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  • linux thread synchronization

    - by johnnycrash
    I am new to linux and linux threads. I have spent some time googling to try to understand the differences between all the functions available for thread synchronization. I still have some questions. I have found all of these different types of synchronizations, each with a number of functions for locking, unlocking, testing the lock, etc. gcc atomic operations futexes mutexes spinlocks seqlocks rculocks conditions semaphores My current (but probably flawed) understanding is this: semaphores are process wide, involve the filesystem (virtually I assume), and are probably the slowest. Futexes might be the base locking mechanism used by mutexes, spinlocks, seqlocks, and rculocks. Futexes might be faster than the locking mechanisms that are based on them. Spinlocks dont block and thus avoid context swtiches. However they avoid the context switch at the expense of consuming all the cycles on a CPU until the lock is released (spinning). They should only should be used on multi processor systems for obvious reasons. Never sleep in a spinlock. The seq lock just tells you when you finished your work if a writer changed the data the work was based on. You have to go back and repeat the work in this case. Atomic operations are the fastest synch call, and probably are used in all the above locking mechanisms. You do not want to use atomic operations on all the fields in your shared data. You want to use a lock (mutex, futex, spin, seq, rcu) or a single atomic opertation on a lock flag when you are accessing multiple data fields. My questions go like this: Am I right so far with my assumptions? Does anyone know the cpu cycle cost of the various options? I am adding parallelism to the app so we can get better wall time response at the expense of running fewer app instances per box. Performances is the utmost consideration. I don't want to consume cpu with context switching, spinning, or lots of extra cpu cycles to read and write shared memory. I am absolutely concerned with number of cpu cycles consumed. Which (if any) of the locks prevent interruption of a thread by the scheduler or interrupt...or am I just an idiot and all synchonization mechanisms do this. What kinds of interruption are prevented? Can I block all threads or threads just on the locking thread's CPU? This question stems from my fear of interrupting a thread holding a lock for a very commonly used function. I expect that the scheduler might schedule any number of other workers who will likely run into this function and then block because it was locked. A lot of context switching would be wasted until the thread with the lock gets rescheduled and finishes. I can re-write this function to minimize lock time, but still it is so commonly called I would like to use a lock that prevents interruption...across all processors. I am writing user code...so I get software interrupts, not hardware ones...right? I should stay away from any functions (spin/seq locks) that have the word "irq" in them. Which locks are for writing kernel or driver code and which are meant for user mode? Does anyone think using an atomic operation to have multiple threads move through a linked list is nuts? I am thinking to atomicly change the current item pointer to the next item in the list. If the attempt works, then the thread can safely use the data the current item pointed to before it was moved. Other threads would now be moved along the list. futexes? Any reason to use them instead of mutexes? Is there a better way than using a condition to sleep a thread when there is no work? When using gcc atomic ops, specifically the test_and_set, can I get a performance increase by doing a non atomic test first and then using test_and_set to confirm? *I know this will be case specific, so here is the case. There is a large collection of work items, say thousands. Each work item has a flag that is initialized to 0. When a thread has exclusive access to the work item, the flag will be one. There will be lots of worker threads. Any time a thread is looking for work, they can non atomicly test for 1. If they read a 1, we know for certain that the work is unavailable. If they read a zero, they need to perform the atomic test_and_set to confirm. So if the atomic test_and_set is 500 cpu cycles because it is disabling pipelining, causes cpu's to communicate and L2 caches to flush/fill .... and a simple test is 1 cycle .... then as long as I had a better ratio of 500 to 1 when it came to stumbling upon already completed work items....this would be a win.* I hope to use mutexes or spinlocks to sparilngly protect sections of code that I want only one thread on the SYSTEM (not jsut the CPU) to access at a time. I hope to sparingly use gcc atomic ops to select work and minimize use of mutexes and spinlocks. For instance: a flag in a work item can be checked to see if a thread has worked it (0=no, 1=yes or in progress). A simple test_and_set tells the thread if it has work or needs to move on. I hope to use conditions to wake up threads when there is work. Thanks!

