stdio performance in tango, stdlib, and perl

[James Dennett]

Andrei Alexandrescu (See Website For Email) wrote:
> Walter Bright wrote:
>> Andrei Alexandrescu (See Website For Email) wrote:
>>> I've ran a couple of simple tests comparing Perl, D's stdlib (the
>>> coming release), and Tango.
>>
>> Can you add a C++ <iostream> to the mix? I think that would be a very
>> useful additional data point.
> 
> Obliged. Darn, I had to wait a *lot* longer.
> 
> #include <string>
> #include <iostream>
> 
> int main() {
>   std::string s;
>   while (getline(std::cin, s)) {
>     std::cout << s << '\n';
>   }
> }
> 
> (C++ makes the same mistake wrt newline.)
> 
> 35.7s        cppcat
> