Gensim is a python module for vector space modeling, and it is extremely neat. It implements an incremental stochastic singular value decomosition algorithm that may be computed on distributed computer networks¹⁾.

Within the IKW network, I set up an local environment with which you can experiment.

Simple use case:

from gensim import corpora, models, similarities
 
# load corpus from List-Of-Words file (each line is a document, first line states number of documents)
# Doesn't have to be bzipped, but it CAN :-)
corpus = corpora.LowCorpus('/net/data/CL/projects/wordspace/software_tests/corpora/editedCorpusLOW.bz2')
 
# corpus will now be represented as a spare bag of words matrix. It also created a dictionary that maps 
# IDs of words to words, which can be used like this:
# >>> corpus.id2word[2923]
 
# Transform this corpus using Latent Semantic Indexing - we want 100 topics.
lsi = models.LsiModel(corpus, numTopics=200, id2word=corpus.id2word)
 
# Done. We could now query for different topic, eg
 
lsimodel.printTopic(10, topN = 5)
 
# would yield
# '-0.340 * "category" + 0.298 * "$M$" + 0.183 * "algebra" + -0.174 * "functor" + -0.168 * "operator"'

I used the EditedCorpora File to benchmark performance. The File was edited into the List-of-Words-Format (by just inserting a line containing the number of documents at the top). Afterwards Latent Semantic Indexing was performed on the corpus. As the algorithm used for singular value decomposition is incremental, the memory load is constant and can be controlled by passing a chunks parameter to the constructor of the LSI model. This parameter controls how many documents will be loaded into RAM at once, the default is 20000. Larger chunks will speed things up, but also require more RAM. In the distributed mode, this is the number of documents which will be passed to the workers over the network, hence we have to factor in the network transmission speed in choosing our chunk size. For the following experiments, a chunk size of 1000 documents was used.

Loading the corpus and transforming it into sparse vectors takes quite exactly 23 minutes on Quickie.

Please refer to the advanced usage section for details on how to setup Gensim in distributed mode. For testing the distributed mode of the algorithm, twelve 2.54 GHz, 4 GB RAM dual core boxes have been used as workers, with one worker per core, totaling 24 workers. LI

Within the IKW network, there is a local installation of gensim (which in turn depends on Numpy >= 1.4 (current version on IKW machines is 1.3) and Pyro >=4.1), residing in

/net/data/CL/projects/wordspace/gensim_local

to use it, change into this directory and run

source bin/activate
python

This loads the libraries installed locally in this directory and starts the python interpreter. To stop using the virtual environment, simpy run

deactivate

In theory, you can copy the entire gensim_local folder to your machine and run it from there, however I won't guarantee that this works in practice. However, it is easy to create such a local environment yourself:

# Skip this step if you already have virtualenv
easy_install virtualenv
# If you don't have sudo rights, install virtualenv locally:
# mkdir ~/opt
# easy_install --instal-dir opt virtualenv
 
# Create a clean virtual environment
virtualenv --no-site-packages myVirtualEnv
 
# And activate it
cd myVirtualEnv
source bin/activate
 
# Now, using pip, install the other stuff
bin/pip install numpy
bin/pip install gensim[distributed]
bin/pip install Pyro4

The distributed mode works using the Pyro4 Library. The idea behind this library is that you can instantiate python objects remotely and then forget that you have instantiated them remotely and just work with them like normal objects. However, these objects will eat their own local resources. Plus, you don't have to rewrite a single line of code if you want to use them locally. Most of Pyro's functionality is neatly wrapped by Gensim and works off the shelf. Of course, there are a few things you'll have to take care of:

- Tell Gensim which computers to use. We'll do this by running a little script on each computer on the network hat we want to work for us. This script will create workers which can be enslaved. I wrapped the script into another script which automatically creates enough workers to match the number of CPUs in the box you're using, each worker will hog onto one CPU. - Workers will get easily lost in the network, hence we'll need a name server that keeps track of all the workers we have. Of course, there's a script for that, too. Behind the scenes, workers will communicate over TCP/IP with the nameserver an - Furthermore, our enslaved workers may be good at maths, but behave like little children when it comes to sharing the data between them. Therefore, we will need a dispatcher that distributes data to process to all workers evenly, and handles the feedback from the worker. So, our algorithm as such will give a task to the dispatcher, he'll break it into chunks and gives to the workers (using the name server to locate them), they report back, we get our results.