[Rd] Why does an empty vector occupy 40 bytes?

Simon Urbanek simon.urbanek at r-project.org
Thu Aug 29 20:12:10 CEST 2013

On Aug 29, 2013, at 12:08 PM, Hadley Wickham wrote:

>> You have not: the start of the vector area also needs to be aligned (since
>> it might hold doubles or CHARSXPs).
> Thanks, that was the key point I was missing. (Also mentioned to me off list by Luke Tierney).

Not to be picky, but that is not the point. The alignment is due to the attrib pointer which is at offset 8 despite the fact that there is only a 4-byte element in front of it. Maybe for better illustration, this is the layout on 64-bit machines:

* 4 bytes: sxpinfo header (= 32 bits)
* 4 bytes: --- padding so next ptr is aligned ---
* 8 bytes: pointer to attributes
* 8 bytes: pointer to next node
* 8 bytes: pointer to previous node
* 4 bytes: length
* 4 bytes: true length

= 40 bytes

This is already aligned so the payload alignment doesn't any extra padding so that has no effect at all.

>> See memory.c, which says, inter alia
>> /* All vector objects must be a multiple of sizeof(SEXPREC_ALIGN)
>>   bytes so that alignment is preserved for all objects */
>> /* Node Classes.  Non-vector nodes are of class zero. Small vector
>>   nodes are in classes 1, ..., NUM_SMALL_NODE_CLASSES, and large
>>   vector nodes are in class LARGE_NODE_CLASS.  For vector nodes the
>>   node header is followed in memory by the vector data, offset from
>>   the header by SEXPREC_ALIGN. */
>> And also, object.size() is only approximate, and documented to be so. In
>> fact many short vectors are using larger blocks of memory incompletely, and
>> of course the OS is supplying memory in pages.
> Yes, the size of an object is a nebulous concept. Do you want the
> marginal size or the average size? Do you want to know how big the
> object is, or the impact of the object on memory allocation?
> Hadley
> -- 
> Chief Scientist, RStudio
> http://had.co.nz/
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