare that to the version using NAct: class QueueProcessorActor : IActor { private readonly ItemProcessor m_ItemProcessor = ... private Queue<object> m_DataQueue = ... private List<object> m_Results = ... public async Task ProcessOne()     {         // We are an actor, it's always thread-safe to access our private fields         var data = m_DataQueue.Dequeue();         var processedData = await m_ItemProcessor.ProcessData(data);         m_Results.Add(processedData);     } } You don't have to explicitly lock anywhere, NAct ensures that your code will only ever run on one thread, because it's an actor. Either way, async is definitely better than traditional synchronous code. Here's a diagram of what a typical synchronous implementation might do: The left side shows what is running on the thread that has the lock required to access the QueueProcessor's data. The red section is where that lock is held, but doesn't need to be. Contrast that with the async version we wrote above: Here, the lock is released in the middle. The QueueProcessor is free to do something else. Most importantly, even if the ItemProcessor sometimes calls the QueueProcessor, they can never deadlock waiting for each other. So I thoroughly recommend you use async for all code that has to wait a while for things. And if you find yourself writing lots of lock statements, think about using actors as well. Using actors and async together really takes the misery out of concurrent programming.

  Read the article

• College Courses through distance learning

  - by Matt

  I realize this isn't really a programming question, but didn't really know where to post this in the stackexchange and because I am a computer science major i thought id ask here. This is pretty unique to the programmer community since my degree is about 95% programming. I have 1 semester left, but i work full time. I would like to finish up in December, but to make things easier i like to take online classes whenever I can. So, my question is does anyone know of any colleges that offer distance learning courses for computer science? I have been searching around and found a few potential classes, but not sure yet. I would like to gather some classes and see what i can get approval for. Class I need: Only need one C SC 437 Geometric Algorithms C SC 445 Algorithms C SC 473 Automata Only need one C SC 452 Operating Systems C SC 453 Compilers/Systems Software While i only need of each of the above courses i still need to take two more electives. These also have to be upper 400 level classes. So i can take multiple in each category. Some other classes I can take are: CSC 447 - Green Computing CSC 425 - Computer Networking CSC 460 - Database Design CSC 466 - Computer Security I hoping to take one or two of these courses over the summer. If not, then online over the regular semester would be ok too. Any help in helping find these classes would be awesome. Maybe you went to a college that offered distance learning. Some of these classes may be considered to be graduate courses too. Descriptions are listed below if you need. Thanks! Descriptions Computer Security This is an introductory course covering the fundamentals of computer security. In particular, the course will cover basic concepts of computer security such as threat models and security policies, and will show how these concepts apply to specific areas such as communication security, software security, operating systems security, network security, web security, and hardware-based security. Computer Networking Theory and practice of computer networks, emphasizing the principles underlying the design of network software and the role of the communications system in distributed computing. Topics include routing, flow and congestion control, end-to-end protocols, and multicast. Database Design Functions of a database system. Data modeling and logical database design. Query languages and query optimization. Efficient data storage and access. Database access through standalone and web applications. Green Computing This course covers fundamental principles of energy management faced by designers of hardware, operating systems, and data centers. We will explore basic energy management option in individual components such as CPUs, network interfaces, hard drives, memory. We will further present the energy management policies at the operating system level that consider performance vs. energy saving tradeoffs. Finally we will consider large scale data centers where energy management is done at multiple layers from individual components in the system to shutting down entries subset of machines. We will also discuss energy generation and delivery and well as cooling issues in large data centers. Compilers/Systems Software Basic concepts of compilation and related systems software. Topics include lexical analysis, parsing, semantic analysis, code generation; assemblers, loaders, linkers; debuggers. Operating Systems Concepts of modern operating systems; concurrent processes; process synchronization and communication; resource allocation; kernels; deadlock; memory management; file systems. Algorithms Introduction to the design and analysis of algorithms: basic analysis techniques (asymptotics, sums, recurrences); basic design techniques (divide and conquer, dynamic programming, greedy, amortization); acquiring an algorithm repertoire (sorting, median finding, strong components, spanning trees, shortest paths, maximum flow, string matching); and handling intractability (approximation algorithms, branch and bound). Automata Introduction to models of computation (finite automata, pushdown automata, Turing machines), representations of languages (regular expressions, context-free grammars), and the basic hierarchy of languages (regular, context-free, decidable, and undecidable languages). Geometric Algorithms The study of algorithms for geometric objects, using a computational geometry approach, with an emphasis on applications for graphics, VLSI, GIS, robotics, and sensor networks. Topics may include the representation and overlaying of maps, finding nearest neighbors, solving linear programming problems, and searching geometric databases.

