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From "Andrew Purtell (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-12790) Support fairness across parallelized scans
Date Mon, 09 Nov 2015 17:27:11 GMT

    [ https://issues.apache.org/jira/browse/HBASE-12790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14996945#comment-14996945
] 

Andrew Purtell commented on HBASE-12790:
----------------------------------------

If we can all conceptually agree on round robin scheduling of connections then the gap here
is pretty small. 

bq. Opening a new HConnection takes three seconds. It's not feasible to pool enough HConnections
to support the concurrency we see over the various types of queries.

Then can we make this faster? 

For example, when creating a new Connection object, it is not strictly necessary this must
map 1:1 with communication channels at the RPC layer or TCP connections on the wire. Why can't
a new Connection transparently re-use the communication channel constructed by a previous
instance? In which case the distinction between the new Connection and previous is a logical
identifier. A logical identifier carried in requests that can be used to schedule round robin
on the server. A logical identifier which does not need to be exposed to the application.
The application has control of round-robin dispatch through its allocation of Connection objects.




> Support fairness across parallelized scans
> ------------------------------------------
>
>                 Key: HBASE-12790
>                 URL: https://issues.apache.org/jira/browse/HBASE-12790
>             Project: HBase
>          Issue Type: New Feature
>            Reporter: James Taylor
>            Assignee: ramkrishna.s.vasudevan
>              Labels: Phoenix
>         Attachments: AbstractRoundRobinQueue.java, HBASE-12790.patch, HBASE-12790_1.patch,
HBASE-12790_5.patch, HBASE-12790_callwrapper.patch, HBASE-12790_trunk_1.patch, PHOENIX_4.5.3-HBase-0.98-2317-SNAPSHOT.zip
>
>
> 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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