This document provides answers to some of the frequently asked questions about TensorFlow. If you have a question that is not covered here, you might find an answer on one of the TensorFlow community resources.
Features and Compatibility
Can I run distributed training on multiple computers?
Yes! TensorFlow gained support for distributed computation in version 0.8. TensorFlow now supports multiple devices (CPUs and GPUs) in one or more computers.
Does TensorFlow work with Python 3?
As of the 0.6.0 release timeframe (Early December 2015), we do support Python 3.3+.
Building a TensorFlow graph
See also the API documentation on building graphs.
c = tf.matmul(a, b) not execute the matrix multiplication immediately?
In the TensorFlow Python API,
tf.Tensor objects. A
Tensor object is
a symbolic handle to the result of an operation, but does not actually hold the
values of the operation's output. Instead, TensorFlow encourages users to build
up complicated expressions (such as entire neural networks and its gradients) as
a dataflow graph. You then offload the computation of the entire dataflow graph
(or a subgraph of it) to a TensorFlow
tf.Session, which is able to execute the
whole computation much more efficiently than executing the operations
How are devices named?
The supported device names are
"/cpu:0") for the CPU
"/gpu:i") for the ith GPU device.
How do I place operations on a particular device?
To place a group of operations on a device, create them within a
with tf.device(name): context. See
the how-to documentation on
using GPUs with TensorFlow for details of how
TensorFlow assigns operations to devices, and the
CIFAR-10 tutorial for an example model that
uses multiple GPUs.
What are the different types of tensors that are available?
TensorFlow supports a variety of different data types and tensor shapes. See the ranks, shapes, and types reference for more details.
Running a TensorFlow computation
See also the API documentation on running graphs.
What's the deal with feeding and placeholders?
Feeding is a mechanism in the TensorFlow Session API that allows you to
substitute different values for one or more tensors at run time. The
tf.Session.run is a
dictionary that maps
tf.Tensor objects to
numpy arrays (and some other types), which will be used as the values of those
tensors in the execution of a step.
Often, you have certain tensors, such as inputs, that will always be fed. The
tf.placeholder op allows you
to define tensors that must be fed, and optionally allows you to constrain
their shape as well. See the
beginners' MNIST tutorial for an
example of how placeholders and feeding can be used to provide the training data
for a neural network.
What is the difference between
# Using `Session.run()`. sess = tf.Session() c = tf.constant(5.0) print(sess.run(c)) # Using `Tensor.eval()`. c = tf.constant(5.0) with tf.Session(): print(c.eval())
In the second example, the session acts as a
which has the effect of installing it as the default session for the lifetime of
with block. The context manager approach can lead to more concise code for
simple use cases (like unit tests); if your code deals with multiple graphs and
sessions, it may be more straightforward to make explicit calls to
Do Sessions have a lifetime? What about intermediate tensors?
Sessions can own resources, such as
tf.ReaderBase; and these resources can use
a significant amount of memory. These resources (and the associated memory) are
released when the session is closed, by calling
The intermediate tensors that are created as part of a call to
Session.run() will be freed at or before the
end of the call.
Does the runtime parallelize parts of graph execution?
The TensorFlow runtime parallelizes graph execution across many different dimensions:
- The individual ops have parallel implementations, using multiple cores in a CPU, or multiple threads in a GPU.
- Independent nodes in a TensorFlow graph can run in parallel on multiple devices, which makes it possible to speed up CIFAR-10 training using multiple GPUs.
- The Session API allows multiple concurrent steps (i.e. calls to
tf.Session.runin parallel. This enables the runtime to get higher throughput, if a single step does not use all of the resources in your computer.
Which client languages are supported in TensorFlow?
TensorFlow is designed to support multiple client languages. Currently, the best-supported client language is Python. Experimental interfaces for executing and constructing graphs are also available for C++, Java and Go.
TensorFlow also has a C-based client API to help build support for more client languages. We invite contributions of new language bindings.
Does TensorFlow make use of all the devices (GPUs and CPUs) available on my machine?
