Multi cores support in Theano

BLAS operation

BLAS is an interface for some mathematic operations between two vectors, a vector and a matrix or two matrices (e.g. the dot product between vector/matrix and matrix/matrix). Many different implementations of that interface exist and some of them are parallelized.

Theano tries to use that interface as frequently as possible for performance reasons. So if Theano links to a parallel implementation, those operations will run in parallel in Theano.

The most frequent way to control the number of threads used is via the OMP_NUM_THREADS environment variable. Set it to the number of threads you want to use before starting the Python process. Some BLAS implementations support other environment variables.

To test if you BLAS supports OpenMP/Multiple cores, you can use the theano/misc/check_blas.py script from the command line like this:

OMP_NUM_THREADS=1 python theano/misc/check_blas.py -q
OMP_NUM_THREADS=2 python theano/misc/check_blas.py -q

Parallel element wise ops with OpenMP

Because element wise ops work on every tensor entry independently they can be easily parallelized using OpenMP.

To use OpenMP you must set the openmp flag to True.

You can use the flag openmp_elemwise_minsize to set the minimum tensor size for which the operation is parallelized because for short tensors using OpenMP can slow down the operation. The default value is 200000.

For simple (fast) operations you can obtain a speed-up with very large tensors while for more complex operations you can obtain a good speed-up also for smaller tensors.

There is a script elemwise_openmp_speedup.py in theano/misc/ which you can use to tune the value of openmp_elemwise_minsize for your machine. The script runs two elemwise operations (a fast one and a slow one) for a vector of size openmp_elemwise_minsize with and without OpenMP and shows the time difference between the cases.

The only way to control the number of threads used is via the OMP_NUM_THREADS environment variable. Set it to the number of threads you want to use before starting the Python process. You can test this with this command:

OMP_NUM_THREADS=2 python theano/misc/elemwise_openmp_speedup.py
#The output

Fast op time without openmp 0.000533s with openmp 0.000474s speedup 1.12
Slow op time without openmp 0.002987s with openmp 0.001553s speedup 1.92