[jira] [Created] (HADOOP-12107) long running apps may have a huge number of StatisticsData instances under FileSystem

["Sangjin Lee (JIRA)" <ji...@apache.org>]

Sangjin Lee created HADOOP-12107:
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             Summary: 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: Minor


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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