Hello, I'm trying to create my own C code for use within R. While optimizing the code I've noticed that even while only using pointers to get my data to C the time needed still depends on data (vector) size.
To test this, I've created an empty C function to which I've send vectors containing various sizes of elements. The time needed for each call is measured and plotted. I would expect a flat line (a little above y=0) since the only thing send are pointers. What I do not expect is to see a linear climbing line when the vector size increases. Initializing the vectors isn't being measured, only the '.C' call to an empty C function, see below. Is there anything I'm missing that can explain this input-size dependent latency? The only reason I can think of is that these vectors are being copied along the way. What follows is both the R and C code which I use only for testing and a plot of both measurements with DUP=TRUE and DUP=FALSE: (RED: DUP=FALSE, GREEN: DUP=TRUE) http://www.nabble.com/file/p20368695/CandR.png R code: ---------- # sequence from 512 to 2^23 with 2^17 stepsize a <- seq(512, 2^23, 2^17) # storage for wall time h <- length(a); j <- length(a) for (i in 1:length(a)) { x <- as.double(1:a[i]) y <- as.double(x) # system.time()[3] is (actual) wall time h[i] <- system.time(.C("commTest", x, y, DUP=FALSE))[3] j[i] <- system.time(.C("commTest", x, y, DUP=TRUE))[3] x <- 0 y <- 0 } # plot: plot(a, h, type="l", col="red", xlab="Vector Size -->", ylab="Time in Seconds -->"); lines(a, j, col="green") C code: ----------- #include<R.h> extern "C" { void commTest(double* a, double* b); } /* * Empty function * Just testing communication costs between R --> C */ void commTest(double* a, double* b) { /* Do ab-so-lute-ly-nothing.. */ } System Details: --------------------- Linux gpu 2.6.18-6-amd64 #1 SMP Thu May 8 06:49:39 UTC 2008 x86_64 GNU/Linux R version 2.7.1 (2008-06-23) -- View this message in context: http://www.nabble.com/.C%28...%2C-DUP%3DFALSE%29-memory-costs-depending-on-input-size--tp20368695p20368695.html Sent from the R devel mailing list archive at Nabble.com. ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
