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https://issues.apache.org/jira/browse/MAHOUT-824?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13119116#comment-13119116
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Lance Norskog commented on MAHOUT-824:
--------------------------------------

Sure, this is fine. 

MemoryDiffStorage exposes the fact that it uses the RunningAverage(AndStdDev) 
classes. This was a vexing implementation leakage, and is why I had to add new 
constructors for FullRunningAverage(AndStdDev). If I went after it more, I 
would correct this.

Anyway- is there a paper somewhere that defines the error loss for the 
inconsequential diff pruning technique? Is there a better subsampler than 'stop 
after the first N entries'?
                
> FastByIDRunningAverage: Optimize SlopeOneRecommender by optimizing 
> MemoryDiffStorage
> ------------------------------------------------------------------------------------
>
>                 Key: MAHOUT-824
>                 URL: https://issues.apache.org/jira/browse/MAHOUT-824
>             Project: Mahout
>          Issue Type: Improvement
>            Reporter: Lance Norskog
>            Assignee: Sean Owen
>            Priority: Trivial
>             Fix For: 0.6
>
>         Attachments: MAHOUT-824.patch, MAHOUT-824.short.patch
>
>
> The SlopeOneRecommender has by far the best RMS of all of the online 
> recommenders in Mahout (that I've found). Unfortunately the implementation 
> also uses much more memory and is unuseable on my laptop.
> This patch optimizes memory (and speed) by folding 
> FastByIDMap<RunningAverage> into one class: FastByIDRunningAverage. This is 
> what it sounds like: a Long-addressable array of running averages (and 
> optionally standard deviation).

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