I have an NSManagedObjectContext declared like so:
- (NSManagedObjectContext *) backgroundMOC {
if (backgroundMOC != nil) {
return backgroundMOC;
}
backgroundMOC = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
return backgroundMOC;
}
Notice that it is declared with a private queue concurrency type, so its tasks should be run on a background thread. I have the following code:
-(void)testThreading
{
/* ok */
[self.backgroundMOC performBlock:^{
assert(![NSThread isMainThread]);
}];
/* CRASH */
[self.backgroundMOC performBlockAndWait:^{
assert(![NSThread isMainThread]);
}];
}
Why does calling performBlockAndWait execute the task on the main thread rather than background thread?
Tossing in another answer, to try an explain why performBlockAndWait will always run in the calling thread.
performBlock is completely asynchronous. It will always enqueue the block onto the queue of the receiving MOC, and then return immediately. Thus,
[moc performBlock:^{
// Foo
}];
[moc performBlock:^{
// Bar
}];
will place two blocks on the queue for moc. They will always execute asynchronously. Some unknown thread will pull blocks off of the queue and execute them. In addition, those blocks are wrapped within their own autorelease pool, and also they will represent a complete Core Data user event (processPendingChanges).
performBlockAndWait does NOT use the internal queue. It is a synchronous operation that executes in the context of the calling thread. Of course, it will wait until the current operations on the queue have been executed, and then that block will execute in the calling thread. This is documented (and reasserted in several WWDC presentations).
Furthermore, performBockAndWait is re-entrant, so nested calls all happen right in that calling thread.
The Core Data engineers have been very clear that the actual thread in which a queue-based MOC operation runs is not important. It's the synchronization by using the performBlock* API that's key.
So, consider 'performBlock' as "This block is being placed on a queue, to be executed at some undetermined time, in some undetermined thread. The function will return to the caller as soon as it has been enqueued"
performBlockAndWait is "This block will be executed at some undetermined time, in this exact same thread. The function will return after this code has completely executed (which will occur after the current queue associated with this MOC has drained)."
EDIT
Are you sure of "performBlockAndWait does NOT use the internal queue"?
I think it does. The only difference is that performBlockAndWait will
wait until the block's completion. And what do you mean by calling
thread? In my understanding, [moc performBlockAndWait] and [moc
performBloc] both run on its private queue (background or main). The
important concept here is moc owns the queue, not the other way
around. Please correct me if I am wrong. – Philip007
It is unfortunate that I phrased the answer as I did, because, taken by itself, it is incorrect. However, in the context of the original question it is correct. Specifically, when calling performBlockAndWait on a private queue, the block will execute on the thread that called the function - it will not be put on the queue and executed on the "private thread."
Now, before I even get into the details, I want to stress that depending on internal workings of libraries is very dangerous. All you should really care about is that you can never expect a specific thread to execute a block, except anything tied to the main thread. Thus, expecting a performBlockAndWait to not execute on the main thread is not advised because it will execute on the thread that called it.
performBlockAndWait uses GCD, but it also has its own layer (e.g., to prevent deadlocks). If you look at the GCD code (which is open source), you can see how synchronous calls work - and in general they synchronize with the queue and invoke the block on the thread that called the function - unless the queue is the main queue or a global queue. Also, in the WWDC talks, the Core Data engineers stress the point that performBlockAndWait will run in the calling thread.
So, when I say it does not use the internal queue, that does not mean it does not use the data structures at all. It must synchronize the call with the blocks already on the queue, and those submitted in other threads and other asynchronous calls. However, when calling performBlockAndWait it does not put the block on the queue... instead it synchronizes access and runs the submitted block on the thread that called the function.
Now, SO is not a good forum for this, because it's a bit more complex than that, especially w.r.t the main queue, and GCD global queues - but the latter is not important for Core Data.
The main point is that when you call any performBlock* or GCD function, you should not expect it to run on any particular thread (except something tied to the main thread) because queues are not threads, and only the main queue will run blocks on a specific thread.
When calling the core data performBlockAndWait the block will execute in the calling thread (but will be appropriately synchronized with everything submitted to the queue).
I hope that makes sense, though it probably just caused more confusion.
