[jira] [Updated] (SYSTEMML-510) Generalize All Existing wdivmm Patterns

["Mike Dusenberry (JIRA)" <ji...@apache.org>]

     [ https://issues.apache.org/jira/browse/SYSTEMML-510?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel ]

Mike Dusenberry updated SYSTEMML-510:
-------------------------------------
    Issue Type: Bug  (was: Sub-task)
        Parent:     (was: SYSTEMML-435)

> Generalize All Existing wdivmm Patterns
> ---------------------------------------
>
>                 Key: SYSTEMML-510
>                 URL: https://issues.apache.org/jira/browse/SYSTEMML-510
>             Project: SystemML
>          Issue Type: Bug
>          Components: Compiler, Parser, Runtime
>            Reporter: Mike Dusenberry
>
> If we look at the inner loop of Poisson nonnegative matrix factorization (PNMF) in general, we update the factors as 
> {code}
> H = (H * (t(W) %*% (V/(W%*%H + 1e-17))))/t(colSums(W)
> W = (W * ((V/(W%*%H + 1e-17)) %*% t(H)))/t(rowSums(H))
> {code}.
> Notice the addition of the "1e-17" epsilon term in the denominators.  Mathematically, we need this in case any cell of W%*%H evaluates to zero so that we can avoid dividing by zero.  R needs this, but SystemML technically does not due to a fused operator, "wdivmm", that takes care of these situations (or this may be done in the general case?).  This fused operator is currently applied to the pattern {{t(W) %*% (V / %* (W %*% H))}}, amongst other similar patterns.  Ideally, this would easily apply to {{t(W) %*% (V/(W%*%H + 1e-17)}}, regardless of the unneeded epsilon term.  Currently, the addition of the epsilon term causes the algorithm to run in non-linear time (quad or exponential).  Initially, the behavior pointed towards the possibility of the optimizer avoiding the rewrite to the fused operator, resulting in naive computation, and non-linear growth in training time.  Further exploration seems to show that the rewrite is indeed still being applied, but there seems to also be a recursive nesting of the same rewrite over various regions of the above statements that is not found when the epsilon term is removed.
> The following is the full PNMF DML script used:
> {code}
> V = read($X)
> max_iteration = $maxiter
> rank = $rank
> n = nrow(V)
> m = ncol(V)
> range = 0.01
> W = Rand(rows=n, cols=rank, min=0, max=range, pdf="uniform")
> H = Rand(rows=rank, cols=m, min=0, max=range, pdf="uniform")
> loglik0 = sum(V*log(W%*%H)) - as.scalar(colSums(W)%*%rowSums(H))
> i=0
> while(i < max_iteration) {
>   # Addition of epsilon (1e-17) term causes script to run in non-linear time:
> 	H = (H * (t(W) %*% (V/(W%*%H + 1e-17))))/t(colSums(W))
> 	W = (W * ((V/(W%*%H + 1e-17)) %*% t(H)))/t(rowSums(H))
>   # Removal of epsilon works correctly:
>   #H = (H * (t(W) %*% (V/(W%*%H))))/t(colSums(W))
>   #W = (W * ((V/(W%*%H)) %*% t(H)))/t(rowSums(H))
> 	i = i + 1;
> 	print(i + "")
> }
> loglik = sum(V*log(W%*%H+1e-17)) - as.scalar(colSums(W)%*%rowSums(H))
> print("pnmf: " + loglik0 + " -> " + loglik)
> {code}



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