[Rd] Cost of method dispatching: was: when can we expect Prof Tierney's compiled R?

[Vadim Ogranovich]

> -----Original Message-----
> From: Prof Brian Ripley [mailto:ripley at stats.ox.ac.uk] 
> Sent: Wednesday, April 27, 2005 1:13 AM
> To: Vadim Ogranovich
> Cc: Luke Tierney; r-devel at stat.math.ethz.ch
> Subject: Re: [Rd] RE: [R] when can we expect Prof Tierney's 
> compiled R?
> 
> On Tue, 26 Apr 2005, Vadim Ogranovich wrote:
> 
...
> > The arithmetic shows that x[i]<- is still the bottleneck. I suspect 
> > that this is due to a very involved dispatching/search for the 
> > appropriate function on the C level. There might be 
> significant gain 
> > if loops somehow cached the result of the initial 
> dispatching. This is 
> > what you probably referred to as additional improvements in 
> the engine itself.
> 
> I'd be surprised if dispatching were the issue: have you 
> (C-level) profiled to find out?  Please do so: these 
> statements do tend to get perpetuated as fact.


For the record, I didn't profile the dispatching, so it is only my guess
and is not verified by C-level profiling.

The guess is based on reading the code and on the following timing on R
level:
> n = 1e6; iA = seq(2,n); x = double(n);
> f1 <- function(x, iA) for (i in iA) x[i] = c(1.0)
> f2 <- function(x, iA) for (i in iA) x = c(1.0)
> last.gc.time = gc.time(TRUE)
> system.time(f1(x, iA), gcFirst=TRUE)
[1] 3.50 0.01 3.52 0.00 0.00
> print(gc.time() - last.gc.time); last.gc.time = gc.time()
[1] 1.25 0.82 1.24 0.00 0.00
> system.time(f2(x, iA), gcFirst=TRUE)
[1] 0.76 0.00 0.77 0.00 0.00
> print(gc.time() - last.gc.time); last.gc.time = gc.time()
[1] 0.25 0.18 0.23 0.00 0.00

f1 and f2 are identical except that the first assigns to an element in
the vector (and thus goes through the method dispatching).

Originally I had thought that the number of allocations in f1 and in f2
must be the same, the c(1.0) call. But gc.time() shows that the number
of allocations in f1 is indeed, as Prof. Ripley suggests, bigger than in
f2. It is not clear to me where these extra allocations come from and
whether they are necessary. All x[i] = c(1.0) needs to do is to create a
new vector c(1.0), which is a step common between f1 and f2, and then
copy from the vector into x[i].

However even after discounting for gc.time the assignment to x[i] seems
to be heavy.

> 
> You cannot cache the result, as [<- can change the class of 
> x, as could other operations done by the rhs (e.g. if it were 
> x[i] <- g(x, i) the function g could change its argument).

Yes, however R may try to use the last method found and only when that
fails go for the full dispatch. This should give a lot of gain in a
typical case when the vars. types do not change.