  Read the article

• Plagued by multithreaded bugs

  - by koncurrency

  On my new team that I manage, the majority of our code is platform, TCP socket, and http networking code. All C++. Most of it originated from other developers that have left the team. The current developers on the team are very smart, but mostly junior in terms of experience. Our biggest problem: multi-threaded concurrency bugs. Most of our class libraries are written to be asynchronous by use of some thread pool classes. Methods on the class libraries often enqueue long running taks onto the thread pool from one thread and then the callback methods of that class get invoked on a different thread. As a result, we have a lot of edge case bugs involving incorrect threading assumptions. This results in subtle bugs that go beyond just having critical sections and locks to guard against concurrency issues. What makes these problems even harder is that the attempts to fix are often incorrect. Some mistakes I've observed the team attempting (or within the legacy code itself) includes something like the following: Common mistake #1 - Fixing concurrency issue by just put a lock around the shared data, but forgetting about what happens when methods don't get called in an expected order. Here's a very simple example: void Foo::OnHttpRequestComplete(statuscode status) { m_pBar->DoSomethingImportant(status); } void Foo::Shutdown() { m_pBar->Cleanup(); delete m_pBar; m_pBar=nullptr; } So now we have a bug in which Shutdown could get called while OnHttpNetworkRequestComplete is occuring on. A tester finds the bug, captures the crash dump, and assigns the bug to a developer. He in turn fixes the bug like this. void Foo::OnHttpRequestComplete(statuscode status) { AutoLock lock(m_cs); m_pBar->DoSomethingImportant(status); } void Foo::Shutdown() { AutoLock lock(m_cs); m_pBar->Cleanup(); delete m_pBar; m_pBar=nullptr; } The above fix looks good until you realize there's an even more subtle edge case. What happens if Shutdown gets called before OnHttpRequestComplete gets called back? The real world examples my team has are even more complex, and the edge cases are even harder to spot during the code review process. Common Mistake #2 - fixing deadlock issues by blindly exiting the lock, wait for the other thread to finish, then re-enter the lock - but without handling the case that the object just got updated by the other thread! Common Mistake #3 - Even though the objects are reference counted, the shutdown sequence "releases" it's pointer. But forgets to wait for the thread that is still running to release it's instance. As such, components are shutdown cleanly, then spurious or late callbacks are invoked on an object in an state not expecting any more calls. There are other edge cases, but the bottom line is this: Multithreaded programming is just plain hard, even for smart people. As I catch these mistakes, I spend time discussing the errors with each developer on developing a more appropriate fix. But I suspect they are often confused on how to solve each issue because of the enormous amount of legacy code that the "right" fix will involve touching. We're going to be shipping soon, and I'm sure the patches we're applying will hold for the upcoming release. Afterwards, we're going to have some time to improve the code base and refactor where needed. We won't have time to just re-write everything. And the majority of the code isn't all that bad. But I'm looking to refactor code such that threading issues can be avoided altogether. One approach I am considering is this. For each significant platform feature, have a dedicated single thread where all events and network callbacks get marshalled onto. Similar to COM apartment threading in Windows with use of a message loop. Long blocking operations could still get dispatched to a work pool thread, but the completion callback is invoked on on the component's thread. Components could possibly even share the same thread. Then all the class libraries running inside the thread can be written under the assumption of a single threaded world. Before I go down that path, I am also very interested if there are other standard techniques or design patterns for dealing with multithreaded issues. And I have to emphasize - something beyond a book that describes the basics of mutexes and semaphores. What do you think? I am also interested in any other approaches to take towards a refactoring process. Including any of the following: Literature or papers on design patterns around threads. Something beyond an introduction to mutexes and semaphores. We don't need massive parallelism either, just ways to design an object model so as to handle asynchronous events from other threads correctly. Ways to diagram the threading of various components, so that it will be easy to study and evolve solutions for. (That is, a UML equivalent for discussing threads across objects and classes) Educating your development team on the issues with multithreaded code. What would you do?

