title: what is Device interconnect StreamExecutor with strength 1 edge matrix
body: <p>I have four NVIDIA GTX 1080 graphic cards and when I'm initializing a session I see following console output:</p>
  
  <pre><code>Adding visible gpu devices: 0, 1, 2, 3
   Device interconnect StreamExecutor with strength 1 edge matrix:
        0 1 2 3 
   0:   N Y N N 
   1:   Y N N N 
   2:   N N N Y 
   3:   N N Y N 
  </code></pre>
  
  <p>And as well I have 2 NVIDIA M60 Tesla graphic cards and the initialization looks like:</p>
  
  <pre><code>Adding visible gpu devices: 0, 1, 2, 3
   Device interconnect StreamExecutor with strength 1 edge matrix:
        0 1 2 3 
   0:   N N N N 
   1:   N N N N 
   2:   N N N N 
   3:   N N N N 
  </code></pre>
  
  <p>And I noticed this output was changed for me since last  update from 1.6 to 1.8 for 1080 gpu. It looked something like this (cannot remember precisely, just memories): </p>
  
  <pre><code> Adding visible gpu devices: 0, 1, 2, 3
  Device interconnect StreamExecutor with strength 1 edge matrix:
       0 1 2 3            0 1 2 3
  0:   Y N N N         0: N N Y N
  1:   N Y N N    or   1: N N N Y
  2:   N N Y N         2: Y N N N
  3:   N N N Y         3: N Y N N
  </code></pre>
  
  <p>My questions are:</p>
  
  <ul>
  <li>what is this <em>Device interconnect</em>? </li>
  <li>what influence it has on computation power? </li>
  <li>why it differ for different GPUs?</li>
  <li>can it change over time due to hardware reasons (failures, drivers inconsistency...)?</li>
  </ul>