EDIT
Furthermore, you can see the unspoken implications of this, in that the way in which performBlockAndWait provides re-entrant support breaks the FIFO ordering of blocks. As an example...
[context performBlockAndWait:^{
NSLog(#"One");
[context performBlock:^{
NSLog(#"Two");
}];
[context performBlockAndWait:^{
NSLog(#"Three");
}];
}];
Note that strict adherence to the FIFO guarantee of the queue would mean that the nested performBlockAndWait ("Three") would run after the asynchronous block ("Two") since it was submitted after the async block was submitted. However, that is not what happens, as it would be impossible... for the same reason a deadlock ensues with nested dispatch_sync calls. Just something to be aware of if using the synchronous version.
In general, avoid sync versions whenever possible because dispatch_sync can cause a deadlock, and any re-entrant version, like performBlockAndWait will have to make some "bad" decision to support it... like having sync versions "jump" the queue.
Why not? Grand Central Dispatch's block concurrency paradigm (which I assume MOC uses internally) is designed so that only the runtime and operating system need to worry about threads, not the developer (because the OS can do it better than you can do to having more detailed information). Too many people assume that queues are the same as threads. They are not.
Queued blocks are not required to run on any given thread (the exception being blocks in the main queue must execute on the main thread). So, in fact, sometimes sync (i.e. performBlockAndWait) queued blocks will run on the main thread if the runtime feels it would be more efficient than creating a thread for it. Since you are waiting for the result anyway, it wouldn't change the way your program functioned if the main thread were to hang for the duration of the operation.
This last part I am not sure if I remember correctly, but in the WWDC 2011 videos about GCD, I believe that it was mentioned that the runtime will make an effort to run on the main thread, if possible, for sync operations because it is more efficient. In the end though, I suppose the answer to "why" can only be answered by the people who designed the system.
I don't think that the MOC is obligated to use a background thread; it's just obligated to ensure that your code will not run into concurrency issues with the MOC if you use performBlock: or performBlockAndWait:. Since performBlockAndWait: is supposed to block the current thread, it seems reasonable to run that block on that thread.
The performBlockAndWait: call only makes sure that you execute the code in such a way that you don't introduce concurrency (i.e. on 2 threads performBlockAndWait: will not run at the same time, they will block each other).
The long and the short of it is that you can't depend on which thread a MOC operation runs on, well basically ever. I've learned the hard way that if you use GCD or just straight up threads, you always have to create local MOCs for each operation and then merge them to the master MOC.
There is a great library (MagicalRecord) that makes that process very simple.
Related
I am using the following code in a loop:
-(void)getUpdatedComments
{
if(checkComments)
{
objParseOperation=[[ParseOperation alloc] initWithUDID:[[NSUserDefaults standardUserDefaults] valueForKey:kDeviceUDID]:self];
[operationQueue addOperation:objParseOperation3];
}
}
where operationQueue is an object of type NSOperationQueue. I am calling this method every few seconds.
If I call this method after the first time, do I need to cancel the previous operation or just do [objParseOperation release]?
If you are going to use the same queue for adding more new operations then there isn't much use in releasing it and creating a new one. Simply canceling all operations and then adding more operations is enough. You can do so by calling cancelAllOperations on the queue. Note that operations that are already running will continue to run unless they check for the cancellation.
This method sends a cancel message to all operations currently in the queue. Queued operations are cancelled before they begin executing. If an operation is already executing, it is up to that operation to recognize the cancellation and stop what it is doing.
Wether or not releasing the queue will immediately release the enqueued operations or not is not documented and hence should be considered as undefined behavior. Therefor you shouldn't assume that it will work either way and you should not rely on it (see dangerous to autorelease NSOperationQueue).
There are however evidence that suggests that operations will retain their queue such as that GCD queues are retained by the system while they have asynchronous blocks running/pending. You can read in the Grand Central Dispatch documentation that "the queue is retaind by the system until the block has run to completion". Still, the documentation doesn't specify the behavior for NSOperationQueues like mentioned above.
dispatch_async
Submits a block for asynchronous execution on a dispatch queue and returns immediately.
void dispatch_async(
dispatch_queue_t queue,
dispatch_block_t block);
Parameters
queue
The queue on which to submit the block. The queue is retained by the system until the block has run to completion. This parameter cannot be NULL.
block
The block to submit to the target dispatch queue. This function performs Block_copy and Block_release on behalf of callers. This parameter cannot be NULL.
