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From "Chris Douglas (JIRA)" <j...@apache.org>
Subject [jira] Commented: (HADOOP-2774) Add counters to show number of key/values that have been sorted and merged in the maps and reduces
Date Fri, 21 Nov 2008 07:06:44 GMT

    [ https://issues.apache.org/jira/browse/HADOOP-2774?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12649617#action_12649617
] 

Chris Douglas commented on HADOOP-2774:
---------------------------------------

bq. The two counters are for counting the number of records read and the number of records
written, not for determining whether the records came from disk/memory.

I'm confused. The map needs to count spilled records as it writes to disk; that's straightforward.
The reduce- since some of its fetched segments are written directly to disk- either needs
a count from the map as in the original patch, or it should count records as it reads them
from disk. Why does the merger need two counters, particularly since the caller is the only
one that knows whether it's going to disk or to a reduce? When would the number of records
read from its segments differ from the number of records it ultimately emits? Passing an optional
object to the merge that gets pinged every time it emits a record is both straightforward
as an API change and sufficient for this particular use case. One could probably add the counter
to the Merger without changing IFile.Reader and get the same semantics. I like the symmetry
of adding counters to both, but either way is fine.

> Add counters to show number of key/values that have been sorted and merged in the maps
and reduces
> --------------------------------------------------------------------------------------------------
>
>                 Key: HADOOP-2774
>                 URL: https://issues.apache.org/jira/browse/HADOOP-2774
>             Project: Hadoop Core
>          Issue Type: Bug
>            Reporter: Owen O'Malley
>            Assignee: Ravi Gummadi
>             Fix For: 0.20.0
>
>         Attachments: HADOOP-2774.patch, HADOOP-2774.patch
>
>
> For each *pass* of the sort and merge, I would like a count of the number of records.
So for example, if the map output 100 records and they were sorted once, the counter would
be 100. If it spilled twice and was merged together, it would be 200. Clearly in a multi-level
merge, it may not be a multiple of the number of map output records. This would let the users
easily see if they have values like io.sort.mb or io.sort.factor set too low.

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