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GCD serial queue does not seem to execute serially

由 只谈情不闲聊 提交于 2019-12-03 07:33:53

Serial Queues ABSOLUTELY will perform serially. They are not guaranteed to perform on the same thread however.

Assuming you are using the same serial queue, the problems is that NSLog is NOT guaranteed to output results in the proper order when called near simultaneously from different threads.

here is an example:

  1. SQ runs on thread X, sends "In processImages"
  2. log prints "In proc"
  3. SQ on thread X, sends "Done with processImages"
  4. SQ runs on thread Y, sends "In processImages"
  5. log prints "essImages\n"

After 5., NSLog doesn't necessarily know which to print, 3. or 4.

If you absolutely need time ordered logging, You need a dedicated queue for logging. In practice, I've had no problems with just using the main queue:

dispatch_async(dispatch_get_main_queue(), ^{
    NSLog(@"whatever");
});

If all NSlog calls are the on the same queue, you shouldn't have this problem.

enumerateGroupsWithTypes:usingBlock:failureBlock: does its work asynchronously on another thread and calls the blocks passed in when it's done (on the main thread I think). Looking at it from another perspective, if it completed all the synchronously by the time the method call was complete, it could just return an enumerator object of the groups instead, for instance, for a simpler API.

From the documentation:

This method is asynchronous. When groups are enumerated, the user may be asked to confirm the application's access to the data; the method, though, returns immediately. You should perform whatever work you want with the assets in enumerationBlock.

I'm not sure why you're trying to accomplish by using the serial queue, but if you just want to prevent simultaneous access, then you could just add a variable somewhere that keeps track of whether we're currently enumerating or not and check that at first, if you don't have to worry about synchronization issues. (If you do, perhaps you should look into using a GCD group, but it's probably overkill for this situation.)

If the question is "Can serial queue perform tasks asynchronously?" then the answer is no. If you think that it can, you should make sure that all tasks are really performing on the same queue. You can add the following line in the block and compare the output:

dispatch_async(self.serialQueue, ^{
    NSLog(@"current queue:%p current thread:%@",dispatch_get_current_queue(),[NSThread currentThread]);

Make sure that you write NSLog in the block that performs on your queue and not in the enumerateGroupsWithTypes:usingBlock:failureBlock: Also you can try to create your queue like this

dispatch_queue_create("label", DISPATCH_QUEUE_SERIAL);

but I don't think that will change anything

EDIT: By the way, method

enumerateGroupsWithTypes:usingBlock:failureBlock:

is asynchronous, why do you call it on another queue?

UPDATE 2: I can suggest something like this:

dispatch_async(queue, ^{
    NSLog(@"queue");

    pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER, *pmutex = &mutex;
    pthread_mutex_lock(pmutex);

    ALAssetsLibraryGroupsEnumerationResultsBlock listGroupBlock = ^(ALAssetsGroup *group, BOOL *stop) {
        NSLog(@"block");
        if (group) {
            [groups addObject:group];
        } else {

            [self.tableView performSelectorOnMainThread:@selector(reloadData) withObject:nil waitUntilDone:NO];
            dispatch_async(dispatch_get_current_queue(), ^{
                pthread_mutex_unlock(pmutex);
            });
        }
        NSLog(@"block end");
    };

    [assetsLibrary enumerateGroupsWithTypes:groupTypes usingBlock:listGroupBlock failureBlock:failureBlock];
    pthread_mutex_lock(pmutex);
    pthread_mutex_unlock(pmutex);
    pthread_mutex_destroy(pmutex);
    NSLog(@"queue end");
});

I hit an issue like this, and the answer for me was to realize that asynchronous calls from a method on the serialized queue goes to another queue for processing -- one that is not serialized.

So you have to wrap all the calls inside the main method with explicit dispatch_async(serializedQueue, ^{}) to ensure that everything is done in the correct order...

Using Swift and semaphores to illustrate an approach to serialization:

Given: a class with an asynchronous ‘run’ method that will be run on multiple objects at once, and the objective is that each not run until the one before it completes.

