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

["Xiangrui Meng (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SPARK-9175?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Xiangrui Meng resolved SPARK-9175.
----------------------------------
       Resolution: Fixed
    Fix Version/s: 1.5.0
                   1.2.3
                   1.3.2
                   1.4.2

Issue resolved by pull request 7503
[https://github.com/apache/spark/pull/7503]

> 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: 1.2.2, 1.3.1, 1.4.1
>            Reporter: Meihua Wu
>            Assignee: Meihua Wu
>            Priority: Critical
>             Fix For: 1.4.2, 1.3.2, 1.2.3, 1.5.0
>
>
> 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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