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From "Mukul Kumar Singh (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-11887) XceiverClientManager should close XceiverClient on eviction from cache
Date Mon, 05 Jun 2017 13:26:04 GMT

    [ https://issues.apache.org/jira/browse/HDFS-11887?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16036933#comment-16036933
] 

Mukul Kumar Singh commented on HDFS-11887:
------------------------------------------

Thanks for your comments [~cheersyang], I really appreciate them. I thought about your comments
and my reply is as follows.

1) A client is leaked if and only if it is not closed. So as you pointed out, if the client
with active reference is evicted from cache, then it will not be closed on eviction. However
please note that this client will be closed as part of {{releaseClient}} on this client, which
will decrement the reference count and will call {{cleanup}}

2) For the second part of the comment, the existing client connection will not be closed because
there is a reference count on the client and hence the "older instance" will not get connection
refused error because of this reference count.

3) To answer that question that the cache should keep all the active clients in the cache.
I feel that it is impossible for a cache with finite size to guarantee that, for example in
the attached patch, the cache max size is 256 connections(configurable). If all of these connections
are active and a new client is created, that will definitely cause eviction of one of the
existing connections even when it has a reference count on it. So the best cache behavior
can either provide a MFU/LRU cache behavior to cache these elements.

4) This patch ensures that the client is closed when both it has been evicted from cache as
well as there no active users of the client. Hence ensuring that there are no leaks.

Again I really appreciate all the comments and please point me to any issues which I may overlook
or have missed.
We can also setup a discussion to discuss this in detail. Thanks for your feedback :)

> XceiverClientManager should close XceiverClient on eviction from cache
> ----------------------------------------------------------------------
>
>                 Key: HDFS-11887
>                 URL: https://issues.apache.org/jira/browse/HDFS-11887
>             Project: Hadoop HDFS
>          Issue Type: Sub-task
>          Components: ozone
>            Reporter: Mukul Kumar Singh
>            Assignee: Mukul Kumar Singh
>         Attachments: HDFS-11887-HDFS-7240.001.patch, HDFS-11887-HDFS-7240.002.patch
>
>
> XceiverClientManager doesn't close client on eviction which can leak resources.
> {code}
> public XceiverClientManager(Configuration conf) {
> .
> .
> .
>             public void onRemoval(
>                 RemovalNotification<String, XceiverClientWithAccessInfo>
>                   removalNotification) {
>               // If the reference count is not 0, this xceiver client should not
>               // be evicted, add it back to the cache.
>               WithAccessInfo info = removalNotification.getValue();
>               if (info.hasRefence()) {
>                 synchronized (XceiverClientManager.this.openClient) {
>                   XceiverClientManager.this
>                       .openClient.put(removalNotification.getKey(), info);
>                 }
>               }
> {code}
> Also a stack overflow can be triggered because of putting the element back in the cache
on eviction.
> {code}
>                 synchronized (XceiverClientManager.this.openClient) {
>                   XceiverClientManager.this
>                       .openClient.put(removalNotification.getKey(), info);
>                 }
> {code}
> This bug will try to fix both of these cases.



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