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  • Postgresql: Implicit lock acquisition from foreign-key constraint evaluation

    - by fennec
    So, I'm being confused about foreign key constraint handling in Postgresql. (version 8.4.4, for what it's worth). We've got a couple of tables, mildly anonymized below: device: (id, blah, blah, blah, blah, blah x 50)… primary key on id whooooole bunch of other junk device_foo: (id, device_id, left, right) Foreign key (device_id) references device(id) on delete cascade; primary key on id btree index on 'left' and 'right' So I set out with two database windows to run some queries. db1> begin; lock table device in exclusive mode; db2> begin; update device_foo set left = left + 1; The db2 connection blocks. It seems odd to me that an update of the 'left' column on device_stuff should be affected by activity on the device table. But it is. In fact, if I go back to db1: db1> select * from device_stuff for update; *** deadlock occurs *** The pgsql log has the following: blah blah blah deadlock blah. CONTEXT: SQL statement "SELECT 1 FROM ONLY "public"."device" x WHERE "id" OPERATOR(pg_catalog.=) $1 FOR SHARE OF X: update device_foo set left = left + 1; I suppose I've got two issues: the first is that I don't understand the precise mechanism by which this sort of locking occurs. I have got a couple of useful queries to query pg_locks to see what sort of locks a statement invokes, but I haven't been able to observe this particular sort of locking when I run the update device_foo command in isolation. (Perhaps I'm doing something wrong, though.) I also can't find any documentation on the lock acquisition behavior of foreign-key constraint checks. All I have is a log message. Am I to infer from this that any change to a row will acquire an update lock on all the tables which it's foreign-keyed against? The second issue is that I'd like to find some way to make it not happen like that. I'm ending up with occasional deadlocks in the actual application. I'd like to be able to run big update statements that impact all rows on device_foo without acquiring a big lock on the device table. (There's a lot of access going on in the device table, and it's kind of an expensive lock to get.)

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  • How to write my own global lock / unlock functions for PostgreSQL

    - by rafalmag
    I have postgresql (in perlu) function getTravelTime(integer, timestamp), which tries to select data for specified ID and timestamp. If there are no data or if data is old, it downloads them from external server (downloading time ~300ms). Multiple process use this database and this function. There is an error when two process do not find data and download them and try to do an insert to travel_time table (id and timestamp pair have to be unique). I thought about locks. Locking whole table would block all processes and allow only one to proceed. I need to lock only on id and timestamp. pg_advisory_lock seems to lock only in "current session". But my processes uses their own sessions. I tried to write my own lock/unlock functions. Am I doing it right? I use active waiting, how can I omit this? Maybe there is a way to use pg_advisory_lock() as global lock? My code: CREATE TABLE travel_time_locks ( id_key integer NOT NULL, time_key timestamp without time zone NOT NULL, UNIQUE (id_key, time_key) ); ------------ -- Function: mylock(integer, timestamp) DROP FUNCTION IF EXISTS mylock(integer, timestamp) CASCADE; -- Usage: SELECT mylock(1, '2010-03-28T19:45'); -- function tries to do a global lock similar to pg_advisory_lock(key, key) CREATE OR REPLACE FUNCTION mylock(id_input integer, time_input timestamp) RETURNS void AS $BODY$ DECLARE rows int; BEGIN LOOP BEGIN -- active waiting here !!!! :( INSERT INTO travel_time_locks (id_key, time_key) VALUES (id_input, time_input); EXCEPTION WHEN unique_violation THEN CONTINUE; END; EXIT; END LOOP; END; $BODY$ LANGUAGE 'plpgsql' VOLATILE COST 1; ------------ -- Function: myunlock(integer, timestamp) DROP FUNCTION IF EXISTS myunlock(integer, timestamp) CASCADE; -- Usage: SELECT myunlock(1, '2010-03-28T19:45'); -- function tries to do a global unlock similar to pg_advisory_unlock(key, key) CREATE OR REPLACE FUNCTION myunlock(id_input integer, time_input timestamp) RETURNS integer AS $BODY$ DECLARE BEGIN DELETE FROM ONLY travel_time_locks WHERE id_key=id_input AND time_key=time_input; RETURN 1; END; $BODY$ LANGUAGE 'plpgsql' VOLATILE COST 1;

