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https://issues.apache.org/jira/browse/MAHOUT-1286?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13754269#comment-13754269
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Peng Cheng commented on MAHOUT-1286:
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Hi Gokhan,

No problem, but it only has two files, I'll post the patch immediately. -Yours 
Peng
                
> Memory-efficient DataModel, supporting fast online updates and element-wise 
> iteration
> -------------------------------------------------------------------------------------
>
>                 Key: MAHOUT-1286
>                 URL: https://issues.apache.org/jira/browse/MAHOUT-1286
>             Project: Mahout
>          Issue Type: Improvement
>          Components: Collaborative Filtering
>    Affects Versions: 0.9
>            Reporter: Peng Cheng
>              Labels: collaborative-filtering, datamodel, patch, recommender
>             Fix For: 0.9
>
>         Attachments: InMemoryDataModel.java, InMemoryDataModelTest.java
>
>   Original Estimate: 336h
>  Remaining Estimate: 336h
>
> Most DataModel implementation in current CF component use hash map to enable 
> fast 2d indexing and update. This is not memory-efficient for big data set. 
> e.g. Netflix prize dataset takes 11G heap space as a FileDataModel.
> Improved implementation of DataModel should use more compact data structure 
> (like arrays), this can trade a little of time complexity in 2d indexing for 
> vast improvement in memory efficiency. In addition, any online recommender or 
> online-to-batch converted recommender will not be affected by this in 
> training process.

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