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https://issues.apache.org/jira/browse/MAHOUT-391?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=12864851#action_12864851
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Sean Owen commented on MAHOUT-391:
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Hmm, I got similar results from a crude test, generating user vectors. The
on-disk size was negligibly different, and the reducer took longer by a notable
amount. I just can't figure out why it isn't a win -- could be a bad test, or
that there's some misunderstood interaction with Hadoop, but I won't commit
until it's sorted. More testing needed.
> Make vector more space efficient with variable-length encoding, et al
> ---------------------------------------------------------------------
>
> Key: MAHOUT-391
> URL: https://issues.apache.org/jira/browse/MAHOUT-391
> Project: Mahout
> Issue Type: Improvement
> Affects Versions: 0.3
> Reporter: Sean Owen
> Assignee: Sean Owen
> Priority: Minor
> Fix For: 0.4
>
> Attachments: MAHOUT-391.patch
>
>
> There are a few things we can do to make Vector representations smaller on
> disk:
> - Use variable-length encoding for integer values like size and element
> indices in sparse representations
> - Further, delta-encode indices in sequential representations
> - Let caller specify that precision isn't crucial in values, allowing it to
> store values as floats
> Since indices are usually small-ish, I'd guess this saves 2 bytes or so on
> average, out of 12 bytes per element now.
> Using floats where applicable saves another 4. Not bad.
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