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https://issues.apache.org/jira/browse/MAHOUT-881?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13149109#comment-13149109
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Sean Owen commented on MAHOUT-881:
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(See my comments on dev@ too)
Why are there fewer operations? Both seem to do an insert if the new item is
known to be among the new top-N, and not otherwise.
Both versions allocate the ArrayList. I think you're quite right about the
addAll() and sort() though. Even if that's an implementation detail, it's real.
Those are a few extra arrays of size 10-ish on average, so I would imagine it's
not significant. But I also don't like the idea.
On the other hand, I have an unfounded suspicion that the implicit heap sort
here is slower in practice here than a simple quicksort, since that is
generally the case.
I would be surprised if this was not all but identical in performance. We could
also make some micro-wins here in other ways to avoid these couple extra
allocations. or we can use the Lucene-based queue and avoid heap sort. I have a
very slight aversion to changing to something equal but different, replacing
very standard java.util.PriorityQueue with merely "standard" Lucene
PriorityQueue.
If I'm wrong that this isn't equal -- I support it. If we all concludes it's
equal, or barely slower, I'm -0 on the change and really not fussed about it.
> Refactor TopItems to use Lucene's PriorityQueue and remove excessive sorting
> ----------------------------------------------------------------------------
>
> Key: MAHOUT-881
> URL: https://issues.apache.org/jira/browse/MAHOUT-881
> Project: Mahout
> Issue Type: Improvement
> Affects Versions: 0.6
> Reporter: Grant Ingersoll
> Assignee: Grant Ingersoll
> Priority: Minor
> Attachments: MAHOUT-881.patch
>
>
> TopItems.getTop*() all do a fair number of excessive operations that can be
> replaced by switching to using Lucene's PriorityQueue implementation, which
> is more efficient and faster than Java's built in PQ implementation.
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