Why use async HTTP request over sync HTTP in a separate thread?

Question

I know about the difference between how each works but i want to know in a performance wise point of view (resources inside the iphone).

Lets say I send an asynch request and wait for the delegate to be called. This won't lock my execution thread. But what is the difference of doing this against just sending a synch request in another thread with GCD.

Like this:

dispatch_queue_t findPicsQueue;

findPicsQueue = dispatch_queue_create("FindPicsQueue", NULL);


dispatch_async(findPicsQueue, ^{

    NSData *theResponse = [NSURLConnection sendSynchronousRequest:theRequest
                                                        returningResponse:&response
                                                                    error:&error];

    NSHTTPURLResponse *httpResponse = (NSHTTPURLResponse*)response;

    if (error) {
       NSLog(@"Error: %@",error)
    }

    if (httpResponse.statusCode == 200) 
    {
        [self parseXMLFile:theResponse]; // Parses Data and modifies picturesFound

        for (PictureData *tmp in picturesFound) {
            NSLog(@"%@",tmp);
        }          
    }  
}

It wont lock my interface since its not being executed in the main thread, but it will lock this specific thread. And I also think GCD runs queues concurrently.

Thanks in advance. I really want to clarify this question.

Answer 1

If you use NSURLConnection with sendAsynchronousRequest, then almost all processing takes place on the main thread, in particular, the XML parsing will be done on the main thread. Your code example however uses a different thread for processing.

This difference is relevant if you have an iPhone or iPad processor with two cores. Then the XML parsing can run in parallel with some UI activity on the main thread (in your example). So it can be completed earlier compared to running everything on the main thread (sendAsynchronousRequest approach).

For older devices with just one core, only one thread will run at a time and the two approaches should behave almost identical.