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  • Postfix unable to create lock file, permission denied

    - by John Bowlinger
    I thought I had my postfix configuration all set up on my Amazon Ubuntu server but I guess not. I'm trying to set up an admin email account for 3 virtually hosted Apache websites. Here's my postfix main.cf file: myhostname = ip-XX-XXX-XX-XXX.us-west-2.compute.internal alias_maps = hash:/etc/aliases alias_database = hash:/etc/aliases myorigin = /etc/mailname mydestination = ip-XX-XXX-XX-XXX.us-west-2.compute.internal, localhost.us-west-2.compute.internal, , localhost relayhost = mynetworks = 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128 mailbox_size_limit = 0 recipient_delimiter = + inet_interfaces = all virtual_mailbox_domains = example1.com, example2.com, example3.com virtual_mailbox_base = /var/mail/vhosts virtual_mailbox_maps = hash:/etc/postfix/vmailbox virtual_minimum_uid = 100 virtual_uid_maps = static:115 virtual_gid_maps = static:115 virtual_alias_maps = hash:/etc/postfix/virtual Here's my vmailbox file: [email protected] example1.com/admin [email protected] example2.com/admin [email protected] example3.com/admin @example1.com example1.com/catchall @example2.com example2.com/catchall @example3.com example3.com/catchall And finally my virtual file: [email protected] postmaster [email protected] postmaster [email protected] postmaster When I try to send an email to through netcat to my one of my domains, I get: unable to create lock file /var/mail/vhosts/example1.com/admin.lock: Permission denied This is despite the fact that I set example1.com group to postfix and also my virtual_uid_maps and virtual_gid_maps are both set to Postfix group id of 115.

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  • fedora12, yum not releasing "lock" after performing an action

    - by James.Elsey
    Hello, This problem has been occurring quite frequently recently and I can't seem to find a way of preventing it. Whenever I perform an action with yum such as to install or remove software, it appears to execute successfully but then I'm unable to move onto the next yum command For example, I executed yum remove skype, it appeared to remove ok, but next when I try to yum search skype it appears that yum is still processing, and I have to manually kill that process via kill 1234 (or whatever the PID is) My output is as follows [root@nevada james]# yum remove skype Loaded plugins: presto, refresh-packagekit Setting up Remove Process Resolving Dependencies --> Running transaction check ---> Package skype.i586 0:2.1.0.47-fc10 set to be erased --> Finished Dependency Resolution Dependencies Resolved ================================================================================ Package Arch Version Repository Size ================================================================================ Removing: skype i586 2.1.0.47-fc10 installed 24 M Transaction Summary ================================================================================ Remove 1 Package(s) Reinstall 0 Package(s) Downgrade 0 Package(s) Is this ok [y/N]: y Downloading Packages: Running rpm_check_debug Running Transaction Test Finished Transaction Test Transaction Test Succeeded Running Transaction Erasing : skype-2.1.0.47-fc10.i586 1/1 Removed: skype.i586 0:2.1.0.47-fc10 Complete! [root@nevada james]# yum search skype Loaded plugins: presto, refresh-packagekit Existing lock /var/run/yum.pid: another copy is running as pid 3639. Another app is currently holding the yum lock; waiting for it to exit... The other application is: PackageKit Memory : 79 M RSS (372 MB VSZ) Started: Fri Dec 18 08:39:18 2009 - 00:01 ago State : Sleeping, pid: 3639 Kernel version : 2.6.31.6-166.fc12.x86_64 Any ideas how I can prevent this behaviour? Thanks

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  • HP G61 Laptop wont boot- display stays off, caps and num lock indicators blink repeatedly