Discussion
This function is the fundamental mechanism for submitting blocks to a dispatch queue. Calls to this function always return immediately after the block has been submitted and never wait for the block to be invoked. The target queue determines whether the block is invoked serially or concurrently with respect to other blocks submitted to that same queue. Independent serial queues are processed concurrently with respect to each other.
This question already has answers here:
Difference between DispatchQueue.main.async and DispatchQueue.main.sync
(4 answers)
Closed 3 years ago.
I'm reading the docs on dispatch queues for GCD, and in it they say that the queues are FIFO, so I am woundering what effect this has on async / sync dispatches?
from my understand async executes things in the order that it gets things while sync executes things serial..
but when you write your GCD code you decide the order in which things happen.. so as long as your know whats going on in your code you should know the order in which things execute..
my questions are, wheres the benefit of async here? am I missing something in my understanding of these two things.
The first answer isn't quite complete, unfortunately. Yes, sync will block and async will not, however there are additional semantics to take into account. Calling dispatch_sync() will also cause your code to wait until each and every pending item on that queue has finished executing, also making it a synchronization point for said work. dispatch_async() will simply submit the work to the queue and return immediately, after which it will be executed "at some point" and you need to track completion of that work in some other way (usually by nesting one dispatch_async inside another dispatch_async - see the man page for example).
sync means the function WILL BLOCK the current thread until it has completed, async means it will be handled in the background and the function WILL NOT BLOCK the current thread.
If you want serial execution of blocks check out the creation of a serial dispatch queue
From the man page:
FUNDAMENTALS
Conceptually, dispatch_sync() is a convenient wrapper around dispatch_async() with the addition of a semaphore to wait for completion of the block, and a wrapper around the block to signal its completion.
See dispatch_semaphore_create(3) for more information about dispatch semaphores. The actual implementation of the dispatch_sync() function may be optimized and differ from the above description.
Tasks can be performed synchronously or asynchronously.
Synchronous function returns the control on the current queue only after task is finished. It blocks the queue and waits until the task is finished.
Asynchronous function returns control on the current queue right after task has been sent to be performed on the different queue. It doesn't wait until the task is finished. It doesn't block the queue.
Only in Asynchronous we can add delay -> asyncAfter(deadline: 10..
My program is something like this.. I wanted to know will I make my program slow, if I call pthread join???
void* a(void *a)
{
do---something();//which is a very long procedure, I mean takes a lot of time...
pthread_exit();
}
main()
{
while(1)
{
pthread_create(a);
pthread_join(a);
}
}
So, if I call pthread_join, will I halt at that point until the child thread finishes it execution, or do I go on and create one more thread ?????
From the POSIX spec:
The pthread_join() function shall suspend execution of the calling
thread until the target thread terminates, unless the target thread
has already terminated.
If this is not what you want, either defer the call to pthread_join (putting all of the pthread_t's in a container so you can join them later) or use pthread_detach so you do not need to join them at all.
pthread_join() blocks until the child thread has exited, but you still want/need to call it in order for the child thread to be cleaned up properly once it has gone away. But as you say you don't want to have your main thread blocked, so how to deal with this? The way I would do it is to have the child thread send a message (via socket-pair or some other mechanism) back to its parent thread just before it exits.... when the parent thread receives this message, then it knows that now is a good time to call pthread_join(), since the child thread is already gone (or almost gone) and therefore pthread_join will never block (or at least not for more than a few milliseconds).
the join call blocks until the thread exits
You probably want to use non-blocking sockets to handle many clients without having to create a thread for each. See often quoted The C10K problem for more details.
I read that GCD synchronous queues (dispatch_sync) should be used to implement critical sections of code. An example would be a block that subtracts transaction amount from account balance. The interesting part of sync calls is a question, how does that affect the work of other blocks on multiple threads?
