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 <matth...@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 ____________________ Racket Users list: http://lists.racket-lang.org/users
