[Rd] Run garbage collector when too many open files

[Jan van der Laan]

Dear Luke,


Thanks. See below


On 07-08-18 17:07, luke-tierney using uiowa.edu wrote:
> In R 3.5 and later you should not need to gc() -- that should happen
> automatically within the connections code.

Could you elaborate on what has changed in R 3.5? As far as I can tell 
my problem also occurs in R 3.5 (my computer is still on 3.4.4; but I 
assume the solaris CRAN machine isn't). And what do you mean with 'the 
connections code'? Is there something I can so/should do to have the 
garbage collector be a bit more aggressive in cleaning up my mess?

>
> Nevertheless, I would recommend redesigning your approach to avoid
> hanging onto open file connections as these are a scarce resource.
> You can keep around your temporary files without having them open and
> only open/close them on access, with the close run in an on.exit or a
> tryCatch/finally clause.

I am afraid that this will have a large performance penalty. The files 
in question are memory mapped files from which code will reading and 
writing continuously in most cases. Of course, there will probably 
objects that are not used for large amounts of time that could be 
temporarily closed , but it will be a bit difficult for the package to 
detect which objects that will be. I would have to write my own 'garbage 
collector'.


Best,
Uwe

>
> Best,
>
> luke
>
> On Tue, 7 Aug 2018, Jan van der Laan wrote:
>
>> Dear Uwe,
>>
>> (When replying to your message, I sent the reply to r-devel and not 
>> r-package-devel, as Martin Meachler suggested that this thread would 
>> be a better fit for r-devel.)
>>
>> Thanks. In the example below I used rm() explicitly, but in general 
>> users wouldn't do that.
>>
>> One of the reasons for the large number of file handles is that 
>> sometimes unnamed temporary objects are created. For example:
>>
>>> library(ldat)
>>> libraty(lvec)
>>>
>>> a <- lvec(10, "integer")
>> OPENFILE '/tmp/RtmpVqkDsw/file32145169fb06/lvec3214753f2af0'
>>> b <- as_rvec(a[1:3])
>> OPENFILE '/tmp/RtmpVqkDsw/file32145169fb06/lvec32146a50f383'
>> OPENFILE '/tmp/RtmpVqkDsw/file32145169fb06/lvec3214484b652c'
>>> print(b)
>> [1] 0 0 0
>>>
>>>
>>> gc()
>> CLOSEFILE '/tmp/RtmpVqkDsw/file32145169fb06/lvec3214484b652c'
>> CLOSEFILE '/tmp/RtmpVqkDsw/file32145169fb06/lvec32146a50f383'
>>          used (Mb) gc trigger (Mb) max used (Mb)
>> Ncells  796936 42.6    1442291 77.1  1168576 62.5
>> Vcells 1519523 11.6    4356532 33.3  4740854 36.2
>>
>>
>> For debugging, I log when files are opened and closed. The call 
>> a[1:3] (which creates a slice of a) creates two temporary objects 
>> [1]. These are only deleted when I explicitly call gc() or on some 
>> other random moment in time.
>>
>> I hope this illustrates the problem better.
>>
>>
>> Best,
>> Jan
>>
>>
>> [1] One improvement would be to create less temporary files; often 
>> these contain only very little information that is better kept in 
>> memory. But that is only a partial solution.
>>
>>
>>
>>
>> On 07-08-18 15:24, Uwe Ligges wrote:
>>> Why not add functionality that allows to delete object + runs 
>>> cleanup code?
>>>
>>> Best,
>>> Uwe Ligges
>>>
>>>
>>>
>>> On 07.08.2018 14:26, Jan van der Laan wrote:
>>>>
>>>>
>>>> In my package I open handles to temporary files from c++, handles 
>>>> to them are returned to R through vptr objects. The files are 
>>>> deleted then the corresponding R-object is deleted and the garbage 
>>>> collector runs:
>>>>
>>>> a <- lvec(10, "integer")
>>>> rm(a)
>>>>
>>>> Then when the garbage collector runs the file is deleted. However, 
>>>> on some platforms (probably with lower limits on the maximum number 
>>>> of file handles a process can have open), I run into the problem 
>>>> that the garbage collector doesn't run often enough. In this case 
>>>> that means that another package of mine using this package 
>>>> generates an error when its tests are run.
>>>>
>>>> The simplest solution is to add some calls to gc() in my tests. But 
>>>> a more general/automatic solution would be nice.
>>>>
>>>> I thought about something in the lines of
>>>>
>>>> robust_lvec <- function(...) {
>>>>    tryCatch({
>>>>      lvec(...)
>>>>    }, error = function(e) {
>>>>      gc()
>>>>      lvec(...) # duplicated code
>>>>    })
>>>> }
>>>>
>>>> e.g. try to open a file, when that fails call the garbage collector 
>>>> and try again. However, this introduces duplicated code (in this 
>>>> case only one line, but that can be more), and doesn't help if it 
>>>> is another function that tries to open a file.
>>>>
>>>> Is there a better solution?
>>>>
>>>> Thanks!
>>>>
>>>> Jan
>>>>
>>>> ______________________________________________
>>>> R-package-devel using r-project.org mailing list
>>>> https://stat.ethz.ch/mailman/listinfo/r-package-devel
>>
>> ______________________________________________
>> R-devel using r-project.org mailing list
>> https://stat.ethz.ch/mailman/listinfo/r-devel
>>
>