When experimenting in an interactive R session, it’s very easy to create a problem instance where the RAM just isn’t quite enough to compute the solution. (At least it happened to me on more than one occasion.) A somewhat trivial example, but you get the idea:

# No problem nowadays
N <- 1000
dim(matrix(nrow = N, ncol = N))
## [1] 1000 1000
# Got enough RAM?
N <- 10000
dim(matrix(nrow = N, ncol = N))
## [1] 10000 10000
# Probably not...
N <- 100000
dim(matrix(nrow = N, ncol = N))
## Error: Swapping a huge amount of memory. Machine unusable, please reboot

Unless your machine is equipped with 40 GiB of RAM or more, running the last two lines will probably render it unusable for quite a while. This would not happen if the memory available to R was restricted to a sensible value.

For restricting memory, the function memory.limit() in the utils package is available. Unfortunately, it is implemented only for Windows – users of POSIX systems are referred to the ulimit functionality of the shell. There seems to be no way to limit the memory of an already running process; this also applies for RStudio sessions.

The ulimit package attemts to fill this gap by providing an interface to the getrlimit(2) and setrlimit(2) APIs, which are also used when you execute the ulimit built-in in your bash shell. Only a single function memory_limit is exported – it allows querying and optionally setting the maximum amount of virtual memory available to the running process (and also to child processes), this corresponds to the RLIMIT_AS resource of the aformentioned APIs. If you add the line

invisible(ulimit::memory_limit(6000))

to your .Rprofile, only 6000 MiB will be available for each newly started R session, including RStudio sessions. (This would be a sensible setting for a computer with 8 GiB RAM.) Now you are safeguarded against this kind of nuisance:

N <- 100000
dim(matrix(nrow = N, ncol = N))
## Error: cannot allocate vector of size 37.3 Gb

Issue another memory_limit call if you need to temporarily allow for more memory usage in this R session.