John,
Your comment about matrix multiplication was forwarded to the mahoutuser
emailing list.
I have a question for you about your approach. Have you considered doing
the multiplication in a single step by storing the the first matrix in
column major order and the second in row major order? The idea would be to
do a blockwise outer product and use the combiner to sum elements ahead of
the reducer. This could be done by reading a (block) column from the left
matrix and a (block) row from the second and emitting all of the (block)
elements of the outer product of this column and row. The key emitted with
these blocks would be the i,j index of the final location of each block in
the final product. The combiner and reducer could add all of the blocks
together and the use of the combiner would serve to substantially minimize
the amount of network traffic.
This alternative is equivalent to inverting the loop nesting in a
conventional matrix multiplication algorithm. It is also very closely
related to the way that cooccurrence counting is typically done in Hadoop.
Is your project an ongoing effort?
 Forwarded message 
From: Robin Anil <robin.anil@gmail.com>
Date: Tue, Dec 8, 2009 at 11:09 AM
Subject: Fwd: A MapReduce Algorithm for Matrix Multiplication
To: mahoutuser@lucene.apache.org
Cc: John Norstad <jnorstad@northwestern.edu>
 Forwarded message 
From: John Norstad <jnorstad@northwestern.edu>
Date: Tue, Dec 8, 2009 at 10:14 PM
Subject: A MapReduce Algorithm for Matrix Multiplication
To: MapReduce <mapreduceuser@hadoop.apache.org>
As an exercise while learning MapReduce, I developed an algorithm for
matrix multiplication and wrote it up on my web site. If you're
interested, it's at:
http://homepage.mac.com/j.norstad/matrixmultiply
I present the algorithm in English, as pseudocode, and as Java source
code for Hadoop.

John Norstad
Academic and Research Technologies
Northwestern University

Ted Dunning, CTO
DeepDyve
