Design Doc: Variable

Variable is also known as blob in MxNet and Caffe2. It is the input and output type of operators, where a neural network is a graph of operators.

Requirements: Lazy Memory Allocation

For the flexibility of a DL system, a variable should be able to contain any typed value -- a tensor in most cases, but could also be some integer IDs or a scope of other variables in the case of RNN.

To use the minimum amount of memory, we would like that a variable allocates memory only when it has to, or, lazy memory allocation. Let's take the following example:

Variable vr, v1, v2;

Tensor* t1 = new Tensor();
Tensor* t2 = new Tensor();

Randomize(
  /* malloc */ v1.GetMutable<Tensor>().mutable_data<float16>(DDim(100,200)),
  /* size */ t1.Size());

Randomize(
  /* malloc */ v2.GetMutable<Tensor>().mutable_data<float16>(DDim(200,300)),
  /* size */ t2.Size());

Mult(
  /*result*/ vr.GetMutable<Tensor>().mutable_data<v1.Type()>(SizeOfMult(v1, v2)),
  /*input1*/ v1.Get<Tensor>().data(),
  /*input2*/ v2.Get<Tensor>().data());

We see that a variable holds nothing until Variable::GetMutable<Tensor>() allocates a tensor and puts it in the variable. Similarly, a tensor gets its memory until Tensor::mutable_data().

This syntax for lazy memory allocation when we call Randomize and Mult, those functions that mutate the variable, so it saves us some line of C++ code.

Implementation: Type Hiding

To make memory allocation lazy, we cannot assume that we know the type held by a variable at definition time. In other words, class Variable cannot be a template template <T> class Variable.

Because we don't know the type T, we cannot save a T* as Variable's data member. Instead, we save an interface object Placeholder, which can return the pointer to the saved object via Placeholder::Ptr() as void*.

But anyway, Variable needs to know T so could it delete<T>(ptr) and so could Variable::Get checks the expected type and the saved object's type.

We save T in PlaceholderImpl, the implementation of Placeholder. Please be aware that PlaceholderImpl is a class template and T is passed in as a template parameter.

Because PlaceholderImpl knows T, it can save and return typeid(T) for the type comparison in Variable::Get and Variable::GetMutable.

Conclusion

The technique type hiding utilizes C++ class templates, interface and derivation, and C++ RTTI (typeid). This combination saves us from defining something like caffe2::TypeMeta, which takes hundreds of lines of C++ code.