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From "Samarth Jain (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (HBASE-12790) Support fairness across parallelized scans
Date Tue, 07 Mar 2017 22:05:38 GMT

    [ https://issues.apache.org/jira/browse/HBASE-12790?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=15900259#comment-15900259

Samarth Jain commented on HBASE-12790:

Thanks for the pointer [~anoop.hbase]. I see that there is already an executor RWQueueRpcExecutor
that gets hold of the ScanRequest in the dispatch method. 

private boolean isScanRequest(final RequestHeader header, final Message param) {
    if (param instanceof ScanRequest) {
      // The first scan request will be executed as a "short read"
      ScanRequest request = (ScanRequest)param;
      return request.hasScannerId();
    return false;

  public boolean dispatch(final CallRunner callTask) throws InterruptedException {
       if (numScanQueues > 0 && isScanRequest(call.getHeader(), call.param)) {
           queueIndex = numWriteQueues + numReadQueues + scanBalancer.getNextQueue();

So yes, there is a way forward by utilizing the scan attribute for this purpose without having
to add an API to Operation. Having said that, looking at the isWriteRequest method in the
same class, I see that things can get gnarly/brittle/inefficient. 
private boolean isWriteRequest(final RequestHeader header, final Message param) {
    // TODO: Is there a better way to do this?
    if (param instanceof MultiRequest) {
      MultiRequest multi = (MultiRequest)param;
      for (RegionAction regionAction : multi.getRegionActionList()) {
        for (Action action: regionAction.getActionList()) {
          if (action.hasMutation()) {
            return true;


> 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_1.patch, HBASE-12790_5.patch,
HBASE-12790_callwrapper.patch, HBASE-12790.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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