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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