  Read the article

• Synchronized IEnumerator<T>

  - by Dan Bryant

  I'm putting together a custom SynchronizedCollection<T> class so that I can have a synchronized Observable collection for my WPF application. The synchronization is provided via a ReaderWriterLockSlim, which, for the most part, has been easy to apply. The case I'm having trouble with is how to provide thread-safe enumeration of the collection. I've created a custom IEnumerator<T> nested class that looks like this: private class SynchronizedEnumerator : IEnumerator<T> { private SynchronizedCollection<T> _collection; private int _currentIndex; internal SynchronizedEnumerator(SynchronizedCollection<T> collection) { _collection = collection; _collection._lock.EnterReadLock(); _currentIndex = -1; } #region IEnumerator<T> Members public T Current { get; private set;} #endregion #region IDisposable Members public void Dispose() { var collection = _collection; if (collection != null) collection._lock.ExitReadLock(); _collection = null; } #endregion #region IEnumerator Members object System.Collections.IEnumerator.Current { get { return Current; } } public bool MoveNext() { var collection = _collection; if (collection == null) throw new ObjectDisposedException("SynchronizedEnumerator"); _currentIndex++; if (_currentIndex >= collection.Count) { Current = default(T); return false; } Current = collection[_currentIndex]; return true; } public void Reset() { if (_collection == null) throw new ObjectDisposedException("SynchronizedEnumerator"); _currentIndex = -1; Current = default(T); } #endregion } My concern, however, is that if the Enumerator is not Disposed, the lock will never be released. In most use cases, this is not a problem, as foreach should properly call Dispose. It could be a problem, however, if a consumer retrieves an explicit Enumerator instance. Is my only option to document the class with a caveat implementer reminding the consumer to call Dispose if using the Enumerator explicitly or is there a way to safely release the lock during finalization? I'm thinking not, since the finalizer doesn't even run on the same thread, but I was curious if there other ways to improve this. EDIT After thinking about this a bit and reading the responses (particular thanks to Hans), I've decided this is definitely a bad idea. The biggest issue actually isn't forgetting to Dispose, but rather a leisurely consumer creating deadlock while enumerating. I now only read-lock long enough to get a copy and return the enumerator for the copy.

  Read the article

• Asynchronous subprocess on Windows

  - by Stigma

  First of all, the overall problem I am solving is a bit more complicated than I am showing here, so please do not tell me 'use threads with blocking' as it would not solve my actual situation without a fair, FAIR bit of rewriting and refactoring. I have several applications which are not mine to modify, which take data from stdin and poop it out on stdout after doing their magic. My task is to chain several of these programs. Problem is, sometimes they choke, and as such I need to track their progress which is outputted on STDERR. pA = subprocess.Popen(CommandA, shell=False, stdout=subprocess.PIPE, stderr=subprocess.PIPE) # ... some more processes make up the chain, but that is irrelevant to the problem pB = subprocess.Popen(CommandB, shell=False, stdout=subprocess.PIPE, stderr=subprocess.PIPE, stdin=pA.stdout ) Now, reading directly through pA.stdout.readline() and pB.stdout.readline(), or the plain read() functions, is a blocking matter. Since different applications output in different paces and different formats, blocking is not an option. (And as I wrote above, threading is not an option unless at a last, last resort.) pA.communicate() is deadlock safe, but since I need the information live, that is not an option either. Thus google brought me to this asynchronous subprocess snippet on ActiveState. All good at first, until I implement it. Comparing the cmd.exe output of pA.exe | pB.exe, ignoring the fact both output to the same window making for a mess, I see very instantaneous updates. However, I implement the same thing using the above snippet and the read_some() function declared there, and it takes over 10 seconds to notify updates of a single pipe. But when it does, it has updates leading all the way upto 40% progress, for example. Thus I do some more research, and see numerous subjects concerning PeekNamedPipe, anonymous handles, and returning 0 bytes available even though there is information available in the pipe. As the subject has proven quite a bit beyond my expertise to fix or code around, I come to Stack Overflow to look for guidance. :) My platform is W7 64-bit with Python 2.6, the applications are 32-bit in case it matters, and compatibility with Unix is not a concern. I can even deal with a full ctypes or pywin32 solution that subverts subprocess entirely if it is the only solution, as long as I can read from every stderr pipe asynchronously with immediate performance and no deadlocks. :)

  Read the article

• Fairness: Where can it be better handled?

