[Rd] R-devel Digest, Vol 83, Issue 2

Sat Jan 2 23:12:16 CET 2010

On 02/01/2010 3:16 PM, Laurent Gautier wrote:
> On 1/2/10 8:53 PM, Duncan Murdoch wrote:
>> Simon Urbanek wrote:
>>> On Jan 2, 2010, at 12:17 PM, Laurent Gautier wrote:
>>>
>>>> On 1/2/10 5:56 PM, Duncan Murdoch wrote:
>>>>> On 02/01/2010 11:36 AM, Laurent Gautier wrote:
>>>>>> [Disclaimer: what is below reflects my understanding from reading the
>>>>>> R source, others will correct where deemed necessary]
>>>>>>
>>>>>> On 1/2/10 12:00 PM, r-devel-request at r-project.org wrote:
>>>> (...)
>>>>
>>>>>>> I'd also be interested if there is some ideas on the relative
>>>>>>> efficiency
>>>>>>> of the preserve/release mechanism compared to PROTECT/UNPROTECT.
>>>>>> PROTECT/UNPROTECT is trading granularity for speed. It is a stack with
>>>>>> only tow operations possible:
>>>>>> - push 1 object into the stack
>>>>>> - pull (unprotect) N last objects from the stack
>>>>> UNPROTECT_PTR is also possible, which does a linear search through the
>>>>> stack and unprotects something possibly deep within it. There is also
>>>>> REPROTECT which allows you to replace an entry within the stack.
>>>>>
>>>>> I would guess that UNPROTECT_PTR is more efficient than
>>>>> RecursiveRelease
>>>>> because it doesn't use so much stack space when it needs to go deep
>>>>> into
>>>>> the stack to release, but it is possible the compiler recognizes the
>>>>> tail recursion and RecursiveRelease is implemented efficiently. In that
>>>>> case it could be more efficient than UNPROTECT_PTR, which has to move
>>>>> all the other entries down to fill the newly vacated space. Really the
>>>>> only reliable way to answer efficiency questions like this is to try
>>>>> both ways and see which works better in your application.
>>>> Thanks. I did not know about UNPROTECT_PTR.
>>>> I had concerns over the stack usage, but so far it did not prove too
>>>> much of a problem. Still, why isn't the tail recursion explicitly
>>>> made an iteration then ? This would take the "may be the compiler
>>>> figures it out, may be not" variable out of the equation.
>>>>
>>> Careful - the protection stack (bookkeeping by R) has nothing to do
>>> with the C function call stack hence it has nothing to do with the
>>> compiler. The protection stack is global so usually you don't run out
>>> of it unless something goes horribly wrong (=infinite loop).
>> I think Laurent was referring to RecursiveRelease, which could use a lot
>> of C stack if you choose to release something that is deep in the list,
>> since it checks the head, and if that doesn't match, calls itself again
>> on the rest of the list. (I checked, and at least one version of gcc
>> doesn't recognize the tail recursion: it really does generate a
>> recursive call.)
>>
>> Laurent asked why it isn't optimized to avoid the recursion: I think the
>> answer is simply because it is so rarely used that nobody has bothered.
> 
> Yes, I was referring to RecursiveRelease. Sorry if this was not clear.
> 
> What are the chances for a patch to be accepted ? At first sight(*), 
> making that tail recursion an iterative function is not a major 
> undertaking, and reviewing the patch be fairly straightforward... but I 
> can always use that time otherwise if the answer to the question is "nil".

I don't think I would want to review such a patch (I don't know the 
memory manager well, I don't know that there is really a case where it 
matters enough to be worth doing), so I'd say if you don't get a message 
from a core member volunteering to do so, you should assume it won't be 
accepted.  But that doesn't mean you shouldn't write the code for your 
own internal use and edification, and if you can put together a demo 
that shows it really makes a big difference in a realistic situation, 
you might get a different response.

Duncan Murdoch

> 
> 
> L.
> 
> 
> 
>> Duncan Murdoch
>>>>> Another possibility is to maintain your own list or environment of
>>>>> objects, and just protect/preserve the list as a whole.
>>>> Interesting idea, this would let one perform his/her own bookkeeping
>>>> on the list/environment. How is R garbage collection checking
>>>> contained objects ? (I am thinking performances here, and may be
>>>> cyclic references).
>>>>
>>> You don't really want to care because the GC is global for all objects
>>> (and cycles are supported by the GC in R) - so whether you keep it
>>> yourself or Preserve/Release is practically irrelevant (the protection
>>> stack is handled separately).
>>>
>>> As for keeping your own list -- if you really care about performance
>>> that much (to be honest in vast majority of cases it doesn't matter)
>>> you have to be careful how you implement it. Technically the fastest
>>> way is preallocated generic vector but it really depends on how you
>>> deal with the access so you can easily perform worse than
>>> Preserve/Release if you're not careful.
>>>
>>> As a side note - the best way (IMHO) to deal with all those issues is
>>> to use external pointers because a) you get very efficient C
>>> finalizers b) you can directly (and very efficiently) tie lifespan of
>>> other objects to the same SEXP and c) as guardians they can nicely
>>> track other objects that hold them.
>>>
>>> Cheers,
>>> Simon
>>>
>>>
>>>
>>>> L.
>>>>
>>>>
>>>>> Duncan Murdoch
>>>>>
>>>>>> HTH,
>>>>>>
>>>>>>
>>>>>> L.
>>>>>>
>>>>>>
>>>>>>
>>>>>>
>>>>>>> Thanks,
>>>>>>>
>>>>>>> Romain
>>>>>>>
>>>>>>> [1]http://lists.r-forge.r-project.org/pipermail/rcpp-devel/
>>>>>>> [2]
>>>>>>> http://r-forge.r-project.org/plugins/scmsvn/viewcvs.php/pkg/src/RObject.cpp?rev=255&root=rcpp&view=markup
>>>>>>>
>>>>>>>
>>>>>>>
>>>>>>> -- Romain Francois Professional R Enthusiast +33(0) 6 28 91 30 30
>>>>>>> http://romainfrancois.blog.free.fr |- http://tr.im/IW9B : C++
>>>>>>> exceptions
>>>>>>> at the R level |- http://tr.im/IlMh : CPP package: exposing C++
>>>>>>> objects
>>>>>>> `- http://tr.im/HlX9 : new package : bibtex
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