R-devel Digest, Vol 83, Issue 2

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".

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

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[2]
http://r-forge.r-project.org/plugins/scmsvn/viewcvs.php/pkg/src/RObject.cpp?rev=255&root=rcpp&view=markup



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