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https://issues.apache.org/jira/browse/ACCUMULO-2827?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14038171#comment-14038171
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Keith Turner commented on ACCUMULO-2827:
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I ran the test against 1.5.2-SNAPSHOT
> HeapIterator optimization
> -------------------------
>
> Key: ACCUMULO-2827
> URL: https://issues.apache.org/jira/browse/ACCUMULO-2827
> Project: Accumulo
> Issue Type: Improvement
> Affects Versions: 1.5.1, 1.6.0
> Reporter: Jonathan Park
> Assignee: Jonathan Park
> Priority: Minor
> Fix For: 1.5.2, 1.6.1, 1.7.0
>
> Attachments: ACCUMULO-2827-compaction-performance-test.patch,
> ACCUMULO-2827.0.patch.txt, accumulo-2827.raw_data, new_heapiter.png,
> old_heapiter.png, together.png
>
>
> We've been running a few performance tests of our iterator stack and noticed
> a decent amount of time spent in the HeapIterator specifically related to
> add/removal into the heap.
> This may not be a general enough optimization but we thought we'd see what
> people thought. Our assumption is that it's more probable that the current
> "top iterator" will supply the next value in the iteration than not. The
> current implementation takes the other assumption by always removing +
> inserting the minimum iterator back into the heap. With the implementation of
> a binary heap that we're using, this can get costly if our assumption is
> wrong because we pay the log penalty of percolating up the iterator in the
> heap upon insertion and again when percolating down upon removal.
> We believe our assumption is a fair one to hold given that as major
> compactions create a log distribution of file sizes, it's likely that we may
> see a long chain of consecutive entries coming from 1 iterator.
> Understandably, taking this assumption comes at an additional cost in the
> case that we're wrong. Therefore, we've run a few benchmarking tests to see
> how much of a cost we pay as well as what kind of benefit we see. I've
> attached a potential patch (which includes a test harness) + image that
> captures the results of our tests. The x-axis represents # of repeated keys
> before switching to another iterator. The y-axis represents iteration time.
> The sets of blue + red lines varies in # of iterators present in the heap.
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