import os
import tensorflow as tf
import keras.backend.tensorflow_backend as KTF
 
def get_session(gpu_fraction=0.3):
 '''Assume that you have 6GB of GPU memory and want to allocate ~2GB'''
 
 num_threads = os.environ.get('OMP_NUM_THREADS')
 gpu_options = tf.GPUOptions(per_process_gpu_memory_fraction=gpu_fraction)
 
 if num_threads:
  return tf.Session(config=tf.ConfigProto(
   gpu_options=gpu_options, intra_op_parallelism_threads=num_threads))
 else:
  return tf.Session(config=tf.ConfigProto(gpu_options=gpu_options))

import tensorflow as tf
from keras.backend.tensorflow_backend import set_session

config = tf.ConfigProto()
config.gpu_options.per_process_gpu_memory_fraction = 0.5 #half of the memory
set_session(tf.Session(config=config))