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
This syntax for lazy memory allocation when we call
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
Variable's data member. Instead, we save an interface object
Placeholder, which can return the pointer to the saved object via
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.
PlaceholderImpl, the implementation of
Placeholder. Please be aware that
PlaceholderImpl is a class template and
T is passed in as a template parameter.
T, it can save and return
typeid(T) for the type comparison in
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.