TensorFlow supports multiple GPUs and CPUs. See the how-to documentation on using GPUs with TensorFlow for details of how TensorFlow assigns operations to devices, and the CIFAR-10 tutorial for an example model that uses multiple GPUs.
Note that TensorFlow only uses GPU devices with a compute capability greater than 3.5.
Session.run() hang when using a reader or a queue?
tf.QueueBase classes provide special operations that
can block until input (or free space in a bounded queue) becomes
available. These operations allow you to build sophisticated
input pipelines, at the cost of making the
TensorFlow computation somewhat more complicated. See the how-to documentation
QueueRunner objects to drive queues and readers
for more information on how to use them.
What is the lifetime of a variable?
How do variables behave when they are concurrently accessed?
Variables allow concurrent read and write operations. The value read from a
variable may change if it is concurrently updated. By default, concurrent
assignment operations to a variable are allowed to run with no mutual exclusion.
To acquire a lock when assigning to a variable, pass
See also the
How can I determine the shape of a tensor in Python?
In TensorFlow, a tensor has both a static (inferred) shape and a dynamic (true)
shape. The static shape can be read using the
method: this shape is inferred from the operations that were used to create the
tensor, and may be
partially complete. If the static
shape is not fully defined, the dynamic shape of a
t can be
determined by evaluating
What is the difference between
x = tf.reshape(x)?
tf.Tensor.set_shape method updates
the static shape of a
Tensor object, and it is typically used to provide
additional shape information when this cannot be inferred directly. It does not
change the dynamic shape of the tensor.
tf.reshape operation creates
a new tensor with a different dynamic shape.
How do I build a graph that works with variable batch sizes?
It is often useful to build a graph that works with variable batch sizes, for example so that the same code can be used for (mini-)batch training, and single-instance inference. The resulting graph can be saved as a protocol buffer and imported into another program.
When building a variable-size graph, the most important thing to remember is not
to encode the batch size as a Python constant, but instead to use a symbolic
Tensor to represent it. The following tips may be useful:
batch_size = tf.shape(input)to extract the batch dimension from a
input, and store it in a
tf.reduce_sum(...) / batch_size.
If you use placeholders for feeding input, you can specify a variable batch dimension by creating the placeholder with
tf.placeholder(..., shape=[None, ...]). The
Noneelement of the shape corresponds to a variable-sized dimension.
How can I visualize a TensorFlow graph?
See the graph visualization tutorial.
What is the simplest way to send data to TensorBoard?
Add summary ops to your TensorFlow graph, and write these summaries to a log directory. Then, start TensorBoard using
python tensorflow/tensorboard/tensorboard.py --logdir=path/to/log-directory
For more details, see the Summaries and TensorBoard tutorial.
Every time I launch TensorBoard, I get a network security popup!
You can change TensorBoard to serve on localhost rather than '0.0.0.0' by the flag --host=localhost. This should quiet any security warnings.
See also the how-to documentation for adding a new operation to TensorFlow.
My data is in a custom format. How do I read it using TensorFlow?
There are two main options for dealing with data in a custom format.
The easier option is to write parsing code in Python that transforms the data
into a numpy array, then feed a
tf.placeholder a tensor with
that data. See the documentation on
using placeholders for input for
more details. This approach is easy to get up and running, but the parsing can
be a performance bottleneck.
How do I define an operation that takes a variable number of inputs?
The TensorFlow op registration mechanism allows you to define inputs that are a single tensor, a list of tensors with the same type (for example when adding together a variable-length list of tensors), or a list of tensors with different types (for example when enqueuing a tuple of tensors to a queue). See the how-to documentation for adding an op with a list of inputs or outputs for more details of how to define these different input types.
What is TensorFlow's coding style convention?
The TensorFlow Python API adheres to the
PEP8 conventions.* In
particular, we use
CamelCase names for classes, and
snake_case names for
functions, methods, and properties. We also adhere to the
Google Python style guide.
The TensorFlow C++ code base adheres to the Google C++ style guide.
(* With one exception: we use 2-space indentation instead of 4-space indentation.)