    - by Benguy12
    I had my HP G61 laptop running in sleep for a while. When I came back to it about a half-hour later, it was no longer in sleep mode - the power light and the Wi-Fi indicator light were on (I keep Wi-Fi off becuase I use a wired connection) - but nothing was showing on screen. In fact, the display wasn't even turned on. So I let it sit for about 10 minutes but nothing happened. I did a force shut down and rebooted. Instead of a normal boot, the display didnt turn on, the Wi-Fi indicator was off, and the Caps Lock and Num Lock lights just blinked repeatedly. On the external keyboard i use, none of the light indicators were blinking or even on. I tried force shut-down again 10 times, then unplugged all connections except for the power cable (my laptop battery dosent hold a charge for more than 2 minutes, so I always must have a wall connection) and tried to boot again but still nothing happened. I unplugged the battery and even then nothing happened. I also tried booting with the disk drive open, and then with it closed again. On the time it was closed, I was able to successfully boot into Windows, but recieved a "Windows did not shut-down sucessfully" notice. Does anybody know why this may have happened? My PC's specs: Windows 7 Home Premium, 64-bit 4GB of physical RAM, 8GB of vRAM (on a flash drive) AMD Vision x64 processor (don't know any other specs about it) ATI Radeon graphics card, 392 MB DVD-R/W lightscribe drive 2 External hard-disks (first one is 1.5TB, second one is 1TB) custom boot-screen and boot-annimation Standard BIOS apps running before sleep: firefox 10.4 itunes 10.6 adobe photoshop extended CS5.1 rockstar games social club (running in background) microsoft powerpoint 2010 professional edition google chrome I was NOT running Aero or any fancy themes - I was using the normal windows classic theme. I have a desktop icon manager application called Stardock Fences that was also running (it runs as a service/process).

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  • Suddenly getting lock timeouts with MySQL

    - by Marc Hughes
    We've got a web app hosted on Amazon Web services. Our database is a multi-az RDS MySQL server running 5.1.57 and 3-4 app servers talk to it. Today, we started seeing a lot of errors along the lines of "Lock wait timeout exceeded; try restarting transaction" - almost 1% of POST requests are seeing this. There have been no modifications to the code running on the site. There have been no schema changes. We haven't had a big spike in traffic. I've been looking at the processes running, and none seem out of control. I tried scaling our RDS instance from a small to a large, with no effect. Two days ago, Amazon had some outages. As part of the recovery from that, our RDS server, and our app servers ended up in different availability zones, but all within the same region. But yesterday, everything was fine so I'm not convinced that's related. The lock timeouts are in different types of requests and occur in different InnoDB tables. I have noticed the number of open connections jumped when we started seeing problems, but they may be a symptom and not a cause. What are my next steps in debugging this?

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  • Creating a mouse drag done observable with Reactive Extensions

    - by juharr
    I have the following var leftMouseDown = Observable.FromEvent<MouseButtonEventArgs>(displayCanvas, "MouseLeftButtonDown"); var leftMouseUp = Observable.FromEvent<MouseButtonEventArgs>(displayCanvas, "MouseLeftButtonUp"); var mouseMove = Observable.FromEvent<MouseEventArgs>(displayCanvas, "MouseMove"); var leftMouseDragging = from down in leftMouseDown let startPoint = down.EventArgs.GetPosition(displayCanvas) from move in mouseMove.TakeUntil(leftMouseUp) let endPoint = move.EventArgs.GetPosition(displayCanvas) select new { Start = startPoint, End = endPoint }; which when I subscribe to it will give me the start point of the drag and the current end point. Now I need to do something once the drag is done. I was unsuccessful in attempting to do this completely with RX and ended up doing leftMouseDragging.Subscribe(value=> { dragging = true; //Some other code }); leftMouseUp.Subscribe(e=> { if(dragging) { MessageBox.Show("Just finished dragging"); dragging = false; } }); This works fine until I do a right mouse button drag. Then when I click the left mouse button I get the message box. If I only do a left button drag I get the message box, and then clicking the left mouse button doesn't produce the box. I'd like to do this without the external state, but if nothing else I'd at least like for it to work properly. FYI: I tried making dragging volatile and using a lock, but that didn't work. EDIT It turns out my problem was with a right click context menu. Once I got rid of that my above code worked. So, now my problem is how to I get to have the context menu and still have my code work. I assume the Context menu was handling the left mouse click and that somehow caused my code to not work, but I'm still puzzling it out.

