Mocha Backends

A backend in Mocha is a component that carries out actual numerical computation. Mocha is designed to support multiple backends, and switching between different backends should be almost transparent to the rest of the world.

Pure Julia CPU Backend

A pure Julia CPU backend is implemented in Julia. This backend is reasonably fast by making heavy use of the Julia’s built-in BLAS matrix computation library and performance annotations to help the LLVM-based JIT compiler producing high performance instructions.

A pure Julia CPU backend could be instantiated by calling the constructor CPUBackend(). Because there is no external dependency, it should runs on any platform that runs Julia.

If you have many cores in your computer, you can play with the number of threads used by the Julia’s BLAS matrix computation library by:


Depending on the problem size and a lot of other factors, using larger N is not necessarily faster.

CPU Backend with Native Extension

Mocha comes with C++ implementations of some bottleneck computations for the CPU backend. In order to use the native extension, you need to build the native code first (if it is not built automatically when installing the package)."Mocha")

After successfully building the native extension, it could be enabled by setting the environment variable. On bash or zsh, execute


before running Mocha. You can also set the environment variable inside the Julia code:


using Mocha

Note you should set the environment variable before loading the Mocha module. Otherwise Mocha will not load the native extension sub-module at all.

The native extension uses OpenMP to do parallel computation on Linux. The number of OpenMP threads used could be controlled by the OMP_NUM_THREADS environment variable. Note this variable is not specific to Mocha. If you have other programs that uses OpenMP, setting this environment variable in a shell will also affect those problems started subsequently. If you want to restrict to Mocha, simply set the variable in the Julia code:


Note setting to 1 disabled the OpenMP parallelization. Depending on the problem size and a lot of other factors, using multi-thread OpenMP parallelization is not necessarily faster because of the overhead of multi-threads.

The parameter for the number of threads used by the BLAS library applies to the CPU backend with native extension, too.

OpenMP on Mac OS X

When compiling the native extension on Mac OS X, you will get a warning that OpenMP is disabled. This is because currently clang, the built-in compiler for OS X, does not officially support OpenMP yet. If you want to try OpenMP on OS X, please refer to Clang-OMP and compile manually (see below).

Native Extension on Windows

The native extension does not support Windows because automatic building script does not work on Windows. However, the native codes themselves does not use any OS specific features. If you have a compiler installed on Windows, you could try to compile the native extension manually. However, I have not tested the native extension on Windows personally.

Compile Native Extension Manually

The native codes are located in the deps directory of Mocha. Use


to find out where Mocha is installed. You should compile it as a shared library (DLL on Windows). However, currently the filename for the library is hard-coded to be, with a .so extension, regardless of the underlying OS.

CUDA Backend

GPU has been shown to be very effective at training large scale deep neural networks. NVidia® recently released a GPU accelerated library of primitives for deep neural networks called cuDNN. Mocha implemented a CUDA backend by combining cuDNN, cuBLAS and plain CUDA kernels.

In order to use the CUDA backend, you need to have CUDA-compatible GPU devices. The CUDA toolkit should be installed in order to compile the Mocha CUDA kernels. cuBLAS is included in CUDA distribution. But cuDNN needs to be installed separately. You could obtain cuDNN from Nvidia’s website by registering as a CUDA developer for free.


cuDNN requires CUDA 6.5 to run, and currently cuDNN is available to Linux and Windows only.

Before using the CUDA backend, Mocha kernels needs to be compiled. The kernels are located in src/cuda/kernels. Please use Pkg.dir("Mocha") to find out where Mocha is installed on your system. We have included a Makefile for convenience, but if you don’t have make installed, the compiling command is as simple as

nvcc -ptx

After compiling the kernels, you can now start to use the CUDA backend by setting the environment variable MOCHA_USE_CUDA. For example:

ENV["MOCHA_USE_CUDA"] = "true"

using Mocha

sys = System(CuDNNBackend())

# ...


Note instead of instantiate a CPUBackend, you now construct a CuDNNBackend. The environment variable should be set before loading Mocha. It is designed to use conditional loading so that the pure CPU backend could still run on machines without any GPU device or CUDA library installed.