Hi all,
I am using Spark 1.3.1 to write a Spectral Clustering algorithm. This really
confused me today. At first I thought my implementation is wrong. It turns
out it's an issue in MLlib. Fortunately, I've figured it out.
I suggest to add a hint on user document of MLlib ( as far as I know, there
have not been such hints yet) that indices of Local Sparse Vector must be
ordered in ascending manner. Because of ignorance of this point, I spent a
lot of time looking for reasons why computeSVD of RowMatrix did not run
correctly on Sparse data. I don't know the influence of Sparse Vector
without ordered indices on other functions, but I believe it is necessary to
let the users know or fix it. Actually, it's very easy to fix. Just add a
sortBy function in internal construction of SparseVector.
Here is an example to reproduce the affect of unordered Sparse Vector on
computeSVD.
================================================
//in spark-shell, Spark 1.3.1
import org.apache.spark.mllib.linalg.distributed.RowMatrix
import org.apache.spark.mllib.linalg.{SparseVector, DenseVector, Vector,
Vectors}
val sparseData_ordered = Seq(
Vectors.sparse(3, Array(1, 2), Array(1.0, 2.0)),
Vectors.sparse(3, Array(0,1,2), Array(3.0, 4.0, 5.0)),
Vectors.sparse(3, Array(0,1,2), Array(6.0, 7.0, 8.0)),
Vectors.sparse(3, Array(0,2), Array(9.0, 1.0))
)
val sparseMat_ordered = new RowMatrix(sc.parallelize(sparseData_ordered,
2))
val sparseData_not_ordered = Seq(
Vectors.sparse(3, Array(1, 2), Array(1.0, 2.0)),
Vectors.sparse(3, Array(2,1,0), Array(5.0,4.0,3.0)),
Vectors.sparse(3, Array(0,1,2), Array(6.0, 7.0, 8.0)),
Vectors.sparse(3, Array(2,0), Array(1.0,9.0))
)
val sparseMat_not_ordered = new
RowMatrix(sc.parallelize(sparseData_not_ordered, 2))
//apparently, sparseMat_ordered and sparseMat_not_ordered are essentially
the same matirx
//however, the computeSVD result of these two matrixes are different. Users
should be notified about this situation.
println(sparseMat_ordered.computeSVD(2,
true).U.rows.collect.mkString("\n"))
println("===================")
println(sparseMat_not_ordered.computeSVD(2,
true).U.rows.collect.mkString("\n"))
======================================================
The results are:
ordered:
[-0.10972870132786407,-0.18850811494220537]
[-0.44712472003608356,-0.24828866611663725]
[-0.784520738744303,-0.3080692172910691]
[-0.4154110101064339,0.8988385762953358]
not ordered:
[-0.10830447119599484,-0.1559341848984378]
[-0.4522713511277327,-0.23449829541447448]
[-0.7962382310594706,-0.3130624059305111]
[-0.43131320303494614,0.8453864703362308]
Looking into this issue, I can see it's reason locates in
RowMatrix.scala(line 629). The implementation of Sparse dspr here requires
ordered indices. Because it is scanning the indices consecutively to skip
empty columns.
-----
Feel the sparking Spark!
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