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# Tensorflow Segmentation Fault With Single Machine Multiple GPUs Training

Recently, I am trying to learn how to use Tensorflow to do the data parallel training and I found a toy example here https://github.com/aymericdamien/TensorFlow-Examples/blob/master/examples/6_MultiGPU/multigpu_cnn.py.

However, I cannot run this example successfully and I got the following error.

```
WARNING:tensorflow:From /usr/local/anaconda3/envs/tensorflow/lib/python3.6/site-packages/tensorflow/contrib/learn/python/learn/datasets/mnist.py:290: DataSet.__init__ (from tensorflow.contrib.learn.python.learn.datasets.mnist) is deprecated and will be removed in a future version.
Instructions for updating:
Please use alternatives such as official/mnist/dataset.py from tensorflow/models.
WARNING:tensorflow:From test_tensorflow.py:138: softmax_cross_entropy_with_logits (from tensorflow.python.ops.nn_ops) is deprecated and will be removed in a future version.
Instructions for updating:
Future major versions of TensorFlow will allow gradients to flow
into the labels input on backprop by default.
See @{tf.nn.softmax_cross_entropy_with_logits_v2}.
2019-01-15 16:08:51.603247: I tensorflow/core/platform/cpu_feature_guard.cc:141] Your CPU supports instructions that this TensorFlow binary was not compiled to use: AVX2 AVX512F FMA
2019-01-15 16:09:01.674855: I tensorflow/core/common_runtime/gpu/gpu_device.cc:1405] Found device 0 with properties:
name: GeForce RTX 2080 Ti major: 7 minor: 5 memoryClockRate(GHz): 1.545
pciBusID: 0000:1b:00.0
totalMemory: 10.73GiB freeMemory: 10.53GiB
2019-01-15 16:09:01.971847: I tensorflow/core/common_runtime/gpu/gpu_device.cc:1405] Found device 1 with properties:
name: GeForce RTX 2080 Ti major: 7 minor: 5 memoryClockRate(GHz): 1.545
pciBusID: 0000:1c:00.0
totalMemory: 10.73GiB freeMemory: 10.53GiB
2019-01-15 16:09:01.972036: I tensorflow/core/common_runtime/gpu/gpu_device.cc:1484] Adding visible gpu devices: 0, 1
2019-01-15 16:09:02.728988: I tensorflow/core/common_runtime/gpu/gpu_device.cc:965] Device interconnect StreamExecutor with strength 1 edge matrix:
2019-01-15 16:09:02.729020: I tensorflow/core/common_runtime/gpu/gpu_device.cc:971] 0 1
2019-01-15 16:09:02.729025: I tensorflow/core/common_runtime/gpu/gpu_device.cc:984] 0: N N
2019-01-15 16:09:02.729027: I tensorflow/core/common_runtime/gpu/gpu_device.cc:984] 1: N N
2019-01-15 16:09:02.729458: I tensorflow/core/common_runtime/gpu/gpu_device.cc:1097] Created TensorFlow device (/job:localhost/replica:0/task:0/device:GPU:0 with 10166 MB memory) -> physical GPU (device: 0, name: GeForce RTX 2080 Ti, pci bus id: 0000:1b:00.0, compute capability: 7.5)
2019-01-15 16:09:02.875709: I tensorflow/core/common_runtime/gpu/gpu_device.cc:1097] Created TensorFlow device (/job:localhost/replica:0/task:0/device:GPU:1 with 10166 MB memory) -> physical GPU (device: 1, name: GeForce RTX 2080 Ti, pci bus id: 0000:1c:00.0, compute capability: 7.5)
Segmentation fault (core dumped)
```

I believe that what I really do is to collect and average gradients from multiple GPUs and then, update the parameters in my model. I am not sure what causes this problem.

