[Python-Dev] collections module

[Tim Peters]

[Michael Chermside]
> ...
> I'm sure this is just pointing out the obvious, but *IF* the list
> header were increased, we could add TWO fields... According to Tim,
> this would still "only" bump list objects up to 24 bytes, but ...

There's something else to note here, in connection with the realloc-avoiding
list patch/hack I posted yesterday:  for that logic to work, a non-empty
list must always contain enough slots for roundupsize(ob->size) elements.
So, for example, a list of length 1 must nevertheless contain room for 8
objects, which on a 32-bit machine is 7*4 = 28 bytes in the list *guts* it
doesn't need today (but does need after that patch).

So if people like that patch, it would be more *memory*-efficient too to
drop the roundupsize() business and just expand the list header to remember
how many slots have been allocated.  Then no overallocation would be needed.
Adding 8 bytes in the list header should be a net win over asking for up to
28 unused bytes in the guts (of small lists; it can be larger than that for
large lists).