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From "James Taylor (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (PHOENIX-2940) Remove STATS RPCs from rowlock
Date Mon, 06 Jun 2016 18:40:21 GMT

    [ https://issues.apache.org/jira/browse/PHOENIX-2940?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15316951#comment-15316951
] 

James Taylor commented on PHOENIX-2940:
---------------------------------------

Nice work, [~elserj]. Couple of ideas:
- How about if we make the size of the stats cache configurable through a new QueryServices
config parameter?
- Might be simplest to populate the cache on demand when the stats are asked for instead of
through a timer task. If the client-side cache is too small, then the client might be querying
too often for the stats, but I'm not sure what the best way to prevent this would be. Ideally,
we really want to have a propensity to cache the stats that are asked for most frequently.
If we go the timer route, and the client-side cache is too small, then it just becomes less
and less likely that the stats are in the cache when we need them (essentially disabling stats).
 It'd help if we had PHOENIX-2675 so that tables that don't need stats wouldn't fill up the
cache.
- I think it's ok to remove the PTable.getTableStats() method, as an old client would just
get no table stats which we handle correctly today.

> 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
>
>
> 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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