<p>It is a very common mistake to forget that the activations, gradients and optimizer moment tracking variables also take VRRAM, not just the parameters, increasing memory usage quite a bit. The backprob calculations themselves make it so the training phase takes almost double the VRAM of forward / inference use of the neural net, and the Adam optimizer triples the space usage.</p>
<p>So, in the beginning when the network is created, only the parameters are allocated. However, when the training starts. the model actiavtions,  backprop computations and the optimizer's tracking variables get allocated, increasing memory use by a large factor.</p>
<p>To allow the training of larger models, people:</p>
<ul>
<li>use <strong>model parallelism</strong> to spread the weights and computations over different accelerators</li>
<li>use <a href="https://medium.com/tensorflow/fitting-larger-networks-into-memory-583e3c758ff9" rel="nofollow noreferrer"><strong>gradient checkpointing</strong></a>, which allows a tradeoff between more computation vs lower memory use during back-propagation.</li>
<li>Potentially use a <strong>memory efficient optimizer</strong> that aims to reduce the number of tracking variables, such as <a href="https://arxiv.org/abs/1804.04235" rel="nofollow noreferrer">Adafactor</a>, for which you will find implementations for all popular deep learning frameworks.</li>
</ul>
<p>Tools to train very large models:</p>
<ul>
<li>Mesh-Tensorflow <a href="https://arxiv.org/abs/1811.02084" rel="nofollow noreferrer">https://arxiv.org/abs/1811.02084</a>
<a href="https://github.com/tensorflow/mesh" rel="nofollow noreferrer">https://github.com/tensorflow/mesh</a></li>
<li>Microsoft DeepSpeed:
<a href="https://github.com/microsoft/DeepSpeed" rel="nofollow noreferrer">https://github.com/microsoft/DeepSpeed</a> <a href="https://www.deepspeed.ai/" rel="nofollow noreferrer">https://www.deepspeed.ai/</a></li>
<li>Facebook FairScale: <a href="https://github.com/facebookresearch/fairscale" rel="nofollow noreferrer">https://github.com/facebookresearch/fairscale</a></li>
<li>Megatron-LM: <a href="https://arxiv.org/abs/1909.08053" rel="nofollow noreferrer">https://arxiv.org/abs/1909.08053</a>
<a href="https://github.com/NVIDIA/Megatron-LM" rel="nofollow noreferrer">https://github.com/NVIDIA/Megatron-LM</a></li>
<li>Article on integration in HuggingFace Transformers: <a href="https://huggingface.co/blog/zero-deepspeed-fairscale" rel="nofollow noreferrer">https://huggingface.co/blog/zero-deepspeed-fairscale</a></li>
</ul>
Keras uses way too much GPU memory when calling train_on_batch, fit, etc
<p>I've been messing with Keras, and like it so far. There's one big issue I have been having, when working with fairly deep networks: When calling model.train_on_batch, or model.fit etc., Keras allocates significantly more GPU memory than what the model itself should need. This is not caused by trying to train on some really large images, it's the network model itself that seems to require a lot of GPU memory. I have created this toy example to show what I mean. Here's essentially what's going on:</p>

<p>I first create a fairly deep network, and use model.summary() to get the total number of parameters needed for the network (in this case 206538153, which corresponds to about 826 MB). I then use nvidia-smi to see how much GPU memory Keras has allocated, and I can see that it makes perfect sense (849 MB).</p>

<p>I then compile the network, and can confirm that this does not increase GPU memory usage. And as we can see in this case, I have almost 1 GB of VRAM available at this point.</p>

<p>Then I try to feed a simple 16x16 image and a 1x1 ground truth to the network, and then everything blows up, because Keras starts allocating lots of memory again, for no reason that is obvious to me. Something about training the network seems to require a lot more memory than just having the model, which doesn't make sense to me. I have trained significantly deeper networks on this GPU in other frameworks, so that makes me think that I'm using Keras wrong (or there's something wrong in my setup, or in Keras, but of course that's hard to know for sure).</p>

<p>Here's the code:</p>



<pre class="lang-python prettyprint-override"><code>from scipy import misc
import numpy as np
from keras.models import Sequential
from keras.layers import Dense, Activation, Convolution2D, MaxPooling2D, Reshape, Flatten, ZeroPadding2D, Dropout
import os

model = Sequential()

model.add(Convolution2D(256, 3, 3, border_mode='same', input_shape=(16,16,1)))
model.add(MaxPooling2D(pool_size=(2,2), strides=(2,2)))
model.add(Convolution2D(512, 3, 3, border_mode='same'))
model.add(MaxPooling2D(pool_size=(2,2), strides=(2,2)))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(Convolution2D(1024, 3, 3, border_mode='same'))
model.add(MaxPooling2D(pool_size=(2,2), strides=(2,2)))
model.add(Convolution2D(256, 3, 3, border_mode='same'))
model.add(Convolution2D(32, 3, 3, border_mode='same'))
model.add(MaxPooling2D(pool_size=(2,2)))
model.add(Flatten())
model.add(Dense(4))
model.add(Dense(1))

model.summary()

os.system("nvidia-smi")
raw_input("Press Enter to continue...")    

