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https://issues.apache.org/jira/browse/HDFS-11738?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=16122878#comment-16122878
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Vinayakumar B commented on HDFS-11738:
--------------------------------------
I see that, Following changes in
{{DFSInputStream#hedgedFetchBlockByteRange(..)}} present in both HDFS-11303 and
this jira,
{code}
futures.add(firstRequest);
+ Future<ByteBuffer> future = null;
try {
- Future<ByteBuffer> future = hedgedService.poll(
+ future = hedgedService.poll(
conf.getHedgedReadThresholdMillis(), TimeUnit.MILLISECONDS);
if (future != null) {
ByteBuffer result = future.get();
@@ -1142,16 +1143,18 @@ private void hedgedFetchBlockByteRange(LocatedBlock
block, long start,
}
DFSClient.LOG.debug("Waited {}ms to read from {}; spawning hedged "
+ "read", conf.getHedgedReadThresholdMillis(), chosenNode.info);
- // Ignore this node on next go around.
- ignored.add(chosenNode.info);
dfsClient.getHedgedReadMetrics().incHedgedReadOps();
// continue; no need to refresh block locations
} catch (ExecutionException e) {
- // Ignore
+ futures.remove(future);
} catch (InterruptedException e) {
throw new InterruptedIOException(
"Interrupted while waiting for reading task");
}
+ // Ignore this node on next go around.
+ // If poll timeout and the request still ongoing, don't consider it
+ // again. If read data failed, don't consider it either.
+ ignored.add(chosenNode.info);
} else {
// We are starting up a 'hedged' read. We have a read already
// ongoing. Call getBestNodeDNAddrPair instead of chooseDataNode.
{code}
I think its fair to commit HDFS-11303 first and give credit for [~jzhuge]'s
efforts, as it has associated test written along with the change.
I can update the patch again once HDFS-11303 committed. Anyway test in my patch
will fail even after HDFS-11303 is committed. i.e. HDFS-11303 is not exactly
the fix for this issue.
So lets get HDFS-11303 committed first.
> Hedged pread takes more time when block moved from initial locations
> --------------------------------------------------------------------
>
> Key: HDFS-11738
> URL: https://issues.apache.org/jira/browse/HDFS-11738
> Project: Hadoop HDFS
> Issue Type: Bug
> Components: hdfs-client
> Reporter: Vinayakumar B
> Assignee: Vinayakumar B
> Attachments: HDFS-11738-01.patch, HDFS-11738-02.patch
>
>
> Scenario :
> Same as HDFS-11708.
> During Hedge read,
> 1. First two locations fails to read the data in hedged mode.
> 2. chooseData refetches locations and adds a future to read from DN3.
> 3. after adding future to DN3, main thread goes for refetching locations in
> loop and stucks there till all 3 retries to fetch locations exhausted, which
> consumes ~20 seconds with exponential retry time.
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