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From "James Taylor (JIRA)" <j...@apache.org>
Subject [jira] [Created] (HBASE-12790) Support fairness across parallelized scans
Date Wed, 31 Dec 2014 03:20:13 GMT
James Taylor created HBASE-12790:
------------------------------------

             Summary: Support fairness across parallelized scans
                 Key: HBASE-12790
                 URL: https://issues.apache.org/jira/browse/HBASE-12790
             Project: HBase
          Issue Type: Bug
            Reporter: James Taylor


Some HBase clients parallelize the execution of a scan to reduce latency in getting back results.
This can lead to starvation with a loaded cluster and interleaved scans, since the RPC queue
will be ordered and processed on a FIFO basis. For example, if there are two clients, A &
B that submit largish scans at the same time. Say each scan is broken down into 100 scans
by the client (broken down into equal depth chunks along the row key), and the 100 scans of
client A are queued first, followed immediately by the 100 scans of client B. In this case,
client B will be starved out of getting any results back until the scans for client A complete.

One solution to this is to use the attached AbstractRoundRobinQueue instead of the standard
FIFO queue. The queue to be used could be (maybe it already is) configurable based on a new
config parameter. Using this queue would require the client to have the same identifier for
all of the 100 parallel scans that represent a single logical scan from the clients point
of view. With this information, the round robin queue would pick off a task from the queue
in a round robin fashion (instead of a strictly FIFO manner) to prevent starvation over interleaved
parallelized scans.



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