On 03/27/2018 09:51 AM, Iñaki Úcar wrote:
2018-03-27 6:02 GMT+02:00 <luke-tier...@uiowa.edu>:
This has nothing to do with printing or dispatch per se. It is the
result of an internal register (R_ReturnedValue) being protected. It
gets rewritten whenever there is a jump, e.g. by an explicit return
call. So a simplified example is
new_foo <- function() {
e <- new.env()
reg.finalizer(e, function(e) message("Finalizer called"))
e
}
bar <- function(x) return(x)
bar(new_foo())
gc() # still in .Last.value
gc() # nothing
UseMethod essentially does a return call so you see the effect there.
Understood. Thanks for the explanation, Luke.
The R_ReturnedValue register could probably be safely cleared in more
places but it isn't clear exactly where. As things stand it will be
cleared on the next use of a non-local transfer of control, and those
happen frequently enough that I'm not convinced this is worth
addressing, at least not at this point in the release cycle.
I barely know the R internals, and I'm sure there's a good reason
behind this change (R 3.2.3 does not show this behaviour), but IMHO
it's, at the very least, confusing. When .Last.value is cleared, that
object loses the last reference, and I'd expect it to be eligible for
gc.
In my case, I was using an object that internally generates a bunch of
data. I discovered this because I was benchmarking the execution, and
I was running out of memory because the memory wasn't been freed as it
was supposed to. So I spent half of the day on this because I thought
I had a memory leak. :-\ (Not blaming anyone here, of course; just
making a case to show that this may be worth addressing at some
point). :-)
From the perspective of the R user/programmer/package developer, please
do not make any assumptions on when finalizers will be run, only that
they indeed won't be run when the object is still alive. Similarly, it
is not good to make any assumptions that "gc()" will actually run a
collection (and a particular type of collection, that it will be
immediately, etc). Such guarantees would too much restrict the design
space and potential optimizations on the R internals side - and for this
reason are typically not given in other managed languages, either. I've
seen R examples where most time had been wasted tracing live objects
because explicit "gc()" had been run in a tight loop. Note in Java for
instance, an explicit call to gc() had been eventually turned into a
hint only.
Once you start debugging when objects are collected, you are debugging R
internals - and surprises/changes between svn versions/etc should be
expected as well as changes in behavior caused very indirectly by code
changes somewhere else. I work on R internals and spend most of my time
debugging - that is unfortunately normal when you work on a language
runtime. Indeed, the runtime should try not to keep references to
objects for too long, but it remains to be seen whether and for what
cost this could be fixed with R_ReturnedValue.
Best
Tomas
Regards,
Iñaki
Best,
luke
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