[Rd] Performance of .C and .Call functions vs. native R code

From: asmahani <alireza.s.mahani_at_gmail.com>
Date: Wed, 13 Jul 2011 06:28:55 -0700 (PDT)


Hello,

I am in the process of writing an R extension for parallelized MCMC, with heavy use of compiled code (C++). I have been getting my feet wet by implementing a simple matrix-vector multiplication function in C++ (which calls a BLAS level 2 function dgemv), and comparing it to the '%*%' operator in R (which apparently calls a BLAS level 3 function dgemm).

Interestingly, I cannot replicate the performance of the R native operator, using either '.C' or '.Call'. The relative times are 17 (R), 30 (.C), and 26 (.Call). In other words, R native operator is 1.5x faster than my compiled code. Can you explain to me why this is? Through testing I strongly suspect that the BLAS function itself isn't what takes the bulk part of the time, but perhaps data transfer and other overhead associated with the calls (.C and .Call) are the main issues. Are there any ways to reach the performance level of native R code in this case?

Thank you,
Alireza Mahani

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