# Computational methods and efficiency¶

Brian has several different methods for running the computations in a simulation. The default mode is Runtime code generation, which runs the simulation loop in Python but compiles and executes the modules doing the actual simulation work (numerical integration, synaptic propagation, etc.) in a defined target language. Brian will select the best available target language automatically. On Windows, to ensure that you get the advantages of compiled code, read the instructions on installing a suitable compiler in Windows. Runtime mode has the advantage that you can combine the computations performed by Brian with arbitrary Python code specified as NetworkOperation.

The fact that the simulation is run in Python means that there is a (potentially big) overhead for each simulated time step. An alternative is to run Brian in with Standalone code generation – this is in general faster (for certain types of simulations much faster) but cannot be used for all kinds of simulations. To enable this mode, add the following line after your Brian import, but before your simulation code:

set_device('cpp_standalone')


For detailed control over the compilation process (both for runtime and standalone code generation), you can change the Compiler settings that are used.

The following topics are not essential for beginners.

## Runtime code generation¶

Code generation means that Brian 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 (it will also raise a warning in this case, set codegen.target to 'numpy' explicitly to avoid this warning). 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.

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.

## Standalone code generation¶

Brian supports generating standalone code for multiple devices. In this mode, running a Brian script generates source code in a project tree for the target device/language. This code can then be compiled and run on the device, and modified if needed. At the moment, the only “device” supported is standalone C++ code. In some cases, the speed gains can be impressive, in particular for smaller networks with complicated spike propagation rules (such as STDP).

To use the C++ standalone mode, you only have to make very small changes to your script. The exact change depends on whether your script has only a single run() (or Network.run()) call, or several of them:

### Single run call¶

At the beginning of the script, i.e. after the import statements, add:

set_device('cpp_standalone')


The CPPStandaloneDevice.build function will be automatically called with default arguments right after the run() call. If you need non-standard arguments then you can specify them as part of the set_device() call:

set_device('cpp_standalone', directory='my_directory', debug=True)


### Multiple run calls¶

At the beginning of the script, i.e. after the import statements, add:

set_device('cpp_standalone', build_on_run=False)


After the last run() call, call device.build() explicitly:

device.build(directory='output', compile=True, run=True, debug=False)


The build function has several arguments to specify the output directory, whether or not to compile and run the project after creating it and whether or not to compile it with debugging support or not.

### Multiple builds¶

To run multiple full simulations (i.e. multiple device.build calls, not just multiple run() calls as discussed above), you have to reinitialize the device again:

device.reinit()
device.activate()


Note that the device “forgets” about all previously set build options provided to set_device() (most importantly the build_on_run option, but also e.g. the directory), you’ll have to specify them as part of the Device.activate call. Also, Device.activate will reset the defaultclock, you’ll therefore have to set its dt after the activate call if you want to use a non-default value.

### Limitations¶

Not all features of Brian will work with C++ standalone, in particular Python based network operations and some array based syntax such as S.w[0, :] = ... will not work. If possible, rewrite these using string based syntax and they should work. Also note that since the Python code actually runs as normal, code that does something like this may not behave as you would like:

results = []
for val in vals:
# set up a network
run()
results.append(result)


The current C++ standalone code generation only works for a fixed number of run statements, not with loops. If you need to do loops or other features not supported automatically, you can do so by inspecting the generated C++ source code and modifying it, or by inserting code directly into the main loop as follows:

device.insert_code('main', '''
cout << "Testing direct insertion of code." << endl;
''')


### Variables¶

After a simulation has been run (after the run() call if set_device() has been called with build_on_run set to True or after the Device.build call with run set to True), state variables and monitored variables can be accessed using standard syntax, with a few exceptions (e.g. string expressions for indexing).

Warning

OpenMP code has not yet been well tested and so may be inaccurate.

When using the C++ standalone mode, you have the opportunity to turn on multi-threading, if your C++ compiler is compatible with OpenMP. By default, this option is turned off and only one thread is used. However, by changing the preferences of the codegen.cpp_standalone object, you can turn it on. To do so, just add the following line in your python script:

prefs.devices.cpp_standalone.openmp_threads = XX


XX should be a positive value representing the number of threads that will be used during the simulation. Note that the speedup will strongly depend on the network, so there is no guarantee that the speedup will be linear as a function of the number of threads. However, this is working fine for networks with not too small timestep (dt > 0.1ms), and results do not depend on the number of threads used in the simulation.

## Compiler settings¶

If using C++ code generation (either via weave, cython or standalone), the compiler settings can make a big difference for the speed of the simulation. By default, Brian uses a set of compiler settings that switches on various optimizations and compiles for running on the same architecture where the code is compiled. 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).

If there are any issues with these compiler settings, for example because you are using an older version of the C++ compiler or because you want to run the generated code on a different architecture, you can change the settings by manually specifying the codegen.cpp.extra_compile_args preference (or by using codegen.cpp.extra_compile_args_gcc or codegen.cpp.extra_compile_args_msvc if you want to specify the settings for either compiler only).