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https://issues.apache.org/jira/browse/HADOOP-12475?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14957106#comment-14957106
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Sangjin Lee commented on HADOOP-12475:
--------------------------------------
+1 on [~Apache9]'s comment. A more correct way of using the ConcurrentHashMap
is to utilize its putIfAbsent() method:
{code}
connection = connections.get(remoteId);
if (connection == null) {
connection = new Connection(remoteId, serviceClass);
Connection existing = connections.putIfAbsent(remoteId, connection);
if (existing != null) {
// ditch the one you just created as there is already a connection
connection.close();
connection = existing;
}
}
{code}
> Replace guava Cache with ConcurrentHashMap for caching Connection in ipc
> Client
> -------------------------------------------------------------------------------
>
> Key: HADOOP-12475
> URL: https://issues.apache.org/jira/browse/HADOOP-12475
> Project: Hadoop Common
> Issue Type: Sub-task
> Components: conf, io, ipc
> Reporter: Walter Su
> Assignee: Walter Su
> Attachments: HADOOP-12475.01.patch
>
>
> quote [~daryn] from HADOOP-11772:
> {quote}
> CacheBuilder is obscenely expensive for concurrent map, and it requires
> generating unnecessary garbage even just to look up a key. Replace it with
> ConcurrentHashMap.
> I identified this issue that impaired my own perf testing under load. The
> slowdown isn't just the sync. It's the expensive of Connection's ctor
> stalling other connections. The expensive of ConnectionId#equals causes
> delays. Synch'ing on connections causes unfair contention unlike a sync'ed
> method. Concurrency simply hides this.
> {quote}
> BTW, guava Cache is heavyweight. Per local test, ConcurrentHashMap has better
> overal performance.
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