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  • Boost::thread mutex issue: Try to lock, access violation

    - by user1419305
    I am currently learning how to multithread with c++, and for that im using boost::thread. I'm using it for a simple gameengine, running three threads. Two of the threads are reading and writing to the same variables, which are stored inside something i call PrimitiveObjects, basicly balls, plates, boxes etc. But i cant really get it to work, i think the problem is that the two threads are trying to access the same memorylocation at the same time, i have tried to avoid this using mutex locks, but for now im having no luck, this works some times, but if i spam it, i end up with this exception: First-chance exception at 0x00cbfef9 in TTTTT.exe: 0xC0000005: Access violation reading location 0xdddddded. Unhandled exception at 0x77d315de in TTTTT.exe: 0xC0000005: Access violation reading location 0xdddddded. These are the functions inside the object that im using for this, and the debugger is also blaming them for the exception. void PrimitiveObj::setPos(glm::vec3 in){ boost::mutex mDisposingMutex; boost::try_mutex::scoped_try_lock lock(mDisposingMutex); if ( lock) { position = in; boost::try_mutex::scoped_try_lock unlock(mDisposingMutex); } } glm::vec3 PrimitiveObj::getPos(){ boost::mutex myMutex; boost::try_mutex::scoped_try_lock lock(myMutex); if ( lock) { glm::vec3 curPos = position; boost::try_mutex::scoped_try_lock unlock(myMutex); return curPos; } return glm::vec3(0,0,0); } Any ideas?

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  • Selenium Webdriver Java - looking for alternatives for Actions and Robot when performing drag-and-drop

    - by Ja-ke Alconcel
    I first tried Actions class and the drag-and-drop does work on different elements, however it was unable to locate the a specific draggable element on it's exact screen/webpage position. Here's the code I've used: Point loc = driver.findElement(By.id("thiselement")).getLocation(); System.out.println(loc); WebElement drag = driver.findElement(By.id("thiselement")); Actions test = new Actions(driver); test.dragAndDropBy(drag, 0, 60).build().perform(); I checked the element with it's pixel location and it prints (837, -52), which was somewhere on top of the webpage and was pixels away from the actual element. Then I tried using the Robot class and works perfectly fine on my script, but can only provide constant successful runs on a single test machine, running it with a different machine with different screen resolution and screen size will render the script to fail due to the dependency of Robot on the pixel location of the element. The sample code of the Robot script I'm using: Robot dragAndDrop = new Robot(); dragAndDrop.mouseMove(945, 166); //actual pixel location of the draggable element dragAndDrop.mousePress(InputEvent.BUTTON1_MASK); sleep(3000); dragAndDrop.mouseMove(945, 226); dragAndDrop.mouseRelease(InputEvent.BUTTON1_MASK); sleep(3000); Is there any alternative for Actions and Robot to automate drag-and-drop? Or maybe a help on working the script to work on Actions as I really can't use Robot. Thanks in advance.

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  • Lock statement vs Monitor.Enter method.

