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From "Chris Schneider (JIRA)" <j...@apache.org>
Subject [jira] Commented: (NUTCH-246) segment size is never as big as topN or crawlDB size in a distributed deployement
Date Tue, 11 Apr 2006 15:13:26 GMT
    [ http://issues.apache.org/jira/browse/NUTCH-246?page=comments#action_12374049 ] 

Chris Schneider commented on NUTCH-246:
---------------------------------------

A few more details:

Stefan and I were able to reproduce this problem using either an injection set of 4500 URLs
or a larger set of DMOZ URLs. With the 4500 URL injection, only 653 URLs were generated for
the first segment, despite the fact that topN was set to 500K. I confirmed that nearly all
of the 4500 injected URLs passed our URL filer and were actually injected into the crawldb.

To eliminate the possibility that the bug had been fixed recently or was due to a code modification
that we'd made ourselves, we deployed yesterday's sandbox version of nutch (2006-04-10), including
hadoop-0.1.1.jar (though I believe that Stefan had to build it himself because the nutch-0.8-dev.jar
didn't match the source). We made the absolute minimum changes to nutch-site.xml, hadoop-site.xml,
and hadoop-env.sh in order to deploy this version properly in our cluster (1 jobtracker/namenode
machine, 10 tasktracker/datanode machines). However, we got the same results (i.e., very few
URLs actually generated).

This bug has apparently been present since at least change 382948, but I suspect that it may
have been present for the entire history of the mapreduce implementation of Nutch. It may
also be the root cause of NUTCH-136, the explanation for which has always left me somewhat
dissatisfied. Just because a nutch-site.xml containing default properties may override the
desired mapred properties (incorrectly) specified in one of the *-default.xml files, and may
therefore set mapred.map.tasks and mapred.reduce.tasks back to the defaults (2 and 1, respectively),
it's not clear to me exactly how/why you'd get only a fraction of topN URLs fetched. As Stefan
has suggested, it would actually seem more plausible if each tasktracker tried to fetch the
entire set of URLs in this case.

I would suggest that someone with a good understanding of the hadoop implementation investigate
the first generation job in fine detail, both for the case where the mapred properties are
specified in an appropriate manner and for the case where nutch-site.xml overrides the desired
properties, setting them back to the defaults.

> segment size is never as big as topN or crawlDB size in a distributed deployement
> ---------------------------------------------------------------------------------
>
>          Key: NUTCH-246
>          URL: http://issues.apache.org/jira/browse/NUTCH-246
>      Project: Nutch
>         Type: Bug

>     Versions: 0.8-dev
>     Reporter: Stefan Groschupf
>     Priority: Blocker
>      Fix For: 0.8-dev

>
> I didn't reopen NUTCH-136 since it is may related to the hadoop split.
> I tested this on two different deployement (with 10 ttrackers + 1 jobtracker and 9 ttracks
and 1 jobtracker).
> Defining map and reduce task number in a mapred-default.xml does not solve the problem.
(is in nutch/conf on all boxes)
> We verified that it is not  a problem of maximum urls per hosts and also not a problem
of the url filter.
> Looks like the first job of the Generator (Selector) already got to less entries to process.

> May be this is somehow releasted to split generation or configuration inside the distributed
jobtracker since it runs in a different jvm as the jobclient.
> However we was not able to find the source for this problem.
> I think that should be fixed before  publishing a nutch 0.8. 

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