  - by Srinivas Nayak

  Hi, I would like to share one of my practical experience with multiprogramming here. Yesterday I had written a multiprogram. Modifications to sharable resources were put under critical sections protected by P(mutex) and V(mutex) and those critical section code were put in a common library. The library will be used by concurrent applications (of my own). I had three applications that will use the common code from library and do their stuff independently. my library --------- work_on_shared_resource { P(mutex) get_shared_resource work_with_it V(mutex) } --------- my application ----------- application1 { *[ work_on_shared_resource do_something_else_non_ctitical ] } application2 { *[ work_on_shared_resource do_something_else_non_ctitical ] } application3 { *[ work_on_shared_resource ] } *[...] denote a loop. ------------ I had to run the applications on Linux OS. I had a thought in my mind, hanging over years, that, OS shall schedule all the processes running under him with all fairness. In other words, it will give all the processes, their pie of resource-usage equally well. When first two applications were put to work, they run perfectly well without deadlock. But when the third application started running, always the third one got the resources, but since it is not doing anything in its non-critical region, it gets the shared resource more often when other tasks are doing something else. So the other two applications were found almost totally halted. When the third application got terminated forcefully, the previous two applications resumed their work as before. I think, this is a case of starvation, first two applications had to starve. Now how can we ensure fairness? Now I started believing that OS scheduler is innocent and blind. It depends upon who won the race; he got the largest pie of CPU and resource. Shall we attempt to ensure fairness of resource users in the critical-section code in library? Or shall we leave it up to the applications to ensure fairness by being liberal, not greedy? To my knowledge, adding code to ensure fairness to the common library shall be an overwhelming task. On the other hand, believing on the applications will also never ensure 100% fairness. The application which does a very little task after working with shared resources shall win the race where as the application which does heavy processing after their work with shared resources shall always starve. What is the best practice in this case? Where we ensure fairness and how? Sincerely, Srinivas Nayak

  Read the article

• Scala actors: receive vs react

  - by jqno

  Let me first say that I have quite a lot of Java experience, but have only recently become interested in functional languages. Recently I've started looking at Scala, which seems like a very nice language. However, I've been reading about Scala's Actor framework in Programming in Scala, and there's one thing I don't understand. In chapter 30.4 it says that using react instead of receive makes it possible to re-use threads, which is good for performance, since threads are expensive in the JVM. Does this mean that, as long as I remember to call react instead of receive, I can start as many Actors as I like? Before discovering Scala, I've been playing with Erlang, and the author of Programming Erlang boasts about spawning over 200,000 processes without breaking a sweat. I'd hate to do that with Java threads. What kind of limits am I looking at in Scala as compared to Erlang (and Java)? Also, how does this thread re-use work in Scala? Let's assume, for simplicity, that I have only one thread. Will all the actors that I start run sequentially in this thread, or will some sort of task-switching take place? For example, if I start two actors that ping-pong messages to each other, will I risk deadlock if they're started in the same thread? According to Programming in Scala, writing actors to use react is more difficult than with receive. This sounds plausible, since react doesn't return. However, the book goes on to show how you can put a react inside a loop using Actor.loop. As a result, you get loop { react { ... } } which, to me, seems pretty similar to while (true) { receive { ... } } which is used earlier in the book. Still, the book says that "in practice, programs will need at least a few receive's". So what am I missing here? What can receive do that react cannot, besides return? And why do I care? Finally, coming to the core of what I don't understand: the book keeps mentioning how using react makes it possible to discard the call stack to re-use the thread. How does that work? Why is it necessary to discard the call stack? And why can the call stack be discarded when a function terminates by throwing an exception (react), but not when it terminates by returning (receive)? I have the impression that Programming in Scala has been glossing over some of the key issues here, which is a shame, because otherwise it's a truly excellent book.

