Exceeding memory limit in R (even with 24GB RAM)

Question

I am trying to merge two dataframes: one has 908450 observations of 33 variables, and the other has 908450 observations of 2 variables.

dataframe2 <-merge(dataframe1, dataframe2, by="id")

I've cleared all other dataframes from working memory, and reset my memory limit (for a brand new desktop with 24 GB of RAM) using the code:

memory.limit(24576)

But, I'm still getting the error Cannot allocate vector of size 173.Mb.

Any thoughts on how to get around this problem?

Answer 1

To follow up on my comments, use data.table. I put together a quick example matching your data to illustrate:

library(data.table)

dt1 <- data.table(id = 1:908450, matrix(rnorm(908450*32), ncol = 32))
dt2 <- data.table(id = 1:908450, rnorm(908450))
#set keys
setkey(dt1, id)
setkey(dt2, id)
#check dims
> dim(dt1)
[1] 908450     33
> dim(dt2)
[1] 908450      2
#merge together and check system time:
> system.time(dt3 <- dt1[dt2])
   user  system elapsed 
   0.43    0.03    0.47 

So it took less than 1/2 second to merge together. I took a before and after screenshot watching my memory. Before the merge, I was using 3.4 gigs of ram. When I merged together, it jumped to 3.7 and leveled off. I think you'll be hard pressed to find something more memory or time efficient than that.

Before:

After:

Answer 2

As far as I can think of there's three solutions:

• Use datatables
• Use swap memory ( can be adjustable on *nix machines)
• Use sampling