title: tensorflow, how does it work in parallel
body: <p>I'm studying machine learning and tensorflow.
  but i have a question
  I saw documents that Tensorflow by default uses multiple threads.
  So I tried to check this out by using log.</p>
  
  <pre><code>with tf.Session(config=tf.ConfigProto(log_device_placement=True)) as sess:
   x = tf.constant(2)
   y2 = x - 66
   y1 = x + 300
   y = y1 + y2
   result = sess.run(y)
   print(result)
  </code></pre>
  
  <p>then I can get result below.</p>
  
  <blockquote>
    <p>Const: (Const): /job:localhost/replica:0/task:0/cpu:0
    I tensorflow/core/common_runtime/simple_placer.cc:827] Const: (Const)/job:localhost/replica:0/task:0/cpu:0
    add: (Add): /job:localhost/replica:0/task:0/cpu:0
    I tensorflow/core/common_runtime/simple_placer.cc:827] add: (Add)/job:localhost/replica:0/task:0/cpu:0
    sub: (Sub): /job:localhost/replica:0/task:0/cpu:0
    I tensorflow/core/common_runtime/simple_placer.cc:827] sub: (Sub)/job:localhost/replica:0/task:0/cpu:0
    add_1: (Add): /job:localhost/replica:0/task:0/cpu:0
    I tensorflow/core/common_runtime/simple_placer.cc:827] add_1: (Add)/job:localhost/replica:0/task:0/cpu:0
    add/y: (Const): /job:localhost/replica:0/task:0/cpu:0
    I tensorflow/core/common_runtime/simple_placer.cc:827] add/y: (Const)/job:localhost/replica:0/task:0/cpu:0
    sub/y: (Const): /job:localhost/replica:0/task:0/cpu:0
    I tensorflow/core/common_runtime/simple_placer.cc:827] sub/y: (Const)/job:localhost/replica:0/task:0/cpu:0
    238</p>
  </blockquote>
  
  <p>It seems like work in parallel and synchronous, is that right?
  Is it default?</p>