Matthias Boehm created SYSTEMML-2169:
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Summary: Spark nary cbind/rbind with broadcasts
Key: SYSTEMML-2169
URL: https://issues.apache.org/jira/browse/SYSTEMML-2169
Project: SystemML
Issue Type: Task
Reporter: Matthias Boehm
The introduction of nary cbind and rbinds in SYSTEMML-1986 added support for
operations like {{E = cbind(A,B,C,D)}} which concatenates the matrices A, B, C,
D column-wise without the need for intermediates as requires by traditional
binary cbind operations ({{cbind(cbind(cbind(A,B),C),D)}}). SystemML also
provides rewrites to automatically collapse chains of cbind or rbind operations
into their nary counter-parts.
However, for distributed spark operations, the binary cbind is still much
better optimized than the nary operation, which only provides a general case
operation based on repartition joins.
This tasks aims to address this by extending {{BuiltinNarySPInstruction}} at
runtime level. Given the unlimited number of inputs, this runtime approach
seems more appropriate than dedicated physical operations at compiler level. In
detail, we need to evaluate if a subset of input fits into the broadcast
budget, and if so provide alternative code path for nary cbind/rbind operations
with broadcast joins.
Note that distributed codegen operations have a similar characteristics of
unlimited inputs and already leverage broadcast variables when possible. Hence,
we can probably use a similar approach as done in {{SpoofSPInstruction}}.
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