Re: [PATCH V2 3/3] strbuf: allow to use preallocated memory

On Mon, Jun 06, 2016 at 04:24:53PM -0700, Junio C Hamano wrote:

This is not about stack vs heap or even "cheaper" (whatever your
definition of cheap is).  The principle applies equally if the
original buffer came from BSS.

Perhaps I made it clearer by using a more exaggerated example e.g. a
typical expected buffer size of 128 bytes, but the third line of
1000 line input file was an anomaly that is 200k bytes long.  I do
not want to keep that 200k bytes after finishing to process that
third line while using its initial 80 bytes for the remaining 977
lines.

Ah, I see. Yes, I can see that argument, though I'd counter that since
we _did_ see a 200k entry, perhaps the hint given in the code is not
actually very sensible. Having seen one big line, might we expect to see
more?

I dunno. I still feel like this whole thing is just micro-optimization
that is not even really going to be measurable outside of pathological
cases.

By the way, William seemed to be unhappy with die(), but I actually
think having a die() in the API may not be a bad thing if the check
were about something more sensible.  For example, even if a strbuf
that can grow dynamically, capping the maximum size and say "Hey
this is a one-lne-at-a-time text interface; if we need to grow the
buffer to 10MB, there is something wrong and a producer of such an
input does not even deserve a nice error message" could be an
entirely sensible attitude.  But that does not mean an initial
allocation should be 10MB.  If the expected typical workload fits
within a lot lower bound, starting from there and allowing it to
grow up to that maximum would be the more natural thing to do.

Yes, I very much agree that "sensible limits" should not be the same
thing as "initial allocated sizes". It is easy to conflate the two when
using static buffers (because it lets you skip allocation entirely,
which is much simpler!), but that usually leads to "sensible limits"
being set way too low as a compromise.

-Peff