[Python-Dev] PyBench DictCreation (was Re: Performance compares)

[Tim Peters]

[MAL]
> It [pybench] doesn't claim "typical use". pybench is aimed at finding
> out performance issues about hot-spots -- there's no such thing as
> a "typical program", so pybench gives you low level performance
> compares for very specific tasks, e.g. dictionary creation or
> for-loop performance.
>
> I have found it to be rather successful at that. At least gives
> some good hints at where to look...

There must be a misunderstanding here.  I understand and appreciate all that!
>From the instant you created it, PyBench became the best performance canary
we have ("canary" in the sense of bringing a bird into the coal mine with
you, because when a potentially fatal buildup of gasses occurs, the canary
will pass out before you even notice).

My point was that making a decision based solely on that PyBench happens to
create millions of dicts of exactly size 3, and relatively few of any other
size, would be crazy -- which I'm sure you understand and appreciate too.

> ...
> I found that instance dictionaries are usual within the 8 slot
> range. You normally have a few heavy wheight instances and
> many light wheight ones which only have two or three attributes
> in their instance dict.

Matches my observations too.

[on dict resize parameters]
> Why not make those parameters macros at the top of dictobject.c
> which can then be tuned to whatever the programmer needs/wants ?!

Bad idea, IMO.  If someone understands the dict implementation well enough to
be *competent* to change these without, e.g., opening a door to infinite
loops, then they already know where these parameters appear, and can change
the hardcoded #s themselves.  Thr max load factor simply wasn't intended to
be adjustable; and if it were, it would be a per-dict decision.