  Read the article

• multithreading with database

  - by Darsin

  I am looking out for a strategy to utilize multithreading (probably asynchronous delegates) to do a synchronous operation. I am new to multithreading so i will outline my scenario first. This synchronous operation right now is done for one set of data (portfolio) based on the the parameters provided. The (psudeo-code) implementation is given below: public DataSet DoTests(int fundId, DateTime portfolioDate) { // Get test results for the portfolio // Call the database adapter method, which in turn is a stored procedure, // which in turns runs a series of "rule" stored procs and fills a local temp table and returns it back. DataSet resultsDataSet = GetTestResults(fundId, portfolioDate); try { // Do some local processing on the results DoSomeProcessing(resultsDataSet); // Save the results in Test, TestResults and TestAllocations tables in a transaction. // Sets a global transaction which is provided to all the adapter methods called below // It is defined in the Base class StartTransaction("TestTransaction"); // Save Test and get a testId int testId = UpdateTest(resultsDataSet); // Adapter method, uses the same transaction // Update testId in the other tables in the dataset UpdateTestId(resultsDataSet, testId); // Update TestResults UpdateTestResults(resultsDataSet); // Adapter method, uses the same transaction // Update TestAllocations UpdateTestAllocations(resultsDataSet); // Adapter method, uses the same transaction // It is defined in the base class CommitTransaction("TestTransaction"); } catch { RollbackTransaction("TestTransaction"); } return resultsDataSet; } Now the requirement is to do it for multiple set of data. One way would be to call the above DoTests() method in a loop and get the data. I would prefer doing it in parallel. But there are certain catches: StartTransaction() method creates a connection (and transaction) every time it is called. All the underlying database tables, procedures are the same for each call of DoTests(). (obviously). Thus my question are: Will using multithreading anyway improve performance? What are the chances of deadlock especially when new TestId's are being created and the Tests, TestResults and TestAllocations are being saved? How can these deadlocked be handled? Is there any other more efficient way of doing the above operation apart from looping over the DoTests() method repeatedly?

  Read the article

• C# Serialization Surrogate - Cannot access a disposed object

  - by crushhawk

  I have an image class (VisionImage) that is a black box to me. I'm attempting to serialize the image object to file using Serialization Surrogates as explained on this page. Below is my surrogate code. sealed class VisionImageSerializationSurrogate : ISerializationSurrogate { public void GetObjectData(Object obj, SerializationInfo info, StreamingContext context) { VisionImage image = (VisionImage)obj; byte[,] temp = image.ImageToArray().U8; info.AddValue("width", image.Width); info.AddValue("height", image.Height); info.AddValue("pixelvalues", temp, temp.GetType()); } public Object SetObjectData(Object obj, SerializationInfo info, StreamingContext context, ISurrogateSelector selector) { VisionImage image = (VisionImage)obj; Int32 width = info.GetInt32("width"); Int32 height = info.GetInt32("height"); byte[,] temp = new byte[height, width]; temp = (byte[,])info.GetValue("pixelvalues", temp.GetType()); PixelValue2D tempPixels = new PixelValue2D(temp); image.ArrayToImage(tempPixels); return image; } } I've stepped through it to write to binary. It seems to be working fine (file is getting bigger as the images are captured). I tried to test it read the file back in. The values read back in are correct as far as the "info" object is concerned. When I get to the line image.ArrayToImage(tempPixels); It throws the "Cannot access a disposed object" exception. Upon further inspection, the object and the resulting image are both marked as disposed. My code behind the form spawns an "acquisitionWorker" and runs the following code. void acquisitionWorker_LoadImages(object sender, DoWorkEventArgs e) { // This is the main function of the acquisition background worker thread. // Perform image processing here instead of the UI thread to avoid a // sluggish or unresponsive UI. BackgroundWorker worker = (BackgroundWorker)sender; try { uint bufferNumber = 0; // Loop until we tell the thread to cancel or we get an error. When this // function completes the acquisitionWorker_RunWorkerCompleted method will // be called. while (!worker.CancellationPending) { VisionImage savedImage = (VisionImage) formatter.Deserialize(fs); CommonAlgorithms.Copy(savedImage, imageViewer.Image); // Update the UI by calling ReportProgress on the background worker. // This will call the acquisition_ProgressChanged method in the UI // thread, where it is safe to update UI elements. Do not update UI // elements directly in this thread as doing so could result in a // deadlock. worker.ReportProgress(0, bufferNumber); bufferNumber++; } } catch (ImaqException ex) { // If an error occurs and the background worker thread is not being // cancelled, then pass the exception along in the result so that // it can be handled in the acquisition_RunWorkerCompleted method. if (!worker.CancellationPending) e.Result = ex; } } What am I missing here? Why would the object be immediately disposed?

