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From "Josh Elser (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (PHOENIX-2940) Remove STATS RPCs from rowlock
Date Tue, 14 Jun 2016 19:04:27 GMT

    [ https://issues.apache.org/jira/browse/PHOENIX-2940?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15330177#comment-15330177
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Josh Elser commented on PHOENIX-2940:
-------------------------------------

I believe the current test failure in PhoenixServerRpcIT#testMetadataQos is because the client
doing the stats refresh isn't hitting the higher RPC priority like it did when it was happening
in the server. Although, I can't seem to understand why the code path is different (I thought
the RpcScheduler should be global for RSRpcServices and thus would have the same path whether
it was a client or the RS issuing the RPC). Maybe there's something different on the request
when it comes from inside the RS that I haven't found yet.

> Remove STATS RPCs from rowlock
> ------------------------------
>
>                 Key: PHOENIX-2940
>                 URL: https://issues.apache.org/jira/browse/PHOENIX-2940
>             Project: Phoenix
>          Issue Type: Improvement
>         Environment: HDP 2.3 + Apache Phoenix 4.6.0
>            Reporter: Nick Dimiduk
>            Assignee: Josh Elser
>             Fix For: 4.9.0
>
>         Attachments: PHOENIX-2940.001.patch, PHOENIX-2940.002.patch
>
>
> We have an unfortunate situation wherein we potentially execute many RPCs while holding
a row lock. This is problem is discussed in detail on the user list thread ["Write path blocked
by MetaDataEndpoint acquiring region lock"|http://search-hadoop.com/m/9UY0h2qRaBt6Tnaz1&subj=Write+path+blocked+by+MetaDataEndpoint+acquiring+region+lock].
During some situations, the [MetaDataEndpoint|https://github.com/apache/phoenix/blob/10909ae502095bac775d98e6d92288c5cad9b9a6/phoenix-core/src/main/java/org/apache/phoenix/coprocessor/MetaDataEndpointImpl.java#L492]
coprocessor will attempt to refresh it's view of the schema definitions and statistics. This
involves [taking a rowlock|https://github.com/apache/phoenix/blob/10909ae502095bac775d98e6d92288c5cad9b9a6/phoenix-core/src/main/java/org/apache/phoenix/coprocessor/MetaDataEndpointImpl.java#L2862],
executing a scan against the [local region|https://github.com/apache/phoenix/blob/10909ae502095bac775d98e6d92288c5cad9b9a6/phoenix-core/src/main/java/org/apache/phoenix/coprocessor/MetaDataEndpointImpl.java#L542],
and then a scan against a [potentially remote|https://github.com/apache/phoenix/blob/10909ae502095bac775d98e6d92288c5cad9b9a6/phoenix-core/src/main/java/org/apache/phoenix/coprocessor/MetaDataEndpointImpl.java#L964]
statistics table.
> This issue is apparently exacerbated by the use of user-provided timestamps (in my case,
the use of the ROW_TIMESTAMP feature, or perhaps as in PHOENIX-2607). When combined with other
issues (PHOENIX-2939), we end up with total gridlock in our handler threads -- everyone queued
behind the rowlock, scanning and rescanning SYSTEM.STATS. Because this happens in the MetaDataEndpoint,
the means by which all clients refresh their knowledge of schema, gridlock in that RS can
effectively stop all forward progress on the cluster.



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