model.compile(optimizer='sgd',
              loss='mse', 
              metrics=['accuracy'])

os.system("nvidia-smi")              
raw_input("Compiled model. Press Enter to continue...")

n_batches = 1
batch_size = 1
for ibatch in range(n_batches):
    x = np.random.rand(batch_size, 16,16,1)
    y = np.random.rand(batch_size, 1)

    os.system("nvidia-smi")
    raw_input("About to train one iteration. Press Enter to continue...")

    model.train_on_batch(x, y)         
    print("Trained one iteration")
</code></pre>

<p>Which gives the following output for me:</p>

<pre class="lang-python prettyprint-override"><code>Using Theano backend.
Using gpu device 0: GeForce GTX 960 (CNMeM is disabled, cuDNN 5103)
/usr/local/lib/python2.7/dist-packages/theano/sandbox/cuda/__init__.py:600: UserWarning: Your cuDNN version is more recent than the one Theano officially supports. If you see any problems, try updating Theano or downgrading cuDNN to version 5.
  warnings.warn(warn)
____________________________________________________________________________________________________
Layer (type)                     Output Shape          Param #     Connected to                     
====================================================================================================
convolution2d_1 (Convolution2D)  (None, 16, 16, 256)   2560        convolution2d_input_1[0][0]      
____________________________________________________________________________________________________
maxpooling2d_1 (MaxPooling2D)    (None, 8, 8, 256)     0           convolution2d_1[0][0]            
____________________________________________________________________________________________________
convolution2d_2 (Convolution2D)  (None, 8, 8, 512)     1180160     maxpooling2d_1[0][0]             
____________________________________________________________________________________________________
maxpooling2d_2 (MaxPooling2D)    (None, 4, 4, 512)     0           convolution2d_2[0][0]            
____________________________________________________________________________________________________
convolution2d_3 (Convolution2D)  (None, 4, 4, 1024)    4719616     maxpooling2d_2[0][0]             
____________________________________________________________________________________________________
convolution2d_4 (Convolution2D)  (None, 4, 4, 1024)    9438208     convolution2d_3[0][0]            
____________________________________________________________________________________________________
convolution2d_5 (Convolution2D)  (None, 4, 4, 1024)    9438208     convolution2d_4[0][0]            
____________________________________________________________________________________________________
convolution2d_6 (Convolution2D)  (None, 4, 4, 1024)    9438208     convolution2d_5[0][0]            
____________________________________________________________________________________________________
convolution2d_7 (Convolution2D)  (None, 4, 4, 1024)    9438208     convolution2d_6[0][0]            
____________________________________________________________________________________________________
convolution2d_8 (Convolution2D)  (None, 4, 4, 1024)    9438208     convolution2d_7[0][0]            
____________________________________________________________________________________________________
convolution2d_9 (Convolution2D)  (None, 4, 4, 1024)    9438208     convolution2d_8[0][0]            
____________________________________________________________________________________________________
convolution2d_10 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_9[0][0]            
____________________________________________________________________________________________________
convolution2d_11 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_10[0][0]           
____________________________________________________________________________________________________
convolution2d_12 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_11[0][0]           
____________________________________________________________________________________________________
convolution2d_13 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_12[0][0]           
____________________________________________________________________________________________________
convolution2d_14 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_13[0][0]           
____________________________________________________________________________________________________
convolution2d_15 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_14[0][0]           
____________________________________________________________________________________________________
convolution2d_16 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_15[0][0]           
____________________________________________________________________________________________________
convolution2d_17 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_16[0][0]           
____________________________________________________________________________________________________
convolution2d_18 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_17[0][0]           
____________________________________________________________________________________________________
convolution2d_19 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_18[0][0]           
____________________________________________________________________________________________________
convolution2d_20 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_19[0][0]           
____________________________________________________________________________________________________
convolution2d_21 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_20[0][0]           
____________________________________________________________________________________________________
convolution2d_22 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_21[0][0]           
____________________________________________________________________________________________________
convolution2d_23 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_22[0][0]           
____________________________________________________________________________________________________
convolution2d_24 (Convolution2D) (None, 4, 4, 1024)    9438208     convolution2d_23[0][0]           
____________________________________________________________________________________________________
maxpooling2d_3 (MaxPooling2D)    (None, 2, 2, 1024)    0           convolution2d_24[0][0]           
____________________________________________________________________________________________________
convolution2d_25 (Convolution2D) (None, 2, 2, 256)     2359552     maxpooling2d_3[0][0]             
____________________________________________________________________________________________________
convolution2d_26 (Convolution2D) (None, 2, 2, 32)      73760       convolution2d_25[0][0]           
____________________________________________________________________________________________________
maxpooling2d_4 (MaxPooling2D)    (None, 1, 1, 32)      0           convolution2d_26[0][0]           
____________________________________________________________________________________________________
flatten_1 (Flatten)              (None, 32)            0           maxpooling2d_4[0][0]             
____________________________________________________________________________________________________
dense_1 (Dense)                  (None, 4)             132         flatten_1[0][0]                  
____________________________________________________________________________________________________
dense_2 (Dense)                  (None, 1)             5           dense_1[0][0]                    
====================================================================================================
Total params: 206538153
____________________________________________________________________________________________________
None
Thu Oct  6 09:05:42 2016       
+------------------------------------------------------+                       
| NVIDIA-SMI 352.63     Driver Version: 352.63         |                       
|-------------------------------+----------------------+----------------------+
| GPU  Name        Persistence-M| Bus-Id        Disp.A | Volatile Uncorr. ECC |
| Fan  Temp  Perf  Pwr:Usage/Cap|         Memory-Usage | GPU-Util  Compute M. |
|===============================+======================+======================|
|   0  GeForce GTX 960     Off  | 0000:01:00.0      On |                  N/A |
| 30%   37C    P2    28W / 120W |   1082MiB /  2044MiB |      9%      Default |
+-------------------------------+----------------------+----------------------+

