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/ > > ______________________________________________ > R-devel@r-project.org mailing list > https://stat.ethz.ch/mailman/listinfo/r-devel > > ______________________________________________ R-devel@r-project.org mailing list https://stat.ethz.ch/mailman/listinfo/r-devel
