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https://issues.apache.org/jira/browse/YARN-4159?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14745201#comment-14745201
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He Tianyi commented on YARN-4159:
---------------------------------

Sorry, wrong project. Please close.

> False-positive of ACK slow log in DFSClient
> -------------------------------------------
>
>                 Key: YARN-4159
>                 URL: https://issues.apache.org/jira/browse/YARN-4159
>             Project: Hadoop YARN
>          Issue Type: Bug
>          Components: client
>    Affects Versions: 2.6.0
>            Reporter: He Tianyi
>            Priority: Minor
>
> This issue is related with code below:
> {noformat}
> if (duration > dfsclientSlowLogThresholdMs
>     && ack.getSeqno() != Packet.HEART_BEAT_SEQNO) {
>   DFSClient.LOG
>       .warn("Slow ReadProcessor read fields took " + duration
>           + "ms (threshold=" + dfsclientSlowLogThresholdMs + "ms); ack: "
>           + ack + ", targets: " + Arrays.asList(targets));
> } else if (DFSClient.LOG.isDebugEnabled()) {
>   DFSClient.LOG.debug("DFSClient " + ack);
> }
> {noformat}
> DFSClient prints slow log when awaited after unexpected amount of time 
> (usually 30000 ms). This is a good indicator for network or I/O performance 
> issue.
> However, there is scenario that this slow log is false-positive, i.e. a 
> reducer, (StageA) iterates over records with identical key, this takes 
> arbitrary amount of time, but generates no output. (StageB) Then, it output 
> arbitrary number of records when meet a different key.
> If one StageA lasts more than 30000 ms (as the example above), there will be 
> one or more slow log generated, which is not related to any HDFS performance 
> issue. 
> In general cases, user should not expect this, as they could be misguided.



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