# Computational methods and efficiency¶

Brian has several different methods for running the computations in a simulation. In particular, Brian uses “runtime code generation” for efficient computation. This means that it takes the Python code and strings in your model and generates code in one of several possible different languages and actually executes that. The target language for this code generation process is set in the codegen.target preference. By default, this preference is set to 'auto', meaning that it will chose a compiled language target if possible and fall back to Python otherwise. There are two compiled language targets for Python 2.x, 'weave' (needing a working installation of a C++ compiler) and 'cython' (needing the Cython package in addition); for Python 3.x, only 'cython' is available. If you want to chose a code generation target explicitly (e.g. because you want to get rid of the warning that only the Python fallback is available), set the preference to 'numpy', 'weave' or 'cython' at the beginning of your script:

from brian2 import *
prefs.codegen.target = 'numpy'  # use the Python fallback


See Preferences for different ways of setting preferences. If you are using a compiled language target, also see the Compiler settings for maximum speed section below.

Both of these code generation targets are still run via Python, which means that there are still overheads due to Python. The fastest way to run Brian is in “standalone mode” (see Devices), although this won’t work for every possible simulation. Note that you can also use multiple threads with standalone mode, which is not possible in the modes described above. This doesn’t always lead to a huge speed improvement, but can occasionally give a higher than linear speed up relative to the number of cores.

You might find that running simulations in weave or Cython modes won’t work or is not as efficient as you were expecting. This is probably because you’re using Python functions which are not compatible with weave or Cython. For example, if you wrote something like this it would not be efficient:

from brian2 import *
prefs.codegen.target = 'cython'
def f(x):
return abs(x)
G = NeuronGroup(10000, 'dv/dt = -x*f(x) : 1')


The reason is that the function f(x) is a Python function and so cannot be called from C++ directly. To solve this problem, you need to provide an implementation of the function in the target language. See Functions.

## Compiler settings for maximum speed¶

If using C++ code generation (either via weave, cython or standalone), you can maximise the efficiency of the generated code in various ways, described below. These can be set in the global preferences file as described in Preferences.

### GCC¶

For the GCC compiler, the fastest options are:

codegen.cpp.extra_compile_args_gcc = ['-w', '-Ofast', '-march=native']


The -Ofast optimisation allows the compiler to disregard strict IEEE standards compliance. In our usage this has never been a problem, but we don’t do this by default for safety. Note that not all versions of gcc include this switch, older versions might require you to write '-O3', '-ffast-math'.

The -march=native sets the computer architecture to be the one available on the machine you are compiling on. This allows the compiler to make use of as many advanced instructions as possible, but reduces portability of the generated executable (which is not usually an issue). Again, this option is not available on all versions of gcc so on an older version you might have to put your architecture in explicitly (check the gcc docs for your version).

### MSVC¶

For the MSVC compiler, the fastest options are:

codegen.cpp.extra_compile_args_msvc = ['/Ox', '/EHsc', '/w', '/arch:AVX2', '/fp:fast']


Note that as above for -Ofast on gcc, /fp:fast will enable the compiler to disregard strict IEEE standards compliance, which has never been a problem in our usage but we leave this off by default for safety.

The /arch:AVX2 option may not be available on your version of MSVC and your computer architecture. The available options (in order from best to worst) are: AVX2, AVX, SSE2, SSE and IA32.