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From "Liang Xie (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HDFS-6766) optimize ack notify mechanism to avoid thundering herd issue
Date Wed, 30 Jul 2014 02:18:39 GMT

    [ https://issues.apache.org/jira/browse/HDFS-6766?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14078791#comment-14078791
] 

Liang Xie commented on HDFS-6766:
---------------------------------

bq. should be the condition timeout configurable
Per my knowledge, those fixed timeout should be fine, because all of them are covered by a
while loop.

> optimize ack notify mechanism to avoid thundering herd issue
> ------------------------------------------------------------
>
>                 Key: HDFS-6766
>                 URL: https://issues.apache.org/jira/browse/HDFS-6766
>             Project: Hadoop HDFS
>          Issue Type: Improvement
>          Components: hdfs-client
>    Affects Versions: 3.0.0
>            Reporter: Liang Xie
>            Assignee: Liang Xie
>         Attachments: HDFS-6766.txt
>
>
> Currently, DFSOutputStream uses wait/notifyAll to coordinate ack receiving and ack waiting,
etc..
> say there're 5 threads(t1,t2,t3,t4,t5) wait for ack seq no: 1,2,3,4,5, once the no. 1
ack arrived, the "notifyAll" be called, so t2/t3/t4/t5 could do nothing except wait again.
> we can rewrite it with Condition class, with a fair policy(fifo), we can just make t1
be notified, so a number of context switch be saved.
> It's possible more than one thread waiting on the same ack seq no(e.g. no more data be
written between two flush operations), so once it happened, we need to notify those threads,
so i introduced a set to remember this seq no.
> In a simple HBase ycsb testing, the context switch number per second was reduced about
15%, and reduced sys cpu% about 6%(My HBase has new write model patch, i think the benefit
will be higher if w/o it)



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