Limiting number of threads

Question

I need to download images through background threads, but limit the number of threads. Maximum thread number must be 5 and in each thread must be just one serial queue. For client-server using socket rocket library. The main trouble is that i don't need NSOperation pluses like canceling operations. Looking for a simple decision, but can found just something like this:

self.limitingSema = dispatch_semaphore_create(kOperationLimit);
    dispatch_queue_t concurentQueue = dispatch_queue_create("limiting queue", DISPATCH_QUEUE_CONCURRENT);
    dispatch_async(concurentQueue, ^{
        dispatch_semaphore_wait(self.limitingSema, DISPATCH_TIME_FOREVER);

        /* upload image here */

        dispatch_semaphore_signal(self.limitingSema);
    });

But then how to limit number of threads and to wait new operations starting, until they are not ready in queue ?

Is it good to control number of queues ?

NSArray *queues = @[dispatch_queue_create("com.YOU.binaryQueue_1", DISPATCH_QUEUE_SERIAL),
                    dispatch_queue_create("com.YOU.binaryQueue_2", DISPATCH_QUEUE_SERIAL),
                    dispatch_queue_create("com.YOU.binaryQueue_3", DISPATCH_QUEUE_SERIAL)
                    ];

NSUInteger randQueue = arc4random() % [queues count];
dispatch_async([queues objectAtIndex:randQueue], ^{
    NSLog(@"Do something");
});
randQueue = arc4random() % [queues count];
dispatch_async([queues objectAtIndex:randQueue], ^{
    NSLog(@"Do something else");
});

Answer 1

GCD has no option to limit the amount of concurrent blocks running.

This will potentially create one thread that just waits for each operation you enqueue. GCD dynamically adjusts the number of threads it uses. If you enqueue another block and GCD has no more threads available it will spin up another thread if it notices there are free CPU cores available. Since the worker thread is sleeping inside your block the CPU is considered free. This will cause many threads using up a lot of memory - each thread gets 512 KB of Stack.

Your best option would be to use NSOperationQueue for this as you can control directly how many operations will be run in parallel using the maxConcurrentOperationCount property. This will be easier (less code for you to write, test and debug) and much more efficient.