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From "Vijay (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-4705) Speculative execution for Reads
Date Tue, 05 Mar 2013 08:30:12 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-4705?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13593230#comment-13593230
] 

Vijay commented on CASSANDRA-4705:
----------------------------------

Hey Wesley, I dont have it anymore, but you can simulate by running stress tool, perform manual
GC's (with some huge heap settings) on some the servers, or shut some of them down... You
will obviously see reduce in throughput because there are duplicate requests sent... but your
latency with SPE should be better than none!
                
> Speculative execution for Reads
> -------------------------------
>
>                 Key: CASSANDRA-4705
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-4705
>             Project: Cassandra
>          Issue Type: Improvement
>            Reporter: Vijay
>            Assignee: Vijay
>             Fix For: 2.0
>
>         Attachments: 0001-CASSANDRA-4705.patch, 0001-CASSANDRA-4705-v2.patch, 0001-CASSANDRA-4705-v3.patch,
0001-Refactor-to-introduce-AbstractReadExecutor.patch, 0002-Add-Speculative-execution-for-Row-Reads.patch
>
>
> When read_repair is not 1.0, we send the request to one node for some of the requests.
When a node goes down or when a node is too busy the client has to wait for the timeout before
it can retry. 
> It would be nice to watch for latency and execute an additional request to a different
node, if the response is not received within average/99% of the response times recorded in
the past.
> CASSANDRA-2540 might be able to solve the variance when read_repair is set to 1.0
> 1) May be we need to use metrics-core to record various Percentiles
> 2) Modify ReadCallback.get to execute additional request speculatively.

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