  Read the article

• TimeOuts with HttpWebRequest when running Selenium concurrently in .NET

  - by domsom

  I have a download worker that uses ThreadPool-threads to download files. After enhancing these to apply some Selenium tests to the downloaded files, I am constantly experiencing TimeOut-exceptions with the file downloaders and delays running the Selenium tests. More precisely: When the program starts, the download threads start downloading and a couple of pages are seamlessly processed via Selenium Shortly after, the first download threads start throwing TimeOut exceptions from HttpWebRequest. At the same time, commands stop flowing to Selenium (as observed in the SeleniumRC log), but the thread running Selenium is not getting any exception This situation holds as long as there are entries in the download list: new download threads are being started and terminate after receiving TimeOuts (without trying to lock Selenium) As soon as no more download threads are being started, Selenium starts receiving commands again and the threads waiting for the lock are processed sequentially as designed Now here's the download code: HttpWebRequest request = null; WebResponse response = null; Stream stream = null; StreamReader sr = null; try { request = (HttpWebRequest) WebRequest.Create(uri); request.ServicePoint.ConnectionLimit = MAX_CONNECTIONS_PER_HOST; response = request.GetResponse(); stream = response.GetResponseStream(); // Read the stream... } finally { if (request != null) request.Abort(); if (response != null) response.Close(); if (stream != null) { stream.Close(); stream.Dispose(); } if (sr != null) { sr.Close(); sr.Dispose(); } } And this is how Selenium is used afterwards in the same thread: lock(SeleniumLock) { selenium.Open(url); // Run some Selenium commands, but no selenium.stop() } Where selenium is a static variable that is initialized in the static constructor of the class (via selenium.start()). I assume I am running into the CLR connection limit, so I added these lines during initalization: ThreadPool.GetMaxThreads (out maxWorkerThreads, out maxCompletionPortThreads); HttpUtility.MAX_CONNECTIONS_PER_HOST = maxWorkerThreads; System.Net.ServicePointManager.DefaultConnectionLimit = maxWorkerThreads + 1; The + 1 is for the connection to the SeleniumRC, due to my guess that the Selenium client code also uses HttpWebRequest. It seems like I'm still running into some kind of deadlock - although the threads waiting for the Selenium lock do not hold any resources. Any ideas on how to get this working?

  Read the article

• Java consistent synchronization

  - by ring0

  We are facing the following problem in a Spring service, in a multi-threaded environment: three lists are freely and independently accessed for Read once in a while (every 5 minutes), they are all updated to new values. There are some dependencies between the lists, making that, for instance, the third one should not be read while the second one is being updated and the first one already has new values ; that would break the three lists consistency. My initial idea is to make a container object having the three lists as properties. Then the synchronization would be first on that object, then one by one on each of the three lists. Some code is worth a thousands words... so here is a draft private class Sync { final List<Something> a = Collections.synchronizedList(new ArrayList<Something>()); final List<Something> b = Collections.synchronizedList(new ArrayList<Something>()); final List<Something> c = Collections.synchronizedList(new ArrayList<Something>()); } private Sync _sync = new Sync(); ... void updateRunOnceEveryFiveMinutes() { final List<Something> newa = new ArrayList<Something>(); final List<Something> newb = new ArrayList<Something>(); final List<Something> newc = new ArrayList<Something>(); ...building newa, newb and newc... synchronized(_sync) { synchronized(_sync.a) { _synch.a.clear(); _synch.a.addAll(newa); } synchronized(_sync.b) { ...same with newb... } synchronized(_sync.c) { ...same with newc... } } // Next is accessed by clients public List<Something> getListA() { return _sync.a; } public List<Something> getListB() { ...same with b... } public List<Something> getListC() { ...same with c... } The question would be, is this draft safe (no deadlock, data consistency)? would you have a better implementation suggestion for that specific problem? update Changed the order of _sync synchronization and newa... building. Thanks

