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https://issues.apache.org/jira/browse/HADOOP-4988?page=com.atlassian.jira.plugin.system.issuetabpanels:all-tabpanel
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Vivek Ratan updated HADOOP-4988:
--------------------------------

    Attachment: 4988.2.patch

bq. If a queue has no capacity, we should not be giving a task. 

Good catch. I had forgotten about this check. That has been added, I've synced 
with trunk, and a new patch (4988.2.patch) is attached. I've run dos2unix on 
it, and the output of ant test-patch is below: 

{code}
     [exec] +1 overall.
     [exec]
     [exec]     +1 @author.  The patch does not contain any @author tags.
     [exec]
     [exec]     +1 tests included.  The patch appears to include 3 new or 
modified tests.
     [exec]
     [exec]     +1 javadoc.  The javadoc tool did not generate any warning 
messages.
     [exec]
     [exec]     +1 javac.  The applied patch does not increase the total number 
of javac compiler warnings.
     [exec]
     [exec]     +1 findbugs.  The patch does not introduce any new Findbugs 
warnings.
     [exec]
     [exec]     +1 Eclipse classpath. The patch retains Eclipse classpath 
integrity.
{code}


> An earlier fix, for HADOOP-4373, results in a problem with reclaiming 
> capacity when one or more queues have a capacity equal to zero
> ------------------------------------------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-4988
>                 URL: https://issues.apache.org/jira/browse/HADOOP-4988
>             Project: Hadoop Core
>          Issue Type: Bug
>          Components: contrib/capacity-sched
>            Reporter: Vivek Ratan
>            Priority: Blocker
>         Attachments: 4988.1.patch, 4988.2.patch
>
>
> HADOOP-4373 introduced a fix for queues with guaranteed capacity (gc) equal 
> to zero. Part of the fix was in the queue comparator used to sort queues. 
> Queues with gc=0 were placed at the end. This causes a problem with the code 
> for reclaiming capacity, which assumes that queues are sorted based on free 
> space available and that a queue with gc=0 is no different than a queue which 
> is running at capacity. Because of this, the following problem can arise: if 
> we have a system with at least one queue whose gc=0, we may fail to reclaim 
> capacity for some queues. 

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