title: When creating NSMutableData, memory usage doesn't change
body: <p>When I create an 8MB <code>NSMutableData</code> object my memory usage does not seem to change. Is it normal?</p>
  
  <p>My code is as following:</p>
  
  <pre><code>NSMutableData  *myData;
  myData = [[NSMutableData alloc] initWithLength:(8*1024*1024)]; // init with 8 MB
  </code></pre>
  
  <p>When I debug step by step, the memory usage stays at 5.8 MB, even after the object has been allocated... It should go up to 13.8, shouldn't it? I am using ARC (but with MRR it is the same...).</p>
  
  <p>If I try to access the data, memory usage remains the same:</p>
  
  <pre><code>uint8_t *fBuffer;
  fBuffer = (uint8_t *)malloc(8*1024*1024);
  [myData getBytes:fBuffer length:8*1024*1024];
  </code></pre>
  
  <p>In fact, if I modify the data, then the memory usage goes up as it should!</p>
  
  <pre><code>for (int i= 0 ; i&lt;8000000 ; i++) {
      fBuffer[i] = 0x32;
  }
  </code></pre>
  
  <p>=> memory usage is now 13.4 MB.</p>
  
  <p>Thank you all for your help, I now understand how it is supposed to work.</p>