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From "Dmitriy Lyubimov (Commented) (JIRA)" <>
Subject [jira] [Commented] (MAHOUT-797) MapReduce SSVD: provide alternative B-pipeline per B=R' ^{-1} Y'A
Date Tue, 29 Nov 2011 19:19:40 GMT


Dmitriy Lyubimov commented on MAHOUT-797:

so what i think is that using R'^-1YA route is more promising overall for B_0 pipeline. 

The reason being that the version that uses explicit Q must pull in both Q and A while the
version that would use R^-1 would only go over A and Omega, but Omega is imaginary. So we
trade slightly more flops for less network I/O. This route may turn out significantly more
beneficial if we believe that size of Q >> size of A. I imagine this is not too common
though, but i have high hopes. 

we may eventually even deploy some automatic default based on subsampling A for sparsity.

For power iterations pipeline perhaps looses some of it since we don't have random matrix
in the works anymore. It seems possible that i might be able to do both Y_i = AB'_i-1 and
Y'_i Y_i   in one MR step there without ever forming AB' and that would be a huge plus instead.

But all of that changes the flow significantly enough to practically build a completely new
set of jobs (which is ok). 
> MapReduce SSVD: provide alternative B-pipeline per B=R' ^{-1} Y'A
> -----------------------------------------------------------------
>                 Key: MAHOUT-797
>                 URL:
>             Project: Mahout
>          Issue Type: Improvement
>    Affects Versions: 0.5, 0.6
>            Reporter: Dmitriy Lyubimov
>            Assignee: Dmitriy Lyubimov
>             Fix For: Backlog
>         Attachments: MAHOUT-797.pdf
>   Original Estimate: 48h
>  Remaining Estimate: 48h
> Since alternative flow using Cholesky decomposition is extremely easy to add to existing
computations of BB', I am thinking of just adding an option that chooses between B-pipeline
with QR step and B-pipeline with Y'Y-Cholesky step. 
> Ongoing work and some initial code (Y'Y step) is here
> I also want to fix what's left unfixed in MAHOUT-638.

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