    - by Vokinneberg
    I suppose it is an interesting code example. We have a class, let's call it Test with Finalize method. In Main method here is two code blocks where i am using lock statement and Monitor.Enter call. Also i have two instances of class Test here. The experiment is pretty simple - nulling Test variable within locking block and try to collect it manually with GC.Collect method call. So, to see the Finilaze call i am calling GC.WaitForPendingFinalizers method. Everything is very simple as you can see. By defenition of lock statement it's opens by compiler to try{...}finally{..} block with Minitor.Enter call inside of try block and Monitor.Exit in finally block. I've tryed to implement try-finally block manually. I've expected the same behaviour in both cases. in case of using lock and in case of unsing Monitor.Enter. But, surprize, surprize - it is different as you can see below. public class Test : IDisposable { private string name; public Test(string name) { this.name = name; } ~Test() { Console.WriteLine(string.Format("Finalizing class name {0}.", name)); } } class Program { static void Main(string[] args) { var test1 = new Test("Test1"); var test2 = new Test("Tesst2"); lock (test1) { test1 = null; Console.WriteLine("Manual collect 1."); GC.Collect(); GC.WaitForPendingFinalizers(); Console.WriteLine("Manual collect 2."); GC.Collect(); } var lockTaken = false; System.Threading.Monitor.Enter(test2, ref lockTaken); try { test2 = null; Console.WriteLine("Manual collect 3."); GC.Collect(); GC.WaitForPendingFinalizers(); Console.WriteLine("Manual collect 4."); GC.Collect(); } finally { System.Threading.Monitor.Exit(test2); } Console.ReadLine(); } } Output of this example is Manual collect 1. Manual collect 2. Manual collect 3. Finalizing class name Test2. Manual collect 4. And null reference exception in last finally block because test2 is null reference. I've was surprised and disasembly my code into IL. So, here is IL dump of Main method. .entrypoint .maxstack 2 .locals init ( [0] class ConsoleApplication2.Test test1, [1] class ConsoleApplication2.Test test2, [2] bool lockTaken, [3] bool <>s__LockTaken0, [4] class ConsoleApplication2.Test CS$2$0000, [5] bool CS$4$0001) L_0000: nop L_0001: ldstr "Test1" L_0006: newobj instance void ConsoleApplication2.Test::.ctor(string) L_000b: stloc.0 L_000c: ldstr "Tesst2" L_0011: newobj instance void ConsoleApplication2.Test::.ctor(string) L_0016: stloc.1 L_0017: ldc.i4.0 L_0018: stloc.3 L_0019: ldloc.0 L_001a: dup L_001b: stloc.s CS$2$0000 L_001d: ldloca.s <>s__LockTaken0 L_001f: call void [mscorlib]System.Threading.Monitor::Enter(object, bool&) L_0024: nop L_0025: nop L_0026: ldnull L_0027: stloc.0 L_0028: ldstr "Manual collect." L_002d: call void [mscorlib]System.Console::WriteLine(string) L_0032: nop L_0033: call void [mscorlib]System.GC::Collect() L_0038: nop L_0039: call void [mscorlib]System.GC::WaitForPendingFinalizers() L_003e: nop L_003f: ldstr "Manual collect." L_0044: call void [mscorlib]System.Console::WriteLine(string) L_0049: nop L_004a: call void [mscorlib]System.GC::Collect() L_004f: nop L_0050: nop L_0051: leave.s L_0066 L_0053: ldloc.3 L_0054: ldc.i4.0 L_0055: ceq L_0057: stloc.s CS$4$0001 L_0059: ldloc.s CS$4$0001 L_005b: brtrue.s L_0065 L_005d: ldloc.s CS$2$0000 L_005f: call void [mscorlib]System.Threading.Monitor::Exit(object) L_0064: nop L_0065: endfinally L_0066: nop L_0067: ldc.i4.0 L_0068: stloc.2 L_0069: ldloc.1 L_006a: ldloca.s lockTaken L_006c: call void [mscorlib]System.Threading.Monitor::Enter(object, bool&) L_0071: nop L_0072: nop L_0073: ldnull L_0074: stloc.1 L_0075: ldstr "Manual collect." L_007a: call void [mscorlib]System.Console::WriteLine(string) L_007f: nop L_0080: call void [mscorlib]System.GC::Collect() L_0085: nop L_0086: call void [mscorlib]System.GC::WaitForPendingFinalizers() L_008b: nop L_008c: ldstr "Manual collect." L_0091: call void [mscorlib]System.Console::WriteLine(string) L_0096: nop L_0097: call void [mscorlib]System.GC::Collect() L_009c: nop L_009d: nop L_009e: leave.s L_00aa L_00a0: nop L_00a1: ldloc.1 L_00a2: call void [mscorlib]System.Threading.Monitor::Exit(object) L_00a7: nop L_00a8: nop L_00a9: endfinally L_00aa: nop L_00ab: call string [mscorlib]System.Console::ReadLine() L_00b0: pop L_00b1: ret .try L_0019 to L_0053 finally handler L_0053 to L_0066 .try L_0072 to L_00a0 finally handler L_00a0 to L_00aa I does not see any difference between lock statement and Monitor.Enter call. So, why i steel have a reference to the instance of test1 in case of lock, and object is not collected by GC, but in case of using Monitor.Enter it is collected and finilized?

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  • Way to turn on keyboard's caps-lock light without actually turning on caps-lock?

