Do environments make copies?
Hi Folks, Thanks for all your replies and input. In particular, thanks Luke, for explaining what is happening under the covers. In retrospect, my example using save and load to demonstrate the problem I was having was a mistake - I was trying to reproduce the problem I was having in a simple enough way and I thought save and load were showing the same problem (i.e. an extra copy was being made). After carefully examining my gc() traces, I've come to realize that while there are copies being made, there is nothing unexpected about it - the failure to allocate memory is really because R is hitting the 3GB address limit imposed by my linux box during processing. So as Luke suggests, maybe 32 bits is not the right platform for handling large data in R. On the other hand, I think the problem can be somewhat alleviated (though not eliminated) if we did garbage collection of temporary variables immediately so that we can reduce the memory footprint and the fragmentation problem that malloc() is going to be faced with (gctorture() is probably too extreme :-). Most of the problems that I am having are in the coercion routines which do create temporary copies. So in code of the form x = as.vector(x), it would be nice if the old value of x was garbage collected (i.e. if there were no references to it) nawaaz
Luke Tierney wrote:
On Thu, 24 Feb 2005, Berton Gunter wrote:
I was hoping that one of the R gurus would reply to this, but as they have't (thus far) I'll try. Caveat emptor! First of all, R passes function arguments by values, so as soon as you call foo(val) you are already making (at least) one other copy of val for the call.
Conceptually you have a copy, but internally R trieas to use a copy-on-modify strategy to avaoid copying unless necessary. THere are conservative approximations involved, so there is more copying than one might like but definitely not as much as this.
Second,you seem to implicitly make the assumption that assign(..., env=) uses a pointer to point to the values in the environment. I do not know how R handles environments and assignments like this internally, but your data seems to indicate that it copies the value and does not merely point to it (this is where R Core folks can shed more authoritative light).
This assignment does just store the pointer.
Finally, it makes perfect sense to me that, as a data structure, the environment itself may be small even if it effectively points to (one of several copies of) large objects, so that object.size(an.environment) could be small although the environment may "contain" huge arguments. Again, the details depend on the precise implementation and need clarification by someone who actually knows what's going on here, which ain't me. I think the important message is that you shouldn't treat R as C, and you shouldn't try to circumvent R's internal data structures and conventions. R is a language designed to implements Chambers's S model of "Programming with Data." Instead of trying to fool R to handle large data sets, maybe you should consider whether you really **need** all the data in R at one time and if sensible partitioning or sampling to analyze only a portion or portions of the data might not be a more effective strategy.
R can do quite a reasonable job with large data sets on a resonable
platform. A 32 bit platform is not a reasonable one on which to use R
with 800 MB chunks of data. Automatic memory management combined with
the immutable vector semantics require more elbow room than that. If
you really must use data of this size on a 32-bit platform you will
probably be muchhappier using a limited amoutn of C code and external
pointers.
As to what is happening in this example: look at the default parent
used by new.env and combine that with the fact that the serialization
code does not preserve sharing of atomic objects. The two references
to the large object are shared in the original session but lead to two
large objects in the saved image and the load. Using
ref <- list(env = new.env(parent = .GlobalEnv))
in new.ref avoids the second copy both in the saved image and after
loading.
luke
-----Original Message----- From: r-help-bounces@stat.math.ethz.ch [mailto:r-help-bounces@stat.math.ethz.ch] On Behalf Of Nawaaz Ahmed Sent: Thursday, February 24, 2005 10:36 AM To: r-help@stat.math.ethz.ch Subject: [R] Do environments make copies? I am using environments to avoid making copies (by keeping references). But it seems like there is a hidden copy going on somewhere - for example in the code fragment below, I am creating a reference to "y" (of size 500MB) and storing the reference in object "data". But when I save "data" and then restore it in another R session, gc() claims it is using twice the amount of memory. Where/How is this happening? Thanks for any help in working around this - my datasets are just not fitting into my 4GB, 32 bit linux machine (even though my actual data size is around 800MB) Nawaaz
new.ref <- function(value = NULL) {
+ ref <- list(env = new.env())
+ class(ref) <- "refObject"
+ assign("value", value, env = ref$env)
+ ref
+ }
object.size(y)
[1] 587941404
y.ref = new.ref(y) object.size(y.ref)
[1] 328
data = list() data$y.ref = y.ref object.size(data)
[1] 492
save(data, "data.RData")
... run R again ===========
load("data.RData")
gc()
used (Mb) gc trigger (Mb) Ncells 141051 3.8 350000 9.4 Vcells 147037925 1121.9 147390241 1124.5
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