[Rd] hash table clean-up

[Gabor Grothendieck]

On Sun, Mar 4, 2012 at 4:19 PM,  <luke-tierney at uiowa.edu> wrote:
> On Sun, 4 Mar 2012, Florent D. wrote:
>
>> Hello,
>>
>> I have noticed that the memory usage inside an R session increases as
>> more and more objects with unique names are created, even after they
>> are removed. Here is a small reproducible example:
>>
>>> gc()
>>
>>        used (Mb) gc trigger (Mb) max used (Mb)
>> Ncells 531720 14.2     899071 24.1   818163 21.9
>> Vcells 247949  1.9     786432  6.0 641735 4.9
>>>
>>>
>>> for (i in 1:100000) {
>>
>> + name <- paste("x", runif(1), sep="")
>> + assign(name, NULL)
>> + rm(list=name)
>> + rm(name)
>> }
>>>
>>>
>>> gc()
>>
>>        used (Mb) gc trigger (Mb) max used (Mb)
>> Ncells 831714 22.3    1368491 36.6  1265230 33.8
>> Vcells 680551  5.2    1300721 10.0   969572  7.4
>>
>> It appears the increase in memory usage is due to the way R's
>> environment hash table operates
>> (http://cran.r-project.org/doc/manuals/R-ints.html#Hash-table): as
>> objects with new names are created, new entries are made in the hash
>> table; but when the objects are removed from the environment, the
>> corresponding entries are not deleted.
>
>
> Your analysis is incorrect. What you are seeing is the fact that thea
> symbol or name objects used as keys are being added to the global
> symbol table and that is not garbage collected. I believe that too
> many internals rely on this for it to be changed any time soon.  It
> may be possible to have some symbols GC protected and others not, but
> again that would require very careful throught and implementation and
> isn't likely to be a priority anty time soon as far as I can see.
>
> There may be some value in having hash tables that use some form of
> uninterned symbols as keys at some point but that is a larger project
> that might be better provided by a contributed package, at least
> initially.
>

Does this apply to lists too or just environments?


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