The complete code is as the following, which is the official code.

```
from __future__ import division, print_function, absolute_import
import numpy as np
import tensorflow as tf
import time
# Import MNIST data
from tensorflow.examples.tutorials.mnist import input_data
mnist = input_data.read_data_sets("/tmp/data/", one_hot=True)
# Training Parameters
num_gpus = 2
num_steps = 200
learning_rate = 0.001
batch_size = 1024
display_step = 10
# Network Parameters
num_input = 784 # MNIST data input (img shape: 28*28)
num_classes = 10 # MNIST total classes (0-9 digits)
dropout = 0.75 # Dropout, probability to keep units
# Build a convolutional neural network
def conv_net(x, n_classes, dropout, reuse, is_training):
# Define a scope for reusing the variables
with tf.variable_scope('ConvNet', reuse=reuse):
# MNIST data input is a 1-D vector of 784 features (28*28 pixels)
# Reshape to match picture format [Height x Width x Channel]
# Tensor input become 4-D: [Batch Size, Height, Width, Channel]
x = tf.reshape(x, shape=[-1, 28, 28, 1])
# Convolution Layer with 64 filters and a kernel size of 5
x = tf.layers.conv2d(x, 64, 5, activation=tf.nn.relu)
# Max Pooling (down-sampling) with strides of 2 and kernel size of 2
x = tf.layers.max_pooling2d(x, 2, 2)
# Convolution Layer with 256 filters and a kernel size of 5
x = tf.layers.conv2d(x, 256, 3, activation=tf.nn.relu)
# Convolution Layer with 512 filters and a kernel size of 5
x = tf.layers.conv2d(x, 512, 3, activation=tf.nn.relu)
# Max Pooling (down-sampling) with strides of 2 and kernel size of 2
x = tf.layers.max_pooling2d(x, 2, 2)
# Flatten the data to a 1-D vector for the fully connected layer
x = tf.contrib.layers.flatten(x)
# Fully connected layer (in contrib folder for now)
x = tf.layers.dense(x, 2048)
# Apply Dropout (if is_training is False, dropout is not applied)
x = tf.layers.dropout(x, rate=dropout, training=is_training)
# Fully connected layer (in contrib folder for now)
x = tf.layers.dense(x, 1024)
# Apply Dropout (if is_training is False, dropout is not applied)
x = tf.layers.dropout(x, rate=dropout, training=is_training)
# Output layer, class prediction
out = tf.layers.dense(x, n_classes)
# Because 'softmax_cross_entropy_with_logits' loss already apply
# softmax, we only apply softmax to testing network
out = tf.nn.softmax(out) if not is_training else out
return out
def average_gradients(tower_grads):
average_grads = []
for grad_and_vars in zip(*tower_grads):
# Note that each grad_and_vars looks like the following:
# ((grad0_gpu0, var0_gpu0), ... , (grad0_gpuN, var0_gpuN))
grads = []
for g, _ in grad_and_vars:
# Add 0 dimension to the gradients to represent the tower.
expanded_g = tf.expand_dims(g, 0)
# Append on a 'tower' dimension which we will average over below.
grads.append(expanded_g)
# Average over the 'tower' dimension.
grad = tf.concat(grads, 0)
grad = tf.reduce_mean(grad, 0)
# Keep in mind that the Variables are redundant because they are shared
# across towers. So .. we will just return the first tower's pointer to
# the Variable.
v = grad_and_vars[0][1]
grad_and_var = (grad, v)
average_grads.append(grad_and_var)
return average_grads
# By default, all variables will be placed on '/gpu:0'
# So we need a custom device function, to assign all variables to '/cpu:0'
# Note: If GPUs are peered, '/gpu:0' can be a faster option
PS_OPS = ['Variable', 'VariableV2', 'AutoReloadVariable']
def assign_to_device(device, ps_device='/cpu:0'):
def _assign(op):
node_def = op if isinstance(op, tf.NodeDef) else op.node_def
if node_def.op in PS_OPS:
return "/" + ps_device
else:
return device
return _assign
# Place all ops on CPU by default
with tf.device('/cpu:0'):
tower_grads = []
reuse_vars = False
# tf Graph input
X = tf.placeholder(tf.float32, [None, num_input])
Y = tf.placeholder(tf.float32, [None, num_classes])
# Loop over all GPUs and construct their own computation graph
for i in range(num_gpus):
with tf.device(assign_to_device('/gpu:{}'.format(i), ps_device='/cpu:0')):
# Split data between GPUs
_x = X[i * batch_size: (i+1) * batch_size]
_y = Y[i * batch_size: (i+1) * batch_size]
# Because Dropout have different behavior at training and prediction time, we
# need to create 2 distinct computation graphs that share the same weights.
# Create a graph for training
logits_train = conv_net(_x, num_classes, dropout,
reuse=reuse_vars, is_training=True)
# Create another graph for testing that reuse the same weights
logits_test = conv_net(_x, num_classes, dropout,
reuse=True, is_training=False)
# Define loss and optimizer (with train logits, for dropout to take effect)
loss_op = tf.reduce_mean(tf.nn.softmax_cross_entropy_with_logits(
logits=logits_train, labels=_y))
optimizer = tf.train.AdamOptimizer(learning_rate=learning_rate)
grads = optimizer.compute_gradients(loss_op)
# Only first GPU compute accuracy
if i == 0:
# Evaluate model (with test logits, for dropout to be disabled)
correct_pred = tf.equal(tf.argmax(logits_test, 1), tf.argmax(_y, 1))
accuracy = tf.reduce_mean(tf.cast(correct_pred, tf.float32))
reuse_vars = True
tower_grads.append(grads)
tower_grads = average_gradients(tower_grads)
train_op = optimizer.apply_gradients(tower_grads)
# Initialize the variables (i.e. assign their default value)
init = tf.global_variables_initializer()
# Start Training
with tf.Session() as sess:
# Run the initializer
sess.run(init)
# Keep training until reach max iterations
for step in range(1, num_steps + 1):
# Get a batch for each GPU
batch_x, batch_y = mnist.train.next_batch(batch_size * num_gpus)
# Run optimization op (backprop)
ts = time.time()
sess.run(train_op, feed_dict={X: batch_x, Y: batch_y})
te = time.time() - ts
if step % display_step == 0 or step == 1:
# Calculate batch loss and accuracy
loss, acc = sess.run([loss_op, accuracy], feed_dict={X: batch_x,
Y: batch_y})
print("Step " + str(step) + ": Minibatch Loss= " + \
"{:.4f}".format(loss) + ", Training Accuracy= " + \
"{:.3f}".format(acc) + ", %i Examples/sec" % int(len(batch_x)/te))
step += 1
print("Optimization Finished!")
# Calculate accuracy for MNIST test images
print("Testing Accuracy:", \
np.mean([sess.run(accuracy, feed_dict={X: mnist.test.images[i:i+batch_size],
Y: mnist.test.labels[i:i+batch_size]}) for i in range(0, len(mnist.test.images), batch_size)]))
```

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## Answer

Actually, I found what the problem is with my Tensorflow. The above error is caused by the mismatching between RTX GPU cards and CUDA driver.

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source: stackoverflow.com

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