On Monday, February 26, 2018 at 4:39:22 AM UTC-6, Steven D'Aprano wrote:
> On Sun, 25 Feb 2018 19:26:12 -0800, Rick Johnson wrote:
>
> > On Friday, February 23, 2018 at 8:48:55 PM UTC-6, Steven D'Aprano wrote:
> > [...]
> > > Take the Fibonacci double-recursion benchmark. Okay, it
> > > tests how well your language does at making millions of
> > > function calls. Why?
> >
> > Because making "millons of function calls" is what
> > software *DOES*!
>
> You're right -- I worded that badly, and ended up saying
> something I didn't really mean. Mea culpa. Of course over
> the course of the entire program run, most non-trivial
> programmers will end up having millions (if not billions)
> of function calls *in total*. But they're not all happening
> *together*.
So what? Latency is latency. And whether it occurs over the
course of one heavily recursive algorithm that constitutes
the depth and breadth of an entire program (a la fib()), or
it is the incremental cumulative consequence of the entire
program execution, the fact remains that function call
overhead contributes to a significant portion of the latency
inherent in some trivial, and *ALL* non-trivial, modern
software.
>
> What I tried to get across is, how often do we use millions
> of function calls to get one value?
All the time! For, in the abstract, the fundamental function
of software is to extract a "value" from one or more inputs
-- is it not? So, whether that "value" be: (1) a document in
an office productivity suite, or (2) a flight path for a
mars lander or a jumbo jet or child's quad-copter, or (3) a
modified image in a image processing program, or (4) a
design drawing or technical schematic for a mars rover bot,
or (5) an HD quality streaming porno flick, or yes, (6) even
a recursive function which unwittingly underscores some of
the less attractive aspects of the Python language (aka:
function call overhead!) -- all of these examples, and
countless others, extract a value from inputs. Of course,
much like physical traits i suppose, some "values" are more
desirable than others.
>
> By default, Python will only allow one thousand function
> calls in a stack before raising RecursionError.
By _default_, yes.
>
> py> def bad():
> ... return bad()
> ...
> py> bad()
> Traceback (most recent call last):
> File "<stdin>", line 1, in <module>
> File "<stdin>", line 2, in bad
> [ previous line repeats 998 times ]
> RecursionError: maximum recursion depth exceeded
>
>
> There's nothing unique about recursion that will trigger
> this error. Any call stack of more than 1000 function calls
> will do it. So forget about code getting a million
> functions deep. The average is probably more like a dozen,
> perhaps a hundred tops.
I'm very glad you brought this up, because it proves my main
point that, Python's maximum recursion error is one feature
they got _right_. If you don't like the default recursion
limit of 1000, or maybe 1000 recursions is breaking a valid
recursive algorithm, you can set the recursion limit to
whatever you like...
## Python 2 ##
>>> import sys
>>> sys.setrecursionlimit
<built-in function setrecursionlimit>
>>> sys.setrecursionlimit(10)
>>> count = 1
>>> def bad():
... global count
... print count
... count += 1
... bad()
...
>>> bad()
1
2
3
4
5
6
7
8
9
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "<stdin>", line 5, in bad
File "<stdin>", line 5, in bad
File "<stdin>", line 5, in bad
File "<stdin>", line 5, in bad
File "<stdin>", line 5, in bad
File "<stdin>", line 5, in bad
File "<stdin>", line 5, in bad
File "<stdin>", line 5, in bad
File "<stdin>", line 5, in bad
RuntimeError: maximum recursion depth exceeded
It is however unfortunate that we get slamed with N
recursive exception messages :-(, but hey, at least we can
modify the maximum recursion limit! It's a step in the right
direction. :-)
> [...]
> > > For most application code, executing the function is far
> > > more costly than the overhead of calling it,
> >
> > What? Your functions are only called _once_ during the
> > lifetime of your code execution? Enough with the leg
> > pulling.
>
> Let me explain what I intended to say. Here is a really
> poor estimate of the overhead of calling a function on my
> computer, measured in iPython.
>
> [...snip valid examples...]
>
> Now I know that this is not always the case. But I maintain
> that for *most* (but not all) practical, real-world code
> that actually does something useful, the overhead of
> calling the functions is *usually* (but not always) low
> compared to the cost of actually doing whatever it is that
> the functions do.
True. But again you failed to address my main point that:
"Lazy features (when feasible) must be offered
_optionally_".
Yes, function call overhead may be tiny in relation to the
code encapsulated within (at least for non-trivial code),
but the fact remains that function calls are inherent in
modern software, and as a result, there is a compelling
argument to be made that function calls should therefore be
as efficient as possible.
In reality, one cannot write modern software without using
lots of functions and calling (at least) some subset of
those functions *MANY* times during the execution of the
program. And we're not just talking about functions who's
parent node is module scope. Nope. Methods are functions
too, albeit, a special bound class of functions. And the
Python std-lib is chalk full of functions, many of which are
utilized every second of every day to do lots of heavy
lifting.
So in conclusion, if anyone out there is not writing tons of
functions to solve their problems (or at least calling tons
of library functions to solve their problems), well then,
their software is either really *REALLY* simplistic, or they
are doing something really *REALLY* wrong.
And from from that conclusion we now come face to face with
the reality that function call efficiency may be the most
important optimization that Python could ever make.
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