Re: [PATCH 1/4] strbuf_split_buf(): use ALLOC_GROW()

[Michael Haggerty <mhagger@alum.mit.edu>]

On 11/04/2012 12:41 PM, Jeff King wrote:
> On Sun, Nov 04, 2012 at 07:46:51AM +0100, Michael Haggerty wrote:
> 
>> Use ALLOC_GROW() rather than inline code to manage memory in
>> strbuf_split_buf().  Rename "pos" to "nr" because it better describes
>> the use of the variable and it better conforms to the "ALLOC_GROW"
>> idiom.
> 
> I suspect this was not used originally because ALLOC_GROW relies on
> alloc_nr, which does fast growth early on. At (x+16)*3/2, we end up with
> 24 slots for the first allocation. We are typically splitting 1 or 2
> values.
> 
> It probably doesn't make a big difference in practice, though, as we're
> talking about wasting less than 200 bytes on a 64-bit platform, and we
> do not tend to keep large numbers of split lists around.

I did a little bit of archeology, and found out that

* ALLOC_GROW() did indeed exist when this code was developed, so it
  *could have* been used.

* OTOH, I didn't find any indication on the mailing list that the
  choice not to use ALLOC_GROW() was a conscious decision.

So history doesn't give us much guidance.

If the size of the initial allocation is a concern, then I would suggest
adding a macro like ALLOC_SET_SIZE(ary,nr,alloc) that could be called to
initialize the size to some number less than 24.  Such a macro might be
useful elsewhere, too.  It wouldn't, of course, slow the growth rate
*after* the first allocation.

FWIW, the "max" parameter of strbuf_split*() is only used in one place,
though strbuf_split*() is used in some other places where not too many
substrings would be expected.

I am working on some patch series that will eliminate even more uses of
strbuf_split*(), so I won't work more on optimizing its resource usage
unless somebody gives me a stronger nudge.

Michael

-- 
Michael Haggerty
mhagger@alum.mit.edu
http://softwareswirl.blogspot.com/