Managing simulation runs and data

Often, you want to run a simulation multiple times with different parameters to generate data for a plot. There are many different ways to manage this, and Brian has a few tools to make it easier.

Saving data by hand

The simplest strategy is to run your simulation, and then save the data with a unique filename using either pickle, writing text or binary data to a file with Python, or with Numpy and Scipy.

Structured data formats

Another option is to use a more structured file type, for example, you could use the high performance HDF5 scientific data file format with PyTables.

Python also includes an object for storing data in a dictionary like database object with the shelve module.

Brian includes a simple modification of Python’s shelves to make it easy to generate data in parallel on a single machine or across several machines. The problem with Python shelves and HDF5 is that they cannot be accessed by several processes on a single machine concurrently (if two processes attempt to write to the file at the same time it gets corrupted). In addition, if you want to run simulations on two computers at once and merge them you have to write a separate program to merge the databases produced. With the DataManager class, you generate a directory containing multiple files, and the data can be distributed amongst these files. To merge the results generated on two different computers just copy the contents of one directory into the other. The way it works is that to write data to a DataManager, you first generate a ‘’session’’ object (which is essentially a Python shelf object) and then write data to that. However, when you want to read data, it will look in all the files in the directory and return merged data from them. Typically, a session file will have the form username.computername so that merging directories across multiple computers/users is straightforward (no name conflicts). You can also create a ‘’locking session’‘. This object can be used in multiple processes concurrently without danger of losing data.

Multiple runs in parallel

The Python multiprocessing module can be used for relatively simply distributing simulation runs over multiple CPUs. Alternatively, you could use Playdoh (produced by our group) to distribute work over multiple CPUs and multiple machines. For other solutions, see the “Parallel and distributed programming” section of the Scipy Topical Software page.

Brian provides a simple, single machine technique that works with the DataManager object, run_tasks(). With this, you provide a function and a sequence of arguments to that function, and the function calls will be evaluated across multiple CPUs, with the results being stored in the data manager. It also features a GUI which gives feedback on simulations as they run, and can be used to safely stop the processes without risking losing any data. A simple example of using this technique:

from brian import *
from import *
from import *

def find_rate(k, report):
    eqs = '''
    dV/dt = (k-V)/(10*ms) : 1
    G = NeuronGroup(1000, eqs, reset=0, threshold=1)
    M = SpikeCounter(G)
    run(30*second, report=report)
    return (k, mean(M.count)/30)

if __name__=='__main__':
    N = 20
    dataman = DataManager('taskfarmexample')
    if dataman.itemcount()<N:
        M = N-dataman.itemcount()
        run_tasks(dataman, find_rate, rand(M)*19+1)
    X, Y = zip(*dataman.values())
    plot(X, Y, '.')
    ylabel('Firing rate (Hz)')

Finally, a more sophisticated solution for “managing and tracking projects, based on numerical simulation or analysis, with the aim of supporting reproducible research” is Sumatra.