Does TensorFlow by default use all available GPUs in the machine?

Question

I have 3 GTX Titan GPUs in my machine. I run the example provided in Cifar10 with cifar10_train.py and got the following output:

I tensorflow/core/common_runtime/gpu/gpu_init.cc:60] cannot enable peer access from device ordinal 0 to device ordinal 1
I tensorflow/core/common_runtime/gpu/gpu_init.cc:60] cannot enable peer access from device ordinal 1 to device ordinal 0
I tensorflow/core/common_runtime/gpu/gpu_init.cc:127] DMA: 0 1 
I tensorflow/core/common_runtime/gpu/gpu_init.cc:137] 0:   Y N 
I tensorflow/core/common_runtime/gpu/gpu_init.cc:137] 1:   N Y 
I tensorflow/core/common_runtime/gpu/gpu_device.cc:694] Creating TensorFlow device (/gpu:0) -> (device: 0, name: GeForce GTX TITAN, pci bus id: 0000:03:00.0)
I tensorflow/core/common_runtime/gpu/gpu_device.cc:694] Creating TensorFlow device (/gpu:1) -> (device: 1, name: GeForce GTX TITAN, pci bus id: 0000:84:00.0)

It looks to me that TensorFlow is trying to initialize itself on two devices (gpu0 and gpu1).

My question is why it only does that on two devices and is there any way to prevent that? (I only want it to run as if there is a single GPU)

Answer 1

See: Using GPUs

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.

# 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. Since a device was not explicitly specified for the MatMul operation, the TensorFlow runtime will choose one based on the operation and available devices (gpu:0 in this example) and automatically copy tensors between devices if required.

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.]]

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Earlier Answer 2.

See: Using GPUs

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:

# 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)

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Earlier Answer 1.

From CUDA_VISIBLE_DEVICES – Masking GPUs

Does your CUDA application need to target a specific GPU? If you are writing GPU enabled code, you would typically use a device query to select the desired GPUs. However, a quick and easy solution for testing is to use the environment variable CUDA_VISIBLE_DEVICES to restrict the devices that your CUDA application sees. This can be useful if you are attempting to share resources on a node or you want your GPU enabled executable to target a specific GPU.

Environment Variable Syntax

Results

CUDA_VISIBLE_DEVICES=1 Only device 1 will be seen CUDA_VISIBLE_DEVICES=0,1 Devices 0 and 1 will be visible CUDA_VISIBLE_DEVICES=”0,1” Same as above, quotation marks are optional CUDA_VISIBLE_DEVICES=0,2,3 Devices 0, 2, 3 will be visible; device 1 is masked

CUDA will enumerate the visible devices starting at zero. In the last case, devices 0, 2, 3 will appear as devices 0, 1, 2. If you change the order of the string to “2,3,0”, devices 2,3,0 will be enumerated as 0,1,2 respectively. If CUDA_VISIBLE_DEVICES is set to a device that does not exist, all devices will be masked. You can specify a mix of valid and invalid device numbers. All devices before the invalid value will be enumerated, while all devices after the invalid value will be masked.

To determine the device ID for the available hardware in your system, you can run NVIDIA’s deviceQuery executable included in the CUDA SDK. Happy programming!

Chris Mason