[racket] Performance. Higher-order function

From: Roman Klochkov (kalimehtar at mail.ru)
Date: Mon Aug 4 02:29:08 EDT 2014

 Thank you! Now it's rather clear for me.

Mon, 4 Aug 2014 06:47:34 +0100 от Matthew Flatt <mflatt at cs.utah.edu>:
>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
>> ____________________
>>   Racket Users list:
>>  http://lists.racket-lang.org/users

Roman Klochkov
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