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From "Branimir Lambov (JIRA)" <>
Subject [jira] [Commented] (CASSANDRA-8915) Improve MergeIterator performance
Date Tue, 05 May 2015 14:19:00 GMT


Branimir Lambov commented on CASSANDRA-8915:

bq. My optimisation... We could instead perform a merge, but even with this imperfect implementation
the result is typically half as many comparisons as NoEqual, and never more.

I think this is better served by other heap types (e.g. Fibonacci), which I was planning to
explore at a later time. Do you prefer to do this now?

You have convinced me, though, that avoiding that second comparison for {{consume}} is something
that must be done, so I added a [{{FlagEqual}} variation|]
of the non-collapsing approach that flags equality when it finds it. The flag is then sufficient
for {{consume}} and skipping over equal elements in the heap, providing equal comparison complexity
with {{AllEqual}} but without the need to remove/readd iterators. The benchmarks show it to
be on par with {{AllEqual}} in the random cases, and better than {{NoEqual}} in the limited
overlap scenarios.

Also, I don't see any links in your comment. Could I also look at your experiments?

> Improve MergeIterator performance
> ---------------------------------
>                 Key: CASSANDRA-8915
>                 URL:
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Branimir Lambov
>            Assignee: Branimir Lambov
>            Priority: Minor
>             Fix For: 3.x
> The implementation of {{MergeIterator}} uses a priority queue and applies a pair of {{poll}}+{{add}}
operations for every item in the resulting sequence. This is quite inefficient as {{poll}}
necessarily applies at least {{log N}} comparisons (up to {{2log N}}), and {{add}} often requires
another {{log N}}, for example in the case where the inputs largely don't overlap (where {{N}}
is the number of iterators being merged).
> This can easily be replaced with a simple custom structure that can perform replacement
of the top of the queue in a single step, which will very often complete after a couple of
comparisons and in the worst case scenarios will match the complexity of the current implementation.
> This should significantly improve merge performance for iterators with limited overlap
(e.g. levelled compaction).

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