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https://issues.apache.org/jira/browse/SYSTEMML-2169?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16391250#comment-16391250
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Chamath Abeysinghe commented on SYSTEMML-2169:
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[~mboehm7], I would like to take this issue
> 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
> Priority: Major
> Labels: beginner
>
> 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 (i.e., within {{processInstruction}}). Given the unlimited
> number of inputs, this runtime approach seems more appropriate than dedicated
> physical operators 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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