The issue is that the run method allocates a lot of memory and uses a lot of system resources that can cause memory pressure among other issues if too many are run at once.

So the idea is: if a serial queue is used then only one will run at a time, one after the other.

Create a serial queue in the global space by the class:

let serialGeneratorQueue: DispatchQueue = DispatchQueue(label: "com.limit-point.serialGeneratorQueue", autoreleaseFrequency: DispatchQueue.AutoreleaseFrequency.workItem)

class Generator {

    func run() {
         asynchronous_method()
    }

    func start() {

        serialGeneratorQueue.async {
            self.run()
        }
    }

    func completed() {
       // to be called by the asynchronous_method() when done
    }
}

The ‘run’ method of this class for which very many objects will be created and run will be processed on the serial queue:

serialGeneratorQueue.async {
    self.run()
}

In this case an autoreleaseFrequency is .workItem to clean up memory after each run.

The run method is of some general form:

func run() {
   asynchronous_method()
}

The problem with this: the run method exits before the asynchronous_method completes, and the next run method in the queue will run, etc. So the objective is not being achieved because each asynchronous_method is running in parallel, not serially after all.

Use a semaphore to fix. In the class declare

let running = DispatchSemaphore(value: 0)

Now the asynchronous_method completes it calls the ‘completed’ method:

func completed() {
   // some cleanup work etc.
}

The semaphore can be used to serialized the chain of asynchronous_method’s by add ‘running.wait()’ to the ‘run’ method:

func run() {
    asynchronous_method()

    running.wait() 
}

And then in the completed() method add ‘running.signal()’

func completed() {
   // some cleanup work etc.

    running.signal()
}

The running.wait() in ‘run’ will prevent it from exiting until signaled by the completed method using running.signal(), which in turn prevents the serial queue from starting the next run method in the queue. This way the chain of asynchronous methods will indeed be run serially.

So now the class is of the form:

class Generator {

    let running = DispatchSemaphore(value: 0)

    func run() {
         asynchronous_method()

         running.wait() 
    }

    func start() {

        serialGeneratorQueue.async {
            self.run()
        }
    }

    func completed() {
       // to be called by the asynchronous_method() when done

       running.signal()
    }
}

I thought a serial queue would wait [until] the first block is done ...

It does. But your first block simply calls enumerateGroupsWithTypes and the documentation warns us that the method runs asynchronously:

This method is asynchronous. When groups are enumerated, the user may be asked to confirm the application's access to the data; the method, though, returns immediately.

(FWIW, whenever you see a method that has a block/closure parameter, that’s a red flag that the method is likely performing something asynchronously. You can always refer to the relevant method’s documentation and confirm, like we have here.)

So, bottom line, your queue is serial, but it is only sequentially launching a series of asynchronous tasks, but obviously not waiting for those asynchronous tasks to finish, defeating the intent of the serial queue.

So, if you really need to have each tasks wait for the prior asynchronous task, there are a number of traditional solutions to this problem:

  1. Use recursive pattern. I.e., write a rendition of processImage that takes an array of images to process and:

    • check to see if there are any images to process;
    • process first image; and
    • when done (i.e. in the completion handler block), remove the first image from the array and then call processImage again.

  2. Rather than dispatch queues, consider using operation queues. Then you can implement your task as an “asynchronous” NSOperation subclass. This is a very elegant way of wrapping an asynchronous task This is illustrated in https://stackoverflow.com/a/21205992/1271826.

  3. You can use semaphores to make this asynchronous task behave synchronously. This is also illustrated in https://stackoverflow.com/a/21205992/1271826.

Option 1 is the simplest, option 2 is the most elegant, and option 3 is a fragile solution that should be avoided if you can.

You might have more than one object, each with its own serial queue. Tasks dispatched to any single serial queue are performed serially, but tasks dispatched to different serial queues will absolutely be interleaved.

Another simple bug would be to create not a serial queue, but a concurrent queue...

标签
ios
queue
grand-central-dispatch
serial-processing
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