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From "Daryn Sharp (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HADOOP-14033) Reduce fair call queue lock contention
Date Thu, 09 Feb 2017 21:05:41 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-14033?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15860202#comment-15860202
] 

Daryn Sharp commented on HADOOP-14033:
--------------------------------------

Sounds reasonable, but would prefer a separate jira since metrics don't exist today.  It'll
be important to ensure that metrics don't re-introduce similar synchronization overhead this
patch intends to remove.

> Reduce fair call queue lock contention
> --------------------------------------
>
>                 Key: HADOOP-14033
>                 URL: https://issues.apache.org/jira/browse/HADOOP-14033
>             Project: Hadoop Common
>          Issue Type: Sub-task
>          Components: ipc
>    Affects Versions: 2.7.0
>            Reporter: Daryn Sharp
>            Assignee: Daryn Sharp
>         Attachments: HADOOP-14033.patch
>
>
> Under heavy load the call queue may run dry yet clients experience high latency.
> The fcq requires producers and consumers to sync via a shared lock.  Polling consumers
hold the lock while scanning all sub-queues.  Consumers are serialized despite the sub-queues
being thread-safe blocking queues.  The effect is to cause other producers/consumers to frequently
park.
> The lock is unfair, so producers/consumers attempt to barge in on the lock.  The outnumbered
producers tend to remain blocked for an extended time.  As load increases and the queues fill,
the barging consumers drain the queues faster than the producers can fill it.
> Server metrics provide an illusion of healthy throughput, response time, and call queue
length due to starvation on the ingress.   Often as the load gets worse, the server looks
better.



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