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From "Mike Drob (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (ACCUMULO-3067) scan performance degrades after compaction
Date Tue, 19 Aug 2014 21:33:18 GMT

    [ https://issues.apache.org/jira/browse/ACCUMULO-3067?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14102878#comment-14102878
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Mike Drob commented on ACCUMULO-3067:
-------------------------------------

JIT call optimization on the iterators shouldn't be happening, since you already have lots
of targets for the methods. It typically happens when you only have single target, e.g. your
code only uses one instance of a {{Runnable}} so the JIT knows that every call to {{run()}}
will resolve to the same object. You have lots (more than 2) of {{SKVI}}s, so the call resolution
isn't happening to begin with.

I haven't looked at your code, but in your screen shot it looks like you have nearly filled
DFS. That usually leads to bad things.

Something happening on a different table indicates that maybe a cache is getting filled with
data that is not yours and you're getting more cache misses than you were before. Try dropping
OS buffers and see if that makes a difference?

> scan performance degrades after compaction
> ------------------------------------------
>
>                 Key: ACCUMULO-3067
>                 URL: https://issues.apache.org/jira/browse/ACCUMULO-3067
>             Project: Accumulo
>          Issue Type: Bug
>          Components: tserver
>         Environment: Macbook Pro 2.6 GHz Intel Core i7, 16GB RAM, SSD, OSX 10.9.4, single
tablet server process, single client process
>            Reporter: Adam Fuchs
>         Attachments: Screen Shot 2014-08-19 at 4.19.37 PM.png, accumulo_query_perf_test.tar.gz
>
>
> I've been running some scan performance tests on 1.6.0, and I'm running into an interesting
situation in which query performance starts at a certain level and then degrades by ~15% after
an event. The test follows roughly the following scenario:
>  # Single tabletserver instance
>  # Load 100M small (~10byte) key/values into a tablet and let it finish major compacting
>  # Disable the garbage collector (this makes the time to _the event_ longer)
>  # Restart the tabletserver
>  # Repeatedly scan from the beginning to the end of the table in a loop
>  # Something happens on the tablet server, like one of {idle compaction of metadata table,
forced flush of metadata table, forced compaction of metadata table, forced flush of trace
table}
>  # Observe that scan rates dropped by 15-20%
>  # Observe that restarting the scan will not improve performance back to original level.
Performance only gets better upon restarting the tablet server.
> I've been able to get this not to happen by removing iterators from the iterator tree.
It doesn't seem to matter which iterators, but removing a certain number both improves performance
(significantly) and eliminates the degradation problem. The default iterator tree includes
(VersioningIterator, SynchronizedIterator, VisibilityFilter, ColumnQualifierFilter, ColumnFamilySkippingIterator,
DeletingIterator, StatsIterator, MultiIterator, (MemoryIterator*, RFile.Reader*)). Narrowing
this to VisibilityFilter, ColumnFamilySkippingIterator, DeletingIterator, StatsIterator, MultiIterator,
(MemoryIterator*, RFile.Reader*)) eliminates the weird condition. There are also other combinations
that perform much better than the default. I haven't been able to isolate this problem to
a single iterator, despite removing each iterator one at a time.
> Anybody know what might be happening here? Best theory so far: the JVM learns that iterators
can be used in a different way after a compaction, and some JVM optimization like JIT compilation,
branch prediction, or automatic inlining stops happening.



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