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From "Gabor Gevay (JIRA)" <j...@apache.org>
Subject [jira] [Assigned] (FLINK-3722) The divisions in the InMemorySorters' swap/compare methods hurt performance
Date Wed, 27 Jul 2016 12:04:20 GMT

     [ https://issues.apache.org/jira/browse/FLINK-3722?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
]

Gabor Gevay reassigned FLINK-3722:
----------------------------------

    Assignee: Gabor Gevay

> The divisions in the InMemorySorters' swap/compare methods hurt performance
> ---------------------------------------------------------------------------
>
>                 Key: FLINK-3722
>                 URL: https://issues.apache.org/jira/browse/FLINK-3722
>             Project: Flink
>          Issue Type: Improvement
>          Components: Local Runtime
>            Reporter: Gabor Gevay
>            Assignee: Gabor Gevay
>            Priority: Minor
>              Labels: performance
>
> NormalizedKeySorter's and FixedLengthRecordSorter's swap and compare methods use divisions
(which take a lot of time \[1\]) to calculate the index of the MemorySegment and the offset
inside the segment. [~greghogan] reported on the mailing list \[2\] measuring a ~12-14% performance
effect in one case.
> A possibility to improve the situation is the following:
> The way that QuickSort mostly uses these compare and swap methods is that it maintains
two indices, and uses them to call compare and swap. The key observation is that these indices
are mostly stepped by one, and _incrementally_ calculating the quotient and modulo is actually
easy when the index changes only by one: increment/decrement the modulo, and check whether
the modulo has reached 0 or the divisor, and if it did, then wrap-around the modulo and increment/decrement
the quotient.
> To implement this, InMemorySorter would have to expose an iterator that would have the
divisor and the current modulo and quotient as state, and have increment/decrement methods.
Compare and swap could then have overloads that take these iterators as arguments.
> \[1\] http://www.agner.org/optimize/instruction_tables.pdf
> \[2\] http://apache-flink-mailing-list-archive.1008284.n3.nabble.com/DISCUSS-Macro-benchmarking-for-performance-tuning-and-regression-detection-td11078.html



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