Hybrid - Faster training and easy deployment

Deep learning frameworks can be roughly divided into two categories: declarative and imperative. With declarative frameworks (including Tensorflow, Theano, etc) users first declare a fixed computation graph and then execute it end-to-end. The benefit of fixed computation graph is it’s portable and runs more efficiently. However, it’s less flexible because any logic must be encoded into the graph as special operators like scan, while_loop and cond. It’s also hard to debug.

Imperative frameworks (including PyTorch, Chainer, etc) are just the opposite: they execute commands one-by-one just like old fashioned Matlab and Numpy. This style is more flexible, easier to debug, but less efficient.

HybridBlock seamlessly combines declarative programming and imperative programming to offer the benefit of both. Users can quickly develop and debug models with imperative programming and switch to efficient declarative execution by simply calling: HybridBlock.hybridize().


HybridBlock is very similar to Block but has a few restrictions:

  • All children layers of HybridBlock must also be HybridBlock.
  • Only methods that are implemented for both NDArray and Symbol can be used. For example you cannot use .asnumpy(), .shape, etc.
  • Operations cannot change from run to run. For example, you cannot do if x: if x is different for each iteration.

To use hybrid support, we subclass the HybridBlock:

import mxnet as mx
from mxnet import gluon
from mxnet.gluon import nn

class Net(gluon.HybridBlock):
    def __init__(self, **kwargs):
        super(Net, self).__init__(**kwargs)
        with self.name_scope:
            # layers created in name_scope will inherit name space
            # from parent layer.
            self.conv1 = nn.Conv2D(6, kernel_size=5)
            self.pool1 = nn.Pool2D(kernel_size=2)
            self.conv2 = nn.Conv2D(16, kernel_size=5)
            self.pool2 = nn.Pool2D(kernel_size=2)
            self.fc1 = nn.Dense(120)
            self.fc2 = nn.Dense(84)
            # You can use a Dense layer for fc3 but we do dot product manually
            # here for illustration purposes.
            self.fc3_weight = self.params.get('fc3_weight', shape=(10, 84))

    def hybrid_forward(self, F, x, fc3_weight):
        # Here `F` can be either mx.nd or mx.sym, x is the input data,
        # and fc3_weight is either self.fc3_weight.data() or
        # self.fc3_weight.var() depending on whether x is Symbol or NDArray
        x = self.pool1(F.relu(self.conv1(x)))
        x = self.pool2(F.relu(self.conv2(x)))
        # 0 means copy over size from corresponding dimension.
        # -1 means infer size from the rest of dimensions.
        x = x.reshape((0, -1))
        x = F.relu(self.fc1(x))
        x = F.relu(self.fc2(x))
        x = F.dot(x, fc3_weight, transpose_b=True)
        return x


By default, HybridBlock runs just like a standard Block. Each time a layer is called, its hybrid_forward will be run:

net = Net()
x = mx.nd.random_normal(shape=(16, 1, 28, 28))
x = mx.nd.random_normal(shape=(16, 1, 28, 28))

Hybrid execution can be activated by simply calling .hybridize() on the top level layer. The first forward call after activation will try to build a computation graph from hybrid_forward and cache it. On subsequent forward calls the cached graph instead of hybrid_forward will be invoked:

x = mx.nd.random_normal(shape=(16, 1, 28, 28))
x = mx.nd.random_normal(shape=(16, 1, 28, 28))

Note that before hybridize, print(x) printed out one NDArray for forward, but after hybridize, only the first forward printed out a Symbol. On subsequent forward hybrid_forward is not called so nothing was printed.

Hybridize will speed up execution and save memory. If the top level layer is not a HybridBlock, you can still call .hybridize() on it and Gluon will try to hybridize its children layers instead.

Serializing trained model for deployment

Models implemented as HybridBlock can be easily serialized for deployment using other language front-ends like C, C++ and Scala. To this end, we simply forward the model with symbolic variables instead of NDArrays and save the output Symbol(s):

x = mx.sym.var('data')
y = net(x)

If your network outputs more than one value, you can use mx.sym.Group to combine them into a grouped Symbol and then save. The saved json and params files can then be loaded with C, C++ and Scala interface for prediction.