Lets imagine the situation where there are 3 threads that use and execute both system and user defined blocks from main and custom queues in asynchronous mode. Those block are all executed in parallel in some order. Now, if a block is put on a custom queue with sync mode, does that mean that all other blocks (including on other threads) are suspended until the successful execution of the block? Or does that mean that only some lock will be put on that block while other will still execute. However, if other blocks use the same data as the sync block then it's inevitable that other blocks will wait until that lock will be released.
IMHO it doesn't matter, is it one or multiple cores, sync mode should freeze the whole app work. However, these are just my thoughts so please comment on that and share your insights :)
Synchronous dispatch suspends the execution of your code until the dispatched block has finished. Asynchronous dispatch returns immediately, the block is executed asynchronously with regard to the calling code:
dispatch_sync(somewhere, ^{ something });
// Reached later, when the block is finished.
dispatch_async(somewhere, ^{ something });
// Reached immediately. The block might be waiting
// to be executed, executing or already finished.
And there are two kinds of dispatch queues, serial and concurrent. The serial ones dispatch the blocks strictly one by one in the order they are being added. When one finishes, another one starts. There is only one thread needed for this kind of execution. The concurrent queues dispatch the blocks concurrently, in parallel. There are more threads being used there.
You can mix and match sync/async dispatch and serial/concurrent queues as you see fit. If you want to use GCD to guard access to a critical section, use a single serial queue and dispatch all operations on the shared data on this queue (synchronously or asynchronously, does not matter). That way there will always be just one block operating with the shared data:
- (void) addFoo: (id) foo {
dispatch_sync(guardingQueue, ^{ [sharedFooArray addObject:foo]; });
}
- (void) removeFoo: (id) foo {
dispatch_sync(guardingQueue, ^{ [sharedFooArray removeObject:foo]; });
}
Now if guardingQueue is a serial queue, the add/remove operations can never clash even if the addFoo: and removeFoo: methods are called concurrently from different threads.
No it doesn't.
The synchronised part is that the block is put on a queue but control does not pass back to the calling function until the block returns.
Many uses of GCD are asynchronous; you put a block on a queue and rather than waiting for the block to complete it's work control is passed back to the calling function.
This has no effect on other queues.
If you need to serialize the access to a certain resource then there are at least two
mechanisms that are accessible to you. If you have an account object (that is unique
for a given account number), then you can do something like:
#synchronize(accountObject) { ... }
If you don't have an object but are using a C structure for which there is only one
such structure for a given account number then you can do the following:
// Should be added to the account structure.
// 1 => at most 1 object can access accountLock at a time.
dispatch_semaphore_t accountLock = dispatch_semaphore_create(1);
// In your block you do the following:
block = ^(void) {
dispatch_semaphore_wait(accountLock,DISPATCH_TIME_FOREVER);
// Do something
dispatch_semaphore_signal(accountLock);
};
// -- Edited: semaphore was leaking.
// At the appropriate time release the lock
// If the semaphore was created in the init then
// the semaphore should be released in the release method.
dispatch_release(accountLock);
With this, regardless of the level of concurrency of your queues, you are guaranteed that only one thread will access an account at any given time.
There are many more types of synchronization objects but these two are easy to use and
quite flexible.
I'm writing a concurrency application for the iPhone.
I wonder if this code:
while(!conditionBoolean)
{
// do nothing
// until another thread makes this variable true.
}
makeWork();
Is equivalent to the following:
[lock lock]; // this lock is locked by another thread
// causing the current to block until it's unlocked
[lock unlock];
makeWork();
If it's not, what's the difference?
Thank you.
You should prefer the second, the first will produce a tight loop and delay or maybe even prevent the variable being set in the way you want/expect. At the very least you would have to introduce a delay in that loop, a sleep of some kind.
Better still would be to wait on a signalling primitive for the work to complete, which then gets signalled by the other thread - the design is then deterministic, versus depending on a mutex or state variable that some other thread might lock or modify before you get your chance. In general, it's better for a multi-threaded design to be event-driven (push model), not check shared state opportunistically (pull model).
My understanding of mutexes is that the lock can occur in less cycles, so for example it's possible that while you read the conditionboolean to become true, it's possible that another thread could still change it to true while you're reading it, and another goes to false before you read it again. This turns into a race condition, which the mutex locking would hope to avoid. Also this could cause your code not to be the "next in line" if you have numerous functions with a similar while loop.