[Rcpp-devel] RcppArmadillo

[Simon Zehnder]

Thanks Dirk! 

I worked a lot on it already yesterday. 

Regarding OpenMP: I haven't yet found the underlying data structure for SEXPs but there should be something in the R Internals Guide.

As the first-touch-principle has to be applied when the memory is allocated, I would rather copy the data from an SEXP to an arma::mat and run the '#pragma' over the arma::mat.  So for allocating I have to pass an arma::mat an array, that is already allocated via first-touch. memptr() should do this work. I hope, though, that it does not copy the array :)

Best

Simon

On Feb 2, 2013, at 4:17 PM, Dirk Eddelbuettel <edd at debian.org> wrote:

> 
> Hi Simon,
> 
> Welcome to the list.
> 
> On 2 February 2013 at 14:47, Simon Zehnder wrote:
> | Dear Rcpp-Devels,
> | 
> | this list was suggested to me by Dirk Eddelbüttel in regard to a question using C++ Extensions in relation with the Armadillo library. 
> | 
> | At first I have to make compliments to the developers of Rcpp/RcppArmadillo. Dirk, Francois, this is a marvelous work! As someone programming a lot in C++ and using R Extensions regularly, it is cleaning away all this cumbersome programming dirt connected to SEXPs. 
> 
> Thanks.
> 
> | There are still remaining questions for me, which can be surely answered by the subscribers to this list: 
> | 
> | 1. I saw the Rcpp.package.skeleton function and I ask myself, if a similar offer is made for RcppArmadillo for including automatically the Armadillo library in a package? 
> 
> Sure -- there is RcppArmadillo.package.skeleton() in the RcppArmadillo
> package. Dito for RcppGSL and RcppEigen.
> 
> | 2. If I want to use Armadillo solely in the C++ files, not via inline functions as shown in the RcppArmadillo paper, should I use solely the Rcpp package? No, right? The RcppArmadillo package provides objects wrappers for Armadillo objects to be passed to R? 
> 
> If you want just C++ from R, use Rcpp.
> 
> If you want C++ and Armadillo from R, use RcppArmadillo. It depends on Rcpp
> and brings it in.
> 
> Etc pp for other libraries. Rcpp and RcppArmadillo should provide the
> scaffolding to build upon. 
> 
> | 3. My package is based on S4 classes and I saw the S4 class wrapping in the Rcpp package. I miss an example on this. Can you refer to any document or website for this issue? 
> 
> Look at the Rcpp documentation, including the recently introduced Rcpp
> Gallery at http://gallery.rcpp.org for some examples.  I don't use S4 all
> that much so I do not write many examples, but eg in RcppEigen context a few
> more are found (as some of that work was motivated bty sparse matrices which
> already have an S4 representation in R).
> 
> | 4. Further: What is your experience regarding performance with S4 classes and OOP in C++: Does it make a difference mapping the S4 class to a struct in C++ or using directly the attributes of the S4 class (like vectors, etc.) as Armadillo vectors etc. in C++?
> | As I work a lot on the HPC in Aachen/Germany together with some of the contributors to the OpenMP API, I am highly influenced by the parsimonious approach, i.e. use only basic objects in C++ to get high performance (although I know, that one of the main work now in the OpenMP API is the extension to complex/user-defined objects inside the #pragmas). 
> 
> As I said, I do not use S4 all that much. But I am in favour of OOP :)
> 
> | 5. Using OpenMP with RcppArmadillo: Up to now I used almost exclusively the Scythe Statistical Library (http://scythe.wustl.edu), which is pretty fast. I encountered lately problems with it, using parallel computing (OpenMP). Also important in this regard is the possibility to apply a 'first-touch-principle' where all my approaches failed in Scythe, due to the object structure. 
> | Now, I would like to use RccpArmadillo with OpenMP and 
> | 		a) I want to get best performance: how would I proceed?
> 
> Just do it. We have OpenMP examples in the Rcpp docs. Ensure you have locks
> from R, do not call back, and it tends to just work to be "multithreaded in
> chunks" at the C++ level..
> 
> Scythe was very important and a first C++ library for R when it came out. I
> could imagine that Armadillo is faster, but I have not seen comparisons.
> 	
> | 		b) I want to apply the 'first-touch-principle': where do I apply it? What is the internal data structure in Rcpp-/RcppArmadillo-Objects that allocates the memory?
> 
> It's whatever you would do with native R objects at the C level, only easier :)
> 
> | I am very excited now to start work with the Rcpp/RcppArmadillo package in my own one, which is at least planned to be pushed to CRAN one day. 
> 
> We look forward to your contributions.  These are open projects, so if you
> can think of patches to code or documentation, please let us know.
> 
> | I am looking forward to your answers
> 
> Hope this helps.
> 
> Dirk
> 
> | 
> | Best
> | 
> | Simon 
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> 
> -- 
> Dirk Eddelbuettel | edd at debian.org | http://dirk.eddelbuettel.com