title: What's the difference between global queues and user queues?
body: <blockquote>
    <p><strong>Possible Duplicate:</strong><br>
    <a href="https://stackoverflow.com/questions/10984885/what-is-the-difference-between-dispatch-get-global-queue-and-dispatch-queue-crea">What is the difference between dispatch_get_global_queue and dispatch_queue_create?</a>  </p>
  </blockquote>
  
  
  
  <p>I see some methods using gcd,but some of them may do it like this:</p>
  
  <pre><code>dispatch_queue_t queue = dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0);
  dispatch_async(queue, ^{
  
      dispatch_sync(dispatch_get_main_queue(), ^{
  
      });
  });
  </code></pre>
  
  <p>but others may do it like this:</p>
  
  <pre><code>imageQueue_ = dispatch_queue_create("com.company.app.imageQueue", NULL);
  dispatch_async(imageQueue_, ^{
          dispatch_async(dispatch_get_main_queue(), ^{
  
          });
      });
  </code></pre>
  
  <p>What's the differences?If i want to download many images from web,which is better?</p>