[Rd] Objects not gc'ed due to caching (?) in R's S3 dispatch mechanism

Tomas Kalibera tomas.kalibera at gmail.com
Tue Mar 27 11:11:26 CEST 2018

On 03/27/2018 09:51 AM, Iñaki Úcar wrote:
> 2018-03-27 6:02 GMT+02:00  <luke-tierney at 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.


> Regards,
> Iñaki
>> Best,
>> luke
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