[Rd] hash table clean-up

[Florent D.]

Hello,

I have noticed that the memory usage inside an R session increases as
more and more objects with unique names are created, even after they
are removed. Here is a small reproducible example:

> gc()
         used (Mb) gc trigger (Mb) max used (Mb)
Ncells 531720 14.2     899071 24.1   818163 21.9
Vcells 247949  1.9     786432  6.0   641735  4.9
>
> for (i in 1:100000) {
+ name <- paste("x", runif(1), sep="")
+ assign(name, NULL)
+ rm(list=name)
+ rm(name)
}
>
> gc()
         used (Mb) gc trigger (Mb) max used (Mb)
Ncells 831714 22.3    1368491 36.6  1265230 33.8
Vcells 680551  5.2    1300721 10.0   969572  7.4

It appears the increase in memory usage is due to the way R's
environment hash table operates
(http://cran.r-project.org/doc/manuals/R-ints.html#Hash-table): as
objects with new names are created, new entries are made in the hash
table; but when the objects are removed from the environment, the
corresponding entries are not deleted.

I hope you will agree the growth in memory size is an undesirable
feature and can address the issue in a future release. If not, please
let me know why you think it should remain this way.

I believe a fix could be made around the time the hash table is
resized, where only non-removed items would be kept. I can try to make
those changes to src/main/envir.c myself, but C is not my area of
expertise. So if you beat me to it, please let me know.

Thank you,
Florent.