[Rd] Objectsize function visiting every element for alt-rep strings

[Travers Ching]

Thanks for the detailed response, Gabriel!

I think that an object_size alt-rep method that package developers
need to implement might be hard to get right.  One alternative could
be an alt-rep method that returns the number of bytes/characters in a
given string element since I believe the object size of a CHARSXP
depends only on string length?  I think two optional alt-string
methods would be nice:

`alt_string_elt_nchars` -- for the `nchar` function in R
`alt_string_elt_nbytes` -- for `object.size` (which might be different
than nchars due to encoding)

Also since it's an issue that mainly affects R-studio, I started an
issue on their github, and it sounds like they'll avoid calling
object.size on alt-rep objects automatically.  That would fix the main
problem I've been having.

Thanks,
Travers

On Fri, Jan 18, 2019 at 2:49 PM Gabriel Becker <gabembecker using gmail.com> wrote:
>
> Travers,
>
> Great to hear you're trying out the ALTREP stuff, good on you :).
>
> Did you mean the get_altstring_Elt_method? I see the code in size.c within utils that grabs each element, but I don't see any setting (and the setters are noops currently anyway they just do things the old way).
>
> One thing we have to decide is what object.size means for an altrep. I tend to think it should mean the size of the alternative representation currently in use in memory, but I see that a small note in ?object.size indicates that size of objects with compact internal representations may be overestimated, so technically this is "as currently documented". The "we" here, of course, is the R-core team so we'll have to see how they feel on the matter.
>
> As for what to do about it, one possibility is to add an object.size method to the ALTREP method table that gets called if object.size is called on an ALTREP object.  In this case, it would be up  to the class to define an appropriate object.size method. That would be relatively easy to do from a technical standpoint on R's side, but what comes out of object.size would be a bit "Wild West-y", without the consistency and correctness guarantees one might expect from a function in utils.
>
> Another option is to to have object.size recurse to calling object.size on the two parts (SEXPS which together make up a CONS cell, I believe) that make up an ALTREP  internally. Roughly speaking one of these is usually the alternative representation while the other is the spot to put an object with the traditional representation if the payload is ever fully materialized in an altrep-unsafe way - e.g., C code grabs a writable dataptr via INTEGER, REAL, DATAPTR, etc. Note there are exceptions to what I said above, though,such as the wrapper ALTREP classes which always have the parent object (typically a traditionally laid-out vector), because the "alternative representation" part is strictly a metadata annotation in that case and contains no representation of the payload data for those classes.
>
> In this second case the result of object.size would be consistent across all ALTREP classes, but in both cases the result of object.size would no longer give any information about the size of a vector payload. This is consistent with how object.size deals with external pointers now, but could lead to some surprise in the case of vectors which the end user may not even know are ALTREPs.
>
> Thoughts from anyone else on this list?
>
> Anyway, thanks for pointing this out. I'll talk with Luke and see what makes sense to do here.
>
> Best,
> ~G
>
> On Wed, Jan 16, 2019 at 3:49 AM Travers Ching <traversc using gmail.com> wrote:
>>
>> I have a toy alt-rep string package that generates randomly seeded strings.
>>
>> example:
>> library(altstringisode)
>> x <- altrandomStrings(1e8)
>> head(x)
>> [1] "2PN0bdwPY7CA8M06zVKEkhHgZVgtV1" "5PN2qmWqBlQ9wQj99nsQzldVI5ZuGX" ... etc
>> object.size(1e8)
>>
>> Object.size will call the set_altstring_Elt_method for every single
>> element, materializing (slowly) every element of the vector.  This is
>> a problem mostly in R-studio since object.size is called
>> automatically, defeating the purpose of alt-rep.
>>
>> Is there a way to avoid the problem of forced materialization in rstudio?
>>
>> PS: Is there a way to tell if a post has been received by the mailing
>> list?  How long does it take to show up in the archives?
>>
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