tf.train.MonitoredSession

Class MonitoredSession

Defined in tensorflow/python/training/monitored_session.py.

See the guides: Threading and Queues > Queue usage overview, Training > Distributed execution

Session-like object that handles initialization, recovery and hooks.

Example usage:

saver_hook = CheckpointSaverHook(...)
summary_hook = SummarySaverHook(...)
with MonitoredSession(session_creator=ChiefSessionCreator(...),
                      hooks=[saver_hook, summary_hook]) as sess:
  while not sess.should_stop():
    sess.run(train_op)

Initialization: At creation time the monitored session does following things in given order:

  • calls hook.begin() for each given hook
  • finalizes the graph via scaffold.finalize()
  • create session
  • initializes the model via initialization ops provided by Scaffold
  • restores variables if a checkpoint exists
  • launches queue runners
  • calls hook.after_create_session()

Run: When run() is called, the monitored session does following things:

  • calls hook.before_run()
  • calls TensorFlow session.run() with merged fetches and feed_dict
  • calls hook.after_run()
  • returns result of session.run() asked by user
  • if AbortedError or UnavailableError occurs, it recovers or reinitializes the session before executing the run() call again

Exit: At the close(), the monitored session does following things in order:

  • calls hook.end()
  • closes the queue runners and the session
  • suppresses OutOfRange error which indicates that all inputs have been processed if the monitored_session is used as a context

How to set tf.Session arguments:

  • In most cases you can set session arguments as follows:
MonitoredSession(
  session_creator=ChiefSessionCreator(master=..., config=...))
  • In distributed setting for a non-chief worker, you can use following:
MonitoredSession(
  session_creator=WorkerSessionCreator(master=..., config=...))

See MonitoredTrainingSession for an example usage based on chief or worker.

  • it cannot be set as default session.
  • it cannot be sent to saver.save.
  • it cannot be sent to tf.train.start_queue_runners.

Args:

  • session_creator: A factory object to create session. Typically a ChiefSessionCreator which is the default one.
  • hooks: An iterable of `SessionRunHook' objects.

Returns:

A MonitoredSession object.

Child Classes

class StepContext

Properties

graph

The graph that was launched in this session.

Methods

__init__

__init__(
    session_creator=None,
    hooks=None,
    stop_grace_period_secs=120
)

__enter__

__enter__()

__exit__

__exit__(
    exception_type,
    exception_value,
    traceback
)

close

close()

run

run(
    fetches,
    feed_dict=None,
    options=None,
    run_metadata=None
)

Run ops in the monitored session.

This method is completely compatible with the tf.Session.run() method.

Args:

  • fetches: Same as tf.Session.run().
  • feed_dict: Same as tf.Session.run().
  • options: Same as tf.Session.run().
  • run_metadata: Same as tf.Session.run().

Returns:

Same as tf.Session.run().

run_step_fn

run_step_fn(step_fn)

Run ops using a step function.

Args:

  • step_fn: A function or a method with a single argument of type StepContext. The function may use methods of the argument to perform computations with access to a raw session.

    The returned value of the step_fn will be returned from run_step_fn, unless a stop is requested. In that case, the next should_stop call will return True.

    Example usage:

    ```python with tf.Graph().as_default(): c = tf.placeholder(dtypes.float32) v = tf.add(c, 4.0) w = tf.add(c, 0.5)

     def step_fn(step_context):
       a = step_context.session.run(fetches=v, feed_dict={c: 0.5})
       if a <= 4.5:
         step_context.request_stop()
       return step_context.run_with_hooks(fetches=w, feed_dict={c: 0.1})
    
     with tf.MonitoredSession() as session:
       while not session.should_stop():
         a = session.run_step_fn(step_fn)
    

    ```

    Hooks interact with the run_with_hooks() call inside the step_fn as they do with a MonitoredSession.run call.

Returns:

Returns the returned value of step_fn.

Raises:

  • StopIteration: if step_fn has called request_stop(). It may be caught by with tf.MonitoredSession() to close the session.
  • ValueError: if step_fn doesn't have a single argument called step_context. It may also optionally have self for cases when it belongs to an object.

should_stop

should_stop()