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https://issues.apache.org/jira/browse/HADOOP-12107?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14596543#comment-14596543
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Colin Patrick McCabe commented on HADOOP-12107:
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Thanks for looking at this, [~sjlee0]. It looks like a good idea to fix this.
As [~mingma] commented, I think we need to have a single thread per JVM to do
this, rather than one thread per FileSystem object. We have a number of Hadoop
applications which create lots of FileSystem objects, and creating a thread per
FS object would just be too many threads. We could end up with a thread leak
which was worse than the memory leak described here. (I agree that
well-written applications should strive to avoid creating too many FileSystem
objects, but that's a separate issue...)
{code}
private synchronized <T> T visitAll(StatisticsAggregator<T> visitor) {
visitor.accept(rootData);
if (allData != null) {
{code}
The null check isn't needed any more since you changed allData to be
initialized in the constructor.
thanks
> 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
> Attachments: HADOOP-12107.001.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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