[racket] Performance. Higher-order function

From: Matthew Flatt (mflatt at cs.utah.edu)
Date: Mon Aug 4 01:47:34 EDT 2014

Well... a function call is expensive relative to some things, such as
adding fixnums to produce a fixnum.

My read of your initial results is that calling an unknown function is
similar to the cost of one iteration in a loop that sets a character in
a string.

More precisely, decompiling the program shows that the compiler unrolls
the loop in `test1` by 4 iterations, so one call to an unknown function
is the same cost as 4 generic `<`s on fixnums, 4 generic `+ 1`s on
fixnums, 4 `integer->char`s, 4 `string-set!s`, and one jump to a known
function (to recur).

The `build-string1` loop is similarly unrolled 4 times, and the call to
`build-string1` is not inlined in `test3`, so we really can compare the
unknown function call in `test3` to the overall loop overhead plus
`string-set!`s of `test1`. And the fact that neither `build-string1`
nor `test2 is inlined is consistent with `test2` and `test3` having the
same performance.

If I change the `for` body of `test1` with a constant, then the time is
cut in half. So, a `string-set!` takes up about half of the time.

Putting that all together, it looks like a call to an unknown function
takes about twice the time of a `string-set!` --- which (due to various
tag and bounds checks for the string assignment) costs the same as a
small pile of generic arithmetic on fixnums. That could be considered
expensive in a tight loop.

I would hesitate to say that higher order functions are always slow,
because many uses do more work around the call than set one character
in a string.

At Mon, 04 Aug 2014 07:42:53 +0400, Roman Klochkov wrote:
>  > unknown function call is expensive
> So, higher order function always slow. 
> Thus, to improve performance, one should use for/fold, for/list, ... and never 
> use map, foldl, build-string, ... with lambda.
> Is it correct?
> Sun, 3 Aug 2014 13:15:57 -0400 от Matthias Felleisen <matthias at ccs.neu.edu>:
> >
> >Because build-string calls an unknown function 1000 x 100000 times, and an 
> unknown function call is expensive. 
> >
> >Racket could possible collapse all modules and perform additional in-lining 
> optimizations eventually, which may help here. But it doesn't yet. 
> >
> >-- Matthias
> >
> >
> >
> >On Aug 3, 2014, at 5:15 AM, Roman Klochkov wrote:
> >>Are higher order function always slow?
> >>...
> >>---
> >>
> >>So I see, that build-string version is about two times slower, than 
> set-in-the-loop. Why so much? I expected about 10-20% difference.
> >>-- 
> >>Roman Klochkov ____________________
> >> Racket Users list:
> >>  http://lists.racket-lang.org/users
> >
> -- 
> Roman Klochkov
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