[jira] [Created] (SYSTEMML-2039) Memory-inefficiency due to breath-first instruction generation

["Matthias Boehm (JIRA)" <ji...@apache.org>]

Matthias Boehm created SYSTEMML-2039:
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             Summary: Memory-inefficiency due to breath-first instruction generation 
                 Key: SYSTEMML-2039
                 URL: https://issues.apache.org/jira/browse/SYSTEMML-2039
             Project: SystemML
          Issue Type: Bug
            Reporter: Matthias Boehm


In large DAGs, the strict topological ordering (depth-first) instruction generation causes unnecessary memory-inefficiency and unnecessary evictions. Depth-first instruction generation ensures that all consumers of a DAG input (bound to a logical variable name) are executed before the logical variable is overwritten by an output. For example, in below script
{code}
E = A * B + C * D
F = (A + B) + (C + D)
{code}
we compute A*B, C*D, A+B, C+D, and subsequently AB+CD, (A+B)+(C+D), which causes unnecessary memory pressure in the lower levels of the DAG. Furthermore, this also causes poor temporal locality.

Instead, we should use a two-level approach where all intermediates are computed in a depth-first manner, and all transient writes are scheduled after these operations.



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