    - by cksubs
    I'm writing a program that uses caps-lock as a toggle switch. It would be nice to set the LED of the key to show that my program is on or off, like the capslock key does naturally. I know that I could just SendInput('Capslock'); or whatever to actually turn caps-lock on and off. But my app is a typing program, and I don't want to have to deal with translating the all-caps keys that turning it on would give me into their lower/upper cases. I might go that route eventually, but not for this version. I would however be interested in just turning on the LED light WITHOUT actually turning on caps-lock. Is there any way to do that? Thank you.

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  • Acquiring Table Lock in Database - Interview Question

    - by harigm
    One of my interview Questions, if multiple users across the world are accessing the application, in which it uses a Table which has a Primary Key as Auto Increment Field. The Question how can you prevent the other user getting the Same Primary key when the other user is executing? My answer was I will obtain the Lock on the table and I will make the user to wait Until that user is released with the Primary key. But the Question How do you acquire the Table lock programmatically and implement this? If there are 1000 users coming every minute to the application, if you explicity hold the lock on the table, then the application will become slower? How do you manage this? Please suggest the possible answers for the above question

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  • Implementing a Mutex Lock in C

    - by Adam
    I'm trying to make a really mutex in C and for some reason I'm getting cases where two threads are getting the lock at the same time, which shouldn't be possible. Any ideas why it's not working? void mutexLock(mutex_t *mutexlock, pid_t owner) { int failure; while(mutexlock->mx_state == 0 || failure || mutexlock->mx_owner != owner) { failure = 1; if (mutexlock->mx_state == 0) { asm( "test:" "movl $0x01,%%eax\n\t" // move 1 to eax "xchg %%eax,%0\n\t" // try to set the lock bit "mov %%eax,%1\n\t" // export our result to a test var "test %%eax,%%eax\n\t" "jnz test\n\t" :"=r"(mutexlock->mx_state),"=r"(failure) :"r"(mutexlock->mx_state) :"%eax" ); } if (failure == 0) { mutexlock->mx_owner = owner; //test to see if we got the lock bit } } }

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  • How to show that the double-checked-lock pattern with Dictionary's TryGetValue is not threadsafe in

    - by Amir
    Recently I've seen some C# projects that use a double-checked-lock pattern on a Dictionary. Something like this: private static readonly object _lock = new object(); private static volatile IDictionary<string, object> _cache = new Dictionary<string, object>(); public static object Create(string key) { object val; if (!_cache.TryGetValue(key, out val)) { lock (_lock) { if (!_cache.TryGetValue(key, out val)) { val = new object(); // factory construction based on key here. _cache.Add(key, val); } } } return val; } This code is incorrect, since the Dictionary can be "growing" the collection in _cache.Add() while _cache.TryGetValue (outside the lock) is iterating over the collection. It might be extremely unlikely in many situations, but is still wrong. Is there a simple program to demonstrate that this code fails? Does it make sense to incorporate this into a unit test? And if so, how?

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  • Using accelerometer to detect iPad lock switch trigger?

    - by DevDevDev
    So for complicated reasons I am managing view rotations myself, and am only implementing UIDeviceOrientationPortrait as the autorotating orientation. Anyway, when one clicks the screen lock on the iPad, the device will rotate to UIDeviceOrientationPortrait, which will cause my code to rotate, which I do not want to happen. For instance of the user is holding the device in "landscape mode", i.e. I have already manually rotated the UI to landscape mode, and he/she clicks the screen lock, it will rotate the UI to portrait mode, even though the user did not move the device at all. So somehow I need a way to differentiate between rotations to portrait mode occuring from the screen lock, and those occuring from device rotation. As such, I would like somehow to use the accelerometer to detect whether or not the device was actually 'rotated' or whether the button was clicked. Thanks!

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  • Oracle global lock across process

    - by Jimm
    I would like to synchronize access to a particular insert. Hence, if multiple applications execute this "one" insert, the inserts should happen one at a time. The reason behind synchronization is that there should only be ONE instance of this entity. If multiple applications try to insert the same entity,only one should succeed and others should fail. One option considered was to create a composite unique key, that would uniquely identify the entity and rely on unique constraint. For some reasons, the dba department rejected this idea. Other option that came to my mind was to create a stored proc for the insert and if the stored proc can obtain a global lock, then multiple applications invoking the same stored proc, though in their seperate database sessions, it is expected that the stored proc can obtain a global lock and hence serialize the inserts. My question is it possible to for a stored proc in oracle version 10/11, to obtain such a lock and any pointers to documentation would be helpful.

