[Rd] Is it possible to increase MAX_NUM_DLLS in future R releases?

[Martin Morgan]

On 05/04/2016 05:15 AM, Prof Brian Ripley wrote:
> On 04/05/2016 08:44, Martin Maechler wrote:
>>>>>>> Qin Zhu <qinzhu at outlook.com>
>>>>>>>      on Mon, 2 May 2016 16:19:44 -0400 writes:
>>
>>      > Hi,
>>      > I’m working on a Shiny app for statistical analysis. I ran into
>> this "maximal number of DLLs reached" issue recently because my app
>> requires importing many other packages.
>>
>>      > I’ve posted my question on stackoverflow
>> (http://stackoverflow.com/questions/36974206/r-maximal-number-of-dlls-reached
>> <http://stackoverflow.com/questions/36974206/r-maximal-number-of-dlls-reached>).
>>
>>
>>      > I’m just wondering is there any reason to set the maximal
>> number of DLLs to be 100, and is there any plan to increase it/not
>> hardcoding it in the future? It seems many people are also running
>> into this problem. I know I can work around this problem by modifying
>> the source, but since my package is going to be used by other people,
>> I don’t think this is a feasible solution.
>>
>>      > Any suggestions would be appreciated. Thanks!
>>      > Qin
>>
>> Increasing that number is of course "possible"... but it also
>> costs a bit (adding to the fixed memory footprint of R).
>
> And not only that.  At the time this was done (and it was once 50) the
> main cost was searching DLLs for symbols.  That is still an issue, and
> few packages exclude their DLL from symbol search so if symbols have to
> searched for a lot of DLLs will be searched.  (Registering all the
> symbols needed in a package avoids a search, and nowadays by default
> searches from a namespace are restricted to that namespace.)
>
> See
> https://cran.r-project.org/doc/manuals/r-release/R-exts.html#Registering-native-routines
> for some further details about the search mechanism.
>
>> I did not set that limit, but I'm pretty sure it was also meant
>> as reminder for the useR to "clean up" a bit in her / his R
>> session, i.e., not load package namespaces unnecessarily. I
>> cannot yet imagine that you need > 100 packages | namespaces
>> loaded in your R session. OTOH, some packages nowadays have a
>> host of dependencies, so I agree that this at least may happen
>> accidentally more frequently than in the past.
>
> I am not convinced that it is needed.  The OP says he imports many
> packages, and I doubt that more than a few are required at any one time.
>   Good practice is to load namespaces as required, using requireNamespace.

Extensive package dependencies in Bioconductor make it pretty easy to 
end up with dozen of packages attached or loaded. For instance

   library(GenomicFeatures)
   library(DESeq2)

 > length(loadedNamespaces())
[1] 63
 > length(getLoadedDLLs())
[1] 41

Qin's use case is a shiny app, presumably trying to provide relatively 
comprehensive access to a particular domain. Even if the app were to 
load / requireNamespace() (this requires considerable programming 
discipline to ensure that the namespace is available on all programming 
paths where it is used), it doesn't seem at all improbable that the user 
in an exploratory analysis would end up accessing dozens of packages 
with orthogonal dependencies. This is also the use case with Karl 
Forner's post 
https://stat.ethz.ch/pipermail/r-devel/2015-May/071104.html (adding 
library(crlmm) to the above gets us to 53 DLLs).

>
>> The real solution of course would be a code improvement that
>> starts with a relatively small number of "DLLinfo" structures
>> (say 32), and then allocates more batches (of size say 32) if
>> needed.
>
> The problem of course is that such code will rarely be exercised, and
> people have made errors on the boundaries (here multiples of 32) many
> times in the past.  (Note too that DLLs can be removed as well as added,
> another point of coding errors.)

That argues for a simple increase in the maximum number of DLLs. This 
would enable some people to have very bulky applications that pay a 
performance cost (but the cost here is in small fractions of a 
second...) in terms of symbol look-up (and collision?), but would have 
no consequence for those of us with more sane use cases.

Martin Morgan

>
>> Patches to the R sources (development trunk in subversion at
>> https://svn.r-project.org/R/trunk/ ) are very welcome!
>>
>> Martin Maechler
>> ETH Zurich  &  R Core Team
>
>
>


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