VTensor: Using Virtual Tensors to Build a Layout-Oblivious AI Programming Framework

Tensors are a popular programming interface for developing artificial intelligence (AI) algorithms. Layout refers to the order of placing tensor data in the memory and will affect performance by affecting data locality; therefore the deep neural network library has a convention on the layout. Since AI applications can use arbitrary layouts, and existing AI systems do not provide programming abstractions to shield the layout conventions of libraries, operator developers need to write a lot of layout-related code, which reduces the efficiency of integrating new libraries or developing new operators. Furthermore, the developer assigns the layout conversion operation to the internal operator to deal with the uncertainty of the input layout, thus losing the opportunity for layout optimization. Based on the idea of polymorphism, we propose a layout-agnostic virtual tensor programming interface, namely the VTensor framework, which enables developers to write new operators without caring about the underlying physical layout of tensors. In addition, the VTensor framework performs global layout inference at runtime to transparently resolve the required layout of virtual tensors, and runtime layout-oriented optimizations to globally minimize the number of layout transformation operations. Experimental results demonstrate that with VTensor, developers can avoid writing layout-dependent code. Compared with TensorFlow, for the 16 operations used in 12 popular networks, VTensor can reduce the lines of code (LOC) of writing a new operation by 47.82% on average, and improve the overall performance by 18.65% on average.