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From "Adam Fuchs (JIRA)" <>
Subject [jira] [Commented] (ACCUMULO-3067) scan performance degrades after compaction
Date Thu, 21 Aug 2014 15:03:12 GMT


Adam Fuchs commented on ACCUMULO-3067:

I may be narrowing in on an explanation to this one. When running the tserver with -XX:+PrintCompilation
I can watch methods get optimized and deoptimized at runtime. The deoptimization/reoptimization
is definitely happening during _the event_. I suspect what is happening is that when only
scans are happening the runtime gathers stats about how the iterators are used and optimizes
accordingly. When the methods are reoptimized during the compaction they get different usage
stats, and are therefore optimized differently.

Is this a plausible hypothesis? If so, what can/should be done about it? Anybody want to write
a native system iterator stack? Should we come up with a way of tricking or seeding the compiler
with better stats?

> scan performance degrades after compaction
> ------------------------------------------
>                 Key: ACCUMULO-3067
>                 URL:
>             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
>  # 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:

>  * SourceSwitchingIterator
>  ** VersioningIterator
>  *** SynchronizedIterator
>  **** VisibilityFilter
>  ***** ColumnQualifierFilter
>  ****** ColumnFamilySkippingIterator
>  ******* DeletingIterator
>  ******** StatsIterator
>  ********* MultiIterator
>  ********** MemoryIterator
>  ********** ProblemReportingIterator
>  *********** HeapIterator
>  ************ RFile.LocalityGroupReader
> We can eliminate the weird condition by narrowing the set of iterators to:
>  * SourceSwitchingIterator
>  ** VisibilityFilter
>  *** ColumnFamilySkippingIterator
>  **** DeletingIterator
>  ***** StatsIterator
>  ****** MultiIterator
>  ******* MemoryIterator
>  ******* ProblemReportingIterator
>  ******** HeapIterator
>  ********* RFile.LocalityGroupReader
> There are other combinations that also 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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