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From "Todd Lipcon (Commented) (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (MAPREDUCE-3184) Improve handling of fetch failures when a tasktracker is not responding on HTTP
Date Tue, 25 Oct 2011 17:32:32 GMT

    [ https://issues.apache.org/jira/browse/MAPREDUCE-3184?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13135261#comment-13135261
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Todd Lipcon commented on MAPREDUCE-3184:
----------------------------------------

bq. eg does it always return user or system time
it returns the sum of the two. The docs aren't well written ("If the implementation distinguishes
between user mode time and system mode time, the returned CPU time is the amount of time that
the thread has executed in user mode or system mode") but looking at the source the intention
is clear.

Worst case, if an implementation returns only user or only system time, then the CPU usage
will be under-estimated, which is OK. As long as we don't over-estimate, it won't cause a
false shutdown.

bq. thread CPU measurement could be expensive in some Java virtual machine implementations

Looking at those sun bugs, I think "expensive" here is a relative term. For example, the bug
says "the submitted test case takes almost 4 seconds to do 100k CPU measurements" - 40us per
call. Given we make the call once every 15 seconds, I'm not too concerned. I think the "may
be expensive" warning is just to warn people not to sprinkle these calls throughout their
performance-sensitive code to do metrics, etc.

bq. Btw I think it's possible for a subsequent call to System#nanoTime to return a smaller
valu
I don't think so - nanotime is "time since a fixed but arbitrary point in the past". On Linux
it's implemented with  clock_gettime(CLOCK_MONOTONIC)
                
> Improve handling of fetch failures when a tasktracker is not responding on HTTP
> -------------------------------------------------------------------------------
>
>                 Key: MAPREDUCE-3184
>                 URL: https://issues.apache.org/jira/browse/MAPREDUCE-3184
>             Project: Hadoop Map/Reduce
>          Issue Type: Improvement
>          Components: jobtracker
>    Affects Versions: 0.20.205.0
>            Reporter: Todd Lipcon
>            Assignee: Todd Lipcon
>         Attachments: mr-3184.txt
>
>
> On a 100 node cluster, we had an issue where one of the TaskTrackers was hit by MAPREDUCE-2386
and stopped responding to fetches. The behavior observed was the following:
> - every reducer would try to fetch the same map task, and fail after ~13 minutes.
> - At that point, all reducers would report this failed fetch to the JT for the same task,
and the task would be re-run.
> - Meanwhile, the reducers would move on to the next map task that ran on the TT, and
hang for another 13 minutes.
> The job essentially made no progress for hours, as each map task that ran on the bad
node was serially marked failed.
> To combat this issue, we should introduce a second type of failed fetch notification,
used when the TT does not respond at all (ie SocketTimeoutException, etc). These fetch failure
notifications should count against the TT at large, rather than a single task. If more than
half of the reducers report such an issue for a given TT, then all of the tasks from that
TT should be re-run.

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