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Jason Lowe commented on HADOOP-12107:
-------------------------------------
I tried pulling this into branch-2.7. It comes in cleanly except for CHANGES.txt, but the
unit test failed. Subsequent runs showed it was flaky, sometimes passing and sometimes not.
Looking around I noticed a number of precommit builds have been complaining about this test.
I can reproduce the flaky test on trunk, so I filed HADOOP-12706. I'd really like this fix
backported, but I'd also rather not add another flaky test to 2.7 and 2.6.
[~sjlee0] or [~mingma] could you take a look into the unit test?
> long running apps may have a huge number of StatisticsData instances under FileSystem
> -------------------------------------------------------------------------------------
>
> Key: HADOOP-12107
> URL: https://issues.apache.org/jira/browse/HADOOP-12107
> Project: Hadoop Common
> Issue Type: Bug
> Components: fs
> Affects Versions: 2.7.0
> Reporter: Sangjin Lee
> Assignee: Sangjin Lee
> Priority: Critical
> Fix For: 2.8.0
>
> Attachments: HADOOP-12107.001.patch, HADOOP-12107.002.patch, HADOOP-12107.003.patch,
HADOOP-12107.004.patch, HADOOP-12107.005.patch
>
>
> We observed with some of our apps (non-mapreduce apps that use filesystems) that they
end up accumulating a huge memory footprint coming from {{FileSystem$Statistics$StatisticsData}}
(in the {{allData}} list of {{Statistics}}).
> Although the thread reference from {{StatisticsData}} is a weak reference, and thus can
get cleared once a thread goes away, the actual {{StatisticsData}} instances in the list won't
get cleared until any of these following methods is called on {{Statistics}}:
> - {{getBytesRead()}}
> - {{getBytesWritten()}}
> - {{getReadOps()}}
> - {{getLargeReadOps()}}
> - {{getWriteOps()}}
> - {{toString()}}
> It is quite possible to have an application that interacts with a filesystem but does
not call any of these methods on the {{Statistics}}. If such an application runs for a long
time and has a large amount of thread churn, the memory footprint will grow significantly.
> The current workaround is either to limit the thread churn or to invoke these operations
occasionally to pare down the memory. However, this is still a deficiency with {{FileSystem$Statistics}}
itself in that the memory is controlled only as a side effect of those operations.
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