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https://issues.apache.org/jira/browse/HDFS-17962?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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ZanderXu resolved HDFS-17962.
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Resolution: Fixed
> NNTop may lose user metrics during expired window cleanup
> ---------------------------------------------------------
>
> Key: HDFS-17962
> URL: https://issues.apache.org/jira/browse/HDFS-17962
> Project: Hadoop HDFS
> Issue Type: Bug
> Reporter: Yue Wang
> Assignee: Yue Wang
> Priority: Major
> Labels: pull-request-available
>
> NNTop stores a RollingWindow for each (metric, user) pair in a
> ConcurrentHashMap.
> In RollingWindowManager#getTopUsersForMetric(), expired user windows are
> removed when getSum(time) returns 0. Previously, the cleanup used an iterator
> to remove the map entry, while RollingWindowManager#recordMetric() obtained a
> RollingWindow from the map and incremented it outside of any per-key atomic
> operation.
> This allows the following race:
> 1. A recordMetric() thread obtains an existing RollingWindow for a user.
> 2. A snapshot thread observes that the window sum is 0 and removes the
> user-to-window mapping.
> 3. The recordMetric() thread increments the RollingWindow it obtained earlier.
> 4. The increment is applied to an object that is no longer in the map, so the
> metric is lost from subsequent NNTop snapshots.
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