+-----------------------------------------------------------------------------+
| Processes:                                                       GPU Memory |
|  GPU       PID  Type  Process name                               Usage      |
|=============================================================================|
|    0      1796    G   /usr/bin/X                                     155MiB |
|    0      2597    G   compiz                                          65MiB |
|    0      5966    C   python                                         849MiB |
+-----------------------------------------------------------------------------+
Press Enter to continue...
Thu Oct  6 09:05:44 2016       
+------------------------------------------------------+                       
| NVIDIA-SMI 352.63     Driver Version: 352.63         |                       
|-------------------------------+----------------------+----------------------+
| GPU  Name        Persistence-M| Bus-Id        Disp.A | Volatile Uncorr. ECC |
| Fan  Temp  Perf  Pwr:Usage/Cap|         Memory-Usage | GPU-Util  Compute M. |
|===============================+======================+======================|
|   0  GeForce GTX 960     Off  | 0000:01:00.0      On |                  N/A |
| 30%   38C    P2    28W / 120W |   1082MiB /  2044MiB |      0%      Default |
+-------------------------------+----------------------+----------------------+

+-----------------------------------------------------------------------------+
| Processes:                                                       GPU Memory |
|  GPU       PID  Type  Process name                               Usage      |
|=============================================================================|
|    0      1796    G   /usr/bin/X                                     155MiB |
|    0      2597    G   compiz                                          65MiB |
|    0      5966    C   python                                         849MiB |
+-----------------------------------------------------------------------------+
Compiled model. Press Enter to continue...
Thu Oct  6 09:05:44 2016       
+------------------------------------------------------+                       
| NVIDIA-SMI 352.63     Driver Version: 352.63         |                       
|-------------------------------+----------------------+----------------------+
| GPU  Name        Persistence-M| Bus-Id        Disp.A | Volatile Uncorr. ECC |
| Fan  Temp  Perf  Pwr:Usage/Cap|         Memory-Usage | GPU-Util  Compute M. |
|===============================+======================+======================|
|   0  GeForce GTX 960     Off  | 0000:01:00.0      On |                  N/A |
| 30%   38C    P2    28W / 120W |   1082MiB /  2044MiB |      0%      Default |
+-------------------------------+----------------------+----------------------+

