Matrix-vector and matrix-matrix calculations fit nicely into the MapReduce style of computing. In this post I will only examine matrix-matrix calculation as described in [1, ch.2].

Suppose we have a p x q matrix M, whose element in row i and column j will be denoted m_ij and a q x r matrix N whose element in row j and column k is donated by n_jk then the product P = MN will be p x r matrix P whose element in row i and column k will be donated by P_ik, where P(i,k) = m_ij*n_jk

Metrices

We represent matrix M as a relation M(I,J,V), with tuples (i,j,m_ij), and matrix N as a relation N(J,K,W), with tuples (j,k,n_jk). Most matrices are sparse so large amount of cells have value zero. When we represent matrices in this form, we do not need to keep entries for the cells that have values of zero to save large amount of disk space. As input data files, we store matrix M and N on HDFS in following format:

M,i,j,m_ij

 M,0,0,10.0
 M,0,2,9.0
 M,0,3,9.0
 M,1,0,1.0
 M,1,1,3.0
 M,1,2,18.0
 M,1,3,25.2
 ....

N,j,k,n_jk

 N,0,0,1.0
 N,0,2,3.0
 N,0,4,2.0
 N,1,0,2.0
 N,3,2,-1.0
 N,3,6,4.0
 N,4,6,5.0
 N,4,0,-1.0
 ....

We will write Map and Reduce functions to process input files. Map and reduce functions will implement the following algorithms: Algorithns

Map function will produce key,value pairs from the input data as it is described in Algorithm 1. Reduce function uses the output of the Map function and performs the calculations and produces key,value pairs as described in Algorithm 2. All outputs are written to HDFS. The value in row i and column k of product matrix P will be:

Pik

Let me examine the algorithms on an example to explain better. Suppose we have two matrices, M, 2x3 matrix, and N, 3x2 matrix as follows:

MxN

The product P of MN will be as follows:

P of MxN

For matrix M, map task (Algorithm 1) will produce key,value pairs as follows:

(i,k), (M, j, m_ij)

m₁₁ = 1
(1,1), (M, 1, 1) for k = 1
(1,2), (M, 1, 1) for k = 2

m₁₂ = 2
(1,1), (M, 2, 2) for k = 1
(1,2), (M, 2, 2) for k = 2

.........

m₂₃ = 6
(2,1), (M, 3, 6) for k = 1
(2,2), (M, 3, 6) for k = 2

For matrix N, map task (Algorithm 2) will produce key,value pairs as follows:

(i,k),(N,j,n_jk)

n₁₁ = a
(1,1),(N,1,a) for i = 1
(2,1),(N,1,a) for i = 2

n₂₁ = c
(1,1),(N,2,c) for i = 1
(2,1),(N,2,c) for i = 2

n₃₁ = e
(1,1),(N,3,e) for i = 1
(2,1),(N,3,e) for i = 2

.......

n₃₂ = f
(1,2),(N,3,f) for i = 1
(2,2),(N,3,f) for i = 2

After combine operation the map task will return key,value pairs will look like as follows:

( (i,k), [ (M, j, m_ij ), (M, j, m_ij ),..., (N, j, n_jk), (N, j, n_jk), .... ] )
(1,1), [(M,1,1),(M,2,2),(M,3,3),(N,1,a,(N,2,c),(N,3,e)]
(1,2), [(M,1,1),(M,2,2),(M,3,3),(N,1,b,(N,2,d),(N,3,f)]
(2,1), [(M,1,4),(M,2,5),(M,3,6),(N,1,a,(N,2,c),(N,3,e)]
(2,2), [(M,1,4),(M,2,5),(M,3,6),(N,1,b,(N,2,d),(N,3,f)]

Note that the entries for the same key are grouped in the same list, which is performed by the framework. This output will be stored in HDFS and feed the reduce task as input.

Reduce task takes the key,value pairs as the input and process one key at a time. For each key it divides the values in two separate lists for M and N. As an example, it will create the following list for key
(1,1): [(M,1,1),(M,2,2),(M,3,3),(N,1,a,(N,2,c),(N,3,e)]

Reduce task sorts the values begin with M in one list and values begin with N in another list as follows:

l_M = [(M,1,1),(M,2,2),(M,3,3)]
l_N = [(N,1,a),(N,2,c),(N,3,e)]

then sums up the multiplication of m_ij and n_jk for each j as follows:

P(1,1) = 1a + 2c + 3e

The same computation applied to all input entries of reduce task.

Pik

for all i and k is then calculated as follows:_3

P(1,1) = 1a + 2c + 3e
P(1,2) = 1b + 2d + 3f
P(2,1) = 4a + 5c + 6e
P(2,2) = 4b + 5d + 6f

The product matrix P of MN is then generated as:

P of MxN

I have setup a single node Hadoop installation with HDFS and run the matrix calculation experiment on this installation. I used 1000 x 100 matrix M and 100 x 1000 matrix N with sparsity level of 0.3. This means each matrix has about 30K entries. The matrix les M and N stored in input directory on HDFS and the output of the computation is stored in output directory on HDFS.

Mapper and reducer classes are Map.java and Reduce.java respectively. Main method in the MatrixMultiply.java class the configuration parameters are being set as well as the input/output directories of MapReduce job [2].

The Mapper class extends org.apache.hadoop.mapreduce.Mapper class and implements the map task described in Algorithm 1 and creates the key,value pairs from the input files.

Reducer class extends org.apache.hadoop.mapreduce.Reducer class and implements the reduce task described in Algorithm 2 and creates the key,value pairs for the product matrix then writes its output to HDFS.

Check out this tutorial to setup your local environment to run Hadoop map-reduce jobs: http://hadoop.apache.org/docs/current/hadoop-mapreduce-client/hadoop-mapreduce-client-core/MapReduceTutorial.html

[1] Anand Rajaraman and Jerey David Ullman. Mining of Massive Datasets. Cambridge University Press, New York, NY, USA, 2011.

[2] One-step matrix multiplication with hadoop, [Online], 2014