## Summary

cuDNN is a library of efficient implementations of deep learning primitives. The main goal of cuDNN is to simplify maintenance of workloads in the fast development of hardware like GPUs and TPUs: the workloads can use cuDNN as arithmetic primitives without concerning about low level implementations. Hence this library needs to: 1. be easy to integrate into existing frameworks; 2. be optimized for performance and memory usage.

cuDNN supports forward and backward propagation variants of all its routines in single and double precision floating-point arithmetic. This include convolution, pooling, and activation functions. Pooling and activation functions are relatively easy to implement so we focus on convolution here. There are many ways to implement convolution efficiently. The goal is to achieve performance as close to matrix multiplication (a well-studied and highly-optimized workload) as possible without using any auxiliary memory from the host machine. GPU memory has high bandwidth but relatively small capacity compared with auxiliary memory. Convolutions are traditionally implemented in one of the 3 ways:

1. Unroll the convolution filter and input images to reduce it to matrix multiplications. As shown in figure below. This approach achieves great performance (since matrix multiplication is well optimized), but incurs high memory overhead as each cell on the input image is duplicated multiple times. To compensate this, the matrix can be created during calculation: we can divide one matrix (corresponding to the input images) into multiple submatrices and multiply one submatrix with another and materialize the next submatrix at the same time.

2. Using Fast Fourier Transform. FFT can significantly lower the work complexity of the convolutions, but it uses a significant amount of temporary memory since the filer needs to be padded to be the same size as the inputs.

3. Compute convolution directly. Note that this can be very efficient but requires a large number of specialized implementations to handle the many corner cases, making it hard to maintain. In addition, such implementations typically work well for some convolutions but not the others depending on the 11 parameters shown in the above figure.

The 3 implementations each perform better than the other 2 under different scenarios, with different parameters, different running environment (e.g., GPU memory available, compute power, etc), or something else. In practice, all 3 are implemented in cuDNN and is configurable through its parameters.

## Strength

- A library of compute primitives reduces the amount of efforts to maintain machine learning models and workloads: it does not require much code change to reoptimize the code on a different hardware.
- This library makes developing new machine learning workloads easy, as they do not need to worry about the detail implementations of these arithmetic primitives.
- cuDNN provides a huge parameter space. It provides flexibility.

## Weakness

- The abstraction of cuDNN library restricts flexibility. The programmer has to perform computations within the framework defined by cuDNN.
- cuDNN is a relatively general library: we may be able to write more optimized code specific to our model.

*Sharan Chetlur, Cliff Woolley, Philippe Vandermersch, Jonathan Cohen, JohnTran, Bryan Catanzaro, and Evan Shelhamer. 2014. cudnn: Efficient primitives fordeep learning.arXiv preprint arXiv:1410.0759(2014)*