# Using GPUs

## Supported devices

On a typical system, there are multiple computing devices. In TensorFlow, the
supported device types are `CPU` and `GPU`.  They are represented as
`strings`. For example:

*  `"/cpu:0"`: The CPU of your machine.
*  `"/gpu:0"`: The GPU of your machine, if you have one.
*  `"/gpu:1"`: The second GPU of your machine, etc.

If a TensorFlow operation has both CPU and GPU implementations, the
GPU devices will be given priority when the operation is assigned to
a device. For example, `matmul` has both CPU and GPU kernels.  On a
system with devices `cpu:0` and `gpu:0`, `gpu:0` will be selected to run
`matmul`.

## Logging Device placement

To find out which devices your operations and tensors are assigned to, create
the session with `log_device_placement` configuration option set to `True`.

```python
# Creates a graph.
a = tf.constant([1.0, 2.0, 3.0, 4.0, 5.0, 6.0], shape=[2, 3], name='a')
b = tf.constant([1.0, 2.0, 3.0, 4.0, 5.0, 6.0], shape=[3, 2], name='b')
c = tf.matmul(a, b)
# Creates a session with log_device_placement set to True.
sess = tf.Session(config=tf.ConfigProto(log_device_placement=True))
# Runs the op.
print sess.run(c)
```

You should see the following output:

```
Device mapping:
/job:localhost/replica:0/task:0/gpu:0 -> device: 0, name: Tesla K40c, pci bus
id: 0000:05:00.0
b: /job:localhost/replica:0/task:0/gpu:0
a: /job:localhost/replica:0/task:0/gpu:0
MatMul: /job:localhost/replica:0/task:0/gpu:0
[[ 22.  28.]
 [ 49.  64.]]

```

## Manual device placement

If you would like a particular operation to run on a device of your
choice instead of what's automatically selected for you, you can use
`with tf.device` to create a device context such that all the operations
within that context will have the same device assignment.

```python
# Creates a graph.
with tf.device('/cpu:0'):
  a = tf.constant([1.0, 2.0, 3.0, 4.0, 5.0, 6.0], shape=[2, 3], name='a')
  b = tf.constant([1.0, 2.0, 3.0, 4.0, 5.0, 6.0], shape=[3, 2], name='b')
  c = tf.matmul(a, b)
# Creates a session with log_device_placement set to True.
sess = tf.Session(config=tf.ConfigProto(log_device_placement=True))
# Runs the op.
print sess.run(c)
```

You will see that now `a` and `b` are assigned to `cpu:0`.

```
Device mapping:
/job:localhost/replica:0/task:0/gpu:0 -> device: 0, name: Tesla K40c, pci bus
id: 0000:05:00.0
b: /job:localhost/replica:0/task:0/cpu:0
a: /job:localhost/replica:0/task:0/cpu:0
MatMul: /job:localhost/replica:0/task:0/gpu:0
[[ 22.  28.]
 [ 49.  64.]]
```

## Allowing GPU memory growth

By default, TensorFlow maps nearly all of the GPU memory of all GPUs visible to
the process.  This is done to more efficiently use the relatively precious GPU
memory resources on the devices by reducing
[memory fragmentation](https://en.wikipedia.org/wiki/Fragmentation_(computing)).

In some cases it is desirable for the process to only allocate a subset of the
available memory, or to only grow the memory usage as is needed by the process.
TensorFlow provides two Config options on the Session to control this.

The first is the `allow_growth` option, which attempts to allocate only as much
GPU memory based on runtime allocations: it starts out allocating very little
memory, and as Sessions get run and more GPU memory is needed, we extend the GPU
memory region needed by the TensorFlow process.  Note that we do not release
memory, since that can lead to even worse memory fragmentation.  To turn this
option on, set the option in the ConfigProto by:

```python
config = tf.ConfigProto()
config.gpu_options.allow_growth = True
session = tf.Session(config=config, ...)
```

The second method is the `per_process_gpu_memory_fraction` option, which
determines the fraction of the overall amount of memory that each visible GPU
should be allocated.  For example, you can tell TensorFlow to only allocate 40%
of the total memory of each GPU by:

```python
config = tf.ConfigProto()
config.gpu_options.per_process_gpu_memory_fraction = 0.4
session = tf.Session(config=config, ...)
```

This is useful if you want to truly bound the amount of GPU memory available to
the TensorFlow process.

