[jira] [Created] (SPARK-9175) BLAS.gemm fails to update matrix C when alpha==0 and beta!=1

["Meihua Wu (JIRA)" <ji...@apache.org>]

Meihua Wu created SPARK-9175:
--------------------------------

             Summary: BLAS.gemm fails to update matrix C when alpha==0 and beta!=1
                 Key: SPARK-9175
                 URL: https://issues.apache.org/jira/browse/SPARK-9175
             Project: Spark
          Issue Type: Bug
          Components: MLlib
    Affects Versions: 1.4.0
            Reporter: Meihua Wu


In the BLAS wrapper, gemm is supposed to update matrix C to be alpha * A * B + beta * C. However, the current implementation will not update C as long as alpha == 0. This is incorrect when beta is not equal to 1. 

Example:
val p = 3 
val a = DenseMatrix.zeros(p,p)
val b = DenseMatrix.zeros(p,p)
var c = DenseMatrix.eye(p)
BLAS.gemm(0, a, b, 5, c)

c is unchanged in the Spark 1.4 even though it should be multiplied by 5 element-wise.

The bug is caused by the following in BLAS.gemm:
if (alpha == 0.0) {
  logDebug("gemm: alpha is equal to 0. Returning C.")
}

Will submit PR to fix this.



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