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  • is it possible to lock oracle 10g database table with C#/(ADO?).NET 2.0

    - by matti
    I have a table that contains a maximum value that needs to be get and set by multiple programs. How can I lock the table for a while when old value is got and new is updated in C#? In other words: string sql = "lock table MaxValueTable in exclusive mode"; using (DbCommand cmd = cnctn.CreateCommand()) { cmd.CommandText = sql; // execute command somehow!! } maxValue = GetMaxValue(); SetMaxValue(maxValue + X); sql = "lock table MaxValueTable in share mode"; using (DbCommand cmd = cnctn.CreateCommand()) { cmd.CommandText = sql; // execute command somehow!! } -BR: Matti

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  • net c# lock statement in data access layer

    - by Pedro Rivera
    I saw a code where they have the data access layer like this: public class CustomerDA{ private static readonly object _sync = new object(); private static readonly CustomerDA _mutex = new CustomerDA(); private CustomerDA(){ } public CustomerDA GetInstance(){ lock(_sync){ return _mutex; } } public DataSet GetCustomers(){ //database SELECT //return a DataSet } public int UpdateCustomer(some parameters){ //update some user } } public class CustomerBO{ public DataSet GetCustomers(){ //some bussiness logic return CustomerDA.GetInstance().GetCustomers(); } } I was using it, but start thinking... "and what if had to build a facebook like application where there are hundreds of thousands of concurrent users? would I be blocking each user from doing his things until the previous user ends his database stuff? and for the Update method, is it useful to LOCK THREADS in the app when database engines already manage concurrency at database server level?" Then I started to think about moving the lock to the GetCustomers and UpdateCustomer methods, but think again: "is it useful at all?"

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  • How can i disable drag event using jquery?

    - by Chirag
    Hi, i wan to disable drag event using jquery and want to bind one another event with same mouse event, but it will shows an error i have attached drag even to <div class="mydiv"></div> and i have used jQuery('.mydiv').draggable("destroy"); it is disabling the drag event and attached one more functionality on mouse down but when i will press mouse on div and dragging the cursor then it shows error $(this).data("draggable") is undefined [Break on this error] var t = $('body'), o = $(this).data('draggable').options; and this is in ui.draggable.js, any buddy have any idea about this??

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  • innodb lock wait timeout

    - by shantanuo
    As per the documentation link given below: When a lock wait timeout occurs, the current statement is not executed. The current transaction is not rolled back. (Until MySQL 5.0.13 InnoDB rolled back the entire transaction if a lock wait timeout happened. You can restore this behavior by starting the server with the --innodb_rollback_on_timeout option, available as of MySQL 5.0.32. http://dev.mysql.com/doc/refman/5.0/en/innodb-parameters.html#sysvar_innodb_lock_wait_timeout Does it mean that when a lock wait timeout occurs, it compromises the transactional integrity? "roollback on timeout" was the default behaviour till 5.0.13 and I guess that was the correct way to handle such situations. Does anyone think that this should be the default behaviour and the user should not be asked to add a parameter for a functionality that is taken for granted?

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  • Drag and drop between HTML 5 and Silverlight 4

    - by Tej
    Hi - Is it possible to drag an HTML 5 object, e.g. an , into a Silverlight 4 control and accept it? We've attempted to build a simple prototype using an HTML 5 example and a trivial Silverlight control but the cursor changes to the no-entry sign whenever we hover over the Silverlight control. We do, however, get drag entry events firing in Silverlight. Our control happily accepts files dragged from the desktop as expected. We think we've got the HTML 5 drag events set up correctly, and I can possibly get our test published somewhere in case that will help. We've successfully implemented dragging inside a Silverlight control but we now need to integrate with non-Silverlight page components. Is this actually possible to set up or are we just doing something wrong? Thanks for any advice!

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