## Using a single GPU on a multi-GPU system

If you have more than one GPU in your system, the GPU with the lowest ID will be
selected by default. If you would like to run on a different GPU, you will need
to specify the preference explicitly:

```python
# Creates a graph.
with tf.device('/gpu:2'):
  a = tf.constant([1.0, 2.0, 3.0, 4.0, 5.0, 6.0], shape=[2, 3], name='a')
  b = tf.constant([1.0, 2.0, 3.0, 4.0, 5.0, 6.0], shape=[3, 2], name='b')
  c = tf.matmul(a, b)
# Creates a session with log_device_placement set to True.
sess = tf.Session(config=tf.ConfigProto(log_device_placement=True))
# Runs the op.
print sess.run(c)
```

If the device you have specified does not exist, you will get
`InvalidArgumentError`:

```
InvalidArgumentError: Invalid argument: Cannot assign a device to node 'b':
Could not satisfy explicit device specification '/gpu:2'
   [[Node: b = Const[dtype=DT_FLOAT, value=Tensor<type: float shape: [3,2]
   values: 1 2 3...>, _device="/gpu:2"]()]]
```

If you would like TensorFlow to automatically choose an existing and
supported device to run the operations in case the specified one doesn't
exist, you can set `allow_soft_placement` to `True` in the configuration
option when creating the session.

```python
# Creates a graph.
with tf.device('/gpu:2'):
  a = tf.constant([1.0, 2.0, 3.0, 4.0, 5.0, 6.0], shape=[2, 3], name='a')
  b = tf.constant([1.0, 2.0, 3.0, 4.0, 5.0, 6.0], shape=[3, 2], name='b')
  c = tf.matmul(a, b)
# Creates a session with allow_soft_placement and log_device_placement set
# to True.
sess = tf.Session(config=tf.ConfigProto(
      allow_soft_placement=True, log_device_placement=True))
# Runs the op.
print sess.run(c)
```

## Using multiple GPUs

If you would like to run TensorFlow on multiple GPUs, you can construct your
model in a multi-tower fashion where each tower is assigned to a different GPU.
For example:

```
# Creates a graph.
c = []
for d in ['/gpu:2', '/gpu:3']:
  with tf.device(d):
    a = tf.constant([1.0, 2.0, 3.0, 4.0, 5.0, 6.0], shape=[2, 3])
    b = tf.constant([1.0, 2.0, 3.0, 4.0, 5.0, 6.0], shape=[3, 2])
    c.append(tf.matmul(a, b))
with tf.device('/cpu:0'):
  sum = tf.add_n(c)
# Creates a session with log_device_placement set to True.
sess = tf.Session(config=tf.ConfigProto(log_device_placement=True))
# Runs the op.
print sess.run(sum)
```

You will see the following output.

```
Device mapping:
/job:localhost/replica:0/task:0/gpu:0 -> device: 0, name: Tesla K20m, pci bus
id: 0000:02:00.0
/job:localhost/replica:0/task:0/gpu:1 -> device: 1, name: Tesla K20m, pci bus
id: 0000:03:00.0
/job:localhost/replica:0/task:0/gpu:2 -> device: 2, name: Tesla K20m, pci bus
id: 0000:83:00.0
/job:localhost/replica:0/task:0/gpu:3 -> device: 3, name: Tesla K20m, pci bus
id: 0000:84:00.0
Const_3: /job:localhost/replica:0/task:0/gpu:3
Const_2: /job:localhost/replica:0/task:0/gpu:3
MatMul_1: /job:localhost/replica:0/task:0/gpu:3
Const_1: /job:localhost/replica:0/task:0/gpu:2
Const: /job:localhost/replica:0/task:0/gpu:2
MatMul: /job:localhost/replica:0/task:0/gpu:2
AddN: /job:localhost/replica:0/task:0/cpu:0
[[  44.   56.]
 [  98.  128.]]
```

The [cifar10 tutorial](../../tutorials/deep_cnn/index.md) is a good example
demonstrating how to do training with multiple GPUs.
