Ke Xu created SSHD-646:
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Summary: ThreadGroup created in ThreadUtils is not destroyed and
causes memory leak
Key: SSHD-646
URL: https://issues.apache.org/jira/browse/SSHD-646
Project: MINA SSHD
Issue Type: Bug
Affects Versions: 1.0.0
Environment: linux
Reporter: Ke Xu
===============
Problem description
===============
In our application, we found that more than 250M memory were occupied by around
700,000 ThreadGroup objects (whose names are SshClient-nio2, timer,
ClientInputStreamPump). The Threads contained in these ThreadGroups and their
sub groups were all null.
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Analysis
===============
In ThreadUtils class there is an inner class named SshdThreadFactory. Inside
this class, it will create a ThreadGroup and attach it to the parent
Threadgroup:
group = new ThreadGroup(parentGroup, "sshd-" + effectiveName + "-group");
However, the destroy() method of the ThreadGroup object is never explicitely
called when relevent resources are closed.
According to ThreadGroup API, when a ThreadGroup is created with parent
ThreadGroup specified, a reference to this child ThreadGroup is created in the
parent. The destroy() method is the only way to make the parent to remove this
reference.
Thus in case the parent ThreadGroup is main, and there is a lot of ThreadGroup
created, the ThreadGoup objects are not collected by the system until program
exits. It will eventually occupy a lot of memory.
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Suggested solution
===============
A suggested solution is to mark the ThreadGroup object as daemon after it is
created in SshdThreadFactory, e.g.:
group = new ThreadGroup(parentGroup, "sshd-" + effectiveName +
"-group");
group.setDaemon(true);
According to API, when a ThreadGroup is marked as daemon, it is automatically
destroyed when its last thread is stopped or its last thread group is destroyed.
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