+-----------------------------------------------------------------------------+
| Processes:                                                       GPU Memory |
|  GPU       PID  Type  Process name                               Usage      |
|=============================================================================|
|    0      1796    G   /usr/bin/X                                     155MiB |
|    0      2597    G   compiz                                          65MiB |
|    0      5966    C   python                                         849MiB |
+-----------------------------------------------------------------------------+
About to train one iteration. Press Enter to continue...
Error allocating 37748736 bytes of device memory (out of memory). Driver report 34205696 bytes free and 2144010240 bytes total 
Traceback (most recent call last):
  File "memtest.py", line 65, in &lt;module&gt;
    model.train_on_batch(x, y)         
  File "/usr/local/lib/python2.7/dist-packages/keras/models.py", line 712, in train_on_batch
    class_weight=class_weight)
  File "/usr/local/lib/python2.7/dist-packages/keras/engine/training.py", line 1221, in train_on_batch
    outputs = self.train_function(ins)
  File "/usr/local/lib/python2.7/dist-packages/keras/backend/theano_backend.py", line 717, in __call__
    return self.function(*inputs)
  File "/usr/local/lib/python2.7/dist-packages/theano/compile/function_module.py", line 871, in __call__
    storage_map=getattr(self.fn, 'storage_map', None))
  File "/usr/local/lib/python2.7/dist-packages/theano/gof/link.py", line 314, in raise_with_op
    reraise(exc_type, exc_value, exc_trace)
  File "/usr/local/lib/python2.7/dist-packages/theano/compile/function_module.py", line 859, in __call__
    outputs = self.fn()
MemoryError: Error allocating 37748736 bytes of device memory (out of memory).
Apply node that caused the error: GpuContiguous(GpuDimShuffle{3,2,0,1}.0)
Toposort index: 338
Inputs types: [CudaNdarrayType(float32, 4D)]
Inputs shapes: [(1024, 1024, 3, 3)]
Inputs strides: [(1, 1024, 3145728, 1048576)]
Inputs values: ['not shown']
Outputs clients: [[GpuDnnConv{algo='small', inplace=True}(GpuContiguous.0, GpuContiguous.0, GpuAllocEmpty.0, GpuDnnConvDesc{border_mode='half', subsample=(1, 1), conv_mode='conv', precision='float32'}.0, Constant{1.0}, Constant{0.0}), GpuDnnConvGradI{algo='none', inplace=True}(GpuContiguous.0, GpuContiguous.0, GpuAllocEmpty.0, GpuDnnConvDesc{border_mode='half', subsample=(1, 1), conv_mode='conv', precision='float32'}.0, Constant{1.0}, Constant{0.0})]]

HINT: Re-running with most Theano optimization disabled could give you a back-trace of when this node was created. This can be done with by setting the Theano flag 'optimizer=fast_compile'. If that does not work, Theano optimizations can be disabled with 'optimizer=None'.
HINT: Use the Theano flag 'exception_verbosity=high' for a debugprint and storage map footprint of this apply node.
</code></pre>

<p>A few things to note: </p>

<ul>
<li>I have tried both Theano and TensorFlow backends. Both have the same problems, and run out of memory at the same line. In TensorFlow, it seems that Keras preallocates a lot of memory (about 1.5 GB) so nvidia-smi doesn't help us track what's going on there, but I get the same out-of-memory exceptions. Again, this points towards an error in (my usage of) Keras (although it's hard to be certain about such things, it could be something with my setup).</li>
<li>I tried using CNMEM in Theano, which behaves like TensorFlow: It preallocates a large amount of memory (about 1.5 GB) yet crashes in the same place.</li>
<li>There are some warnings about the CudNN-version. I tried running the Theano backend with CUDA but not CudNN and I got the same errors, so that is not the source of the problem.</li>
<li>If you want to test this on your own GPU, you might want to make the network deeper/shallower depending on how much GPU memory you have to test this.</li>
<li>My configuration is as follows: Ubuntu 14.04, GeForce GTX 960, CUDA 7.5.18, CudNN 5.1.3, Python 2.7, Keras 1.1.0 (installed via pip)</li>
<li>I've tried changing the compilation of the model to use different optimizers and losses, but that doesn't seem to change anything.</li>
<li>I've tried changing the train_on_batch function to use fit instead, but it has the same problem.</li>
<li>I saw one similar question here on StackOverflow - <a href="https://stackoverflow.com/questions/35757151/why-does-this-keras-model-require-over-6gb-of-memory">Why does this Keras model require over 6GB of memory?</a> - but as far as I can tell, I don't have those issues in my configuration. I've never had multiple versions of CUDA installed, and I've double checked my PATH, LD_LIBRARY_PATH and CUDA_ROOT variables more times than I can count.</li>
<li>Julius suggested that the activation parameters themselves take up GPU memory. If this is true, can somebody explain it a bit more clearly? I have tried changing the activation function of my convolution layers to functions that are clearly hard-coded with no learnable parameters as far as I can tell, and that doesn't change anything. Also, it seems unlikely that these parameters would take up almost as much memory as the rest of the network itself.</li>
<li>After thorough testing, the largest network I can train is about 453 MB of parameters, out of my ~2 GB of GPU RAM. Is this normal? </li>
<li>After testing Keras on some smaller CNNs that do fit in my GPU, I can see that there are very sudden spikes in GPU RAM usage. If I run a network with about 100 MB of parameters, 99% of the time during training it'll be using less than 200 MB of GPU RAM. But every once in a while, memory usage spikes to about 1.3 GB. It seems safe to assume that it's these spikes that are causing my problems. I've never seen these spikes in other frameworks, but they might be there for a good reason? <strong>If anybody knows what causes them, and if there's a way to avoid them, please chime in!</strong></li>
</ul>