  Read the article

• probelm with recv() on a tcp connection

  - by michael

  Hi, I am simulating TCP communication on windows in C I have sender and a receiver communicating. sender sends packets of specific size to receiver. receiver gets them and send an ACK for each packet it received back to the sender. If the sender didn't get a specific packet (they are numbered in a header inside the packet) it sends the packet again to the receiver. Here is the getPacket function on the receiver side: //get the next packet from the socket. set the packetSize to -1 //if it's the first packet. //return: total bytes read // return: 0 if socket has shutdown on sender side, -1 error, else number of bytes received int getPakcet(char *chunkBuff,int packetSize,SOCKET AcceptSocket){ int totalChunkLen = 0; int bytesRecv=-1; bool firstTime=false; if (packetSize==-1) { packetSize=MAX_PACKET_LENGTH; firstTime=true; } int needToGet=packetSize; do { char* recvBuff; recvBuff = (char*)calloc(needToGet,sizeof(char)); if(recvBuff == NULL){ fprintf(stderr,"Memory allocation problem\n"); return -1; } bytesRecv = recv(AcceptSocket, recvBuff, needToGet, 0); if (bytesRecv == SOCKET_ERROR){ fprintf(stderr,"recv() error %ld.\n", WSAGetLastError()); totalChunkLen=-1; return -1; } if (bytesRecv == 0){ fprintf(stderr,"recv(): socket has shutdown on sender side"); return 0; } else if(bytesRecv > 0) { memcpy(chunkBuff + totalChunkLen,recvBuff,bytesRecv); totalChunkLen+=bytesRecv; } needToGet-=bytesRecv; } while ((totalChunkLen < packetSize) && (!firstTime)); return totalChunkLen; } i use firstTime because for the first time the receiver doesn't know the normal package size that the sender is going to send to it, so i use a MAX_PACKET_LENGTH to get a package and then set the normal package size to the num of bytes i have received my problem is the last package. it's size is less than the package size so lets say last package size is 2 and the normal package size is 4. so recv() gets two bytes, continues to the while condition, then totalChunkLen < packetSize because 2<4 so it iterates the loop again and the gets stuck in recv() because it's blocking because the sender has nothing to send. on the sender side i can't close the connection because i didn't ACK back, so it's kind of a deadlock. receiver is stuck because it's waiting for more packages but sender has nothing to send. i don't want to use a timeout for recv() or to insert a special character to the package header to mark that it is the last one what can i do ? thanks

  Read the article

• Hibernate/Spring: getHibernateTemplate().save(...) Freezes/Hangs

  - by ashes999

  I'm using Hibernate and Spring with the DAO pattern (all Hibernate dependencies in a *DAO.java class). I have nine unit tests (JUnit) which create some business objects, save them, and perform operations on them; the objects are in a hash (so I'm reusing the same objects all the time). My JUnit setup method calls my DAO.deleteAllObjects() method which calls getSession().createSQLQuery("DELETE FROM <tablename>").executeUpdate() for my business object table (just one). One of my unit tests (#8/9) freezes. I presumed it was a database deadlock, because the Hibernate log file shows my delete statement last. However, debugging showed that it's simply HibernateTemplate.save(someObject) that's freezing. (Eclipse shows that it's freezing on HibernateTemplate.save(Object), line 694.) Also interesting to note is that running this test by itself (not in the suite of 9 tests) doesn't cause any problems. How on earth do I troubleshoot and fix this? Also, I'm using @Entity annotations, if that matters. Edit: I removed reuse of my business objects (use unique objects in every method) -- didn't make a difference (still freezes). Edit: This started trickling into other tests, too (can't run more than one test class without getting something freezing) Transaction configuration: <bean id="txManager" class="org.springframework.jdbc.datasource.DataSourceTransactionManager"> <property name="dataSource" ref="dataSource" /> </bean> <tx:advice id="txAdvice" transaction-manager="txManager"> <!-- the transactional semantics... --> <tx:attributes> <!-- all methods starting with 'get' are read-only --> <tx:method name="get*" read-only="true" /> <tx:method name="find*" read-only="true" /> <!-- other methods use the default transaction settings (see below) --> <tx:method name="*" /> </tx:attributes> </tx:advice> <!-- my bean which is exhibiting the hanging behavior --> <aop:config> <aop:pointcut id="beanNameHere" expression="execution(* com.blah.blah.IMyDAO.*(..))" /> <aop:advisor advice-ref="txAdvice" pointcut-ref="beanNameHere" /> </aop:config>

  Read the article

< Previous Page | 6 7 8 9 10 11 12  | Next Page >