On Sat, 2 Jan 2010, 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-requ...@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.)
I believe current gcc -O2 does optimize tail recursive calls (more
generally sibling calls) but RecursiveRelease isn't written as a tail
recursion -- the return vaule is used in an assignment. In any case
it would probably make more sense to rewrite RecursiveRelease to not
use recursion at all, but I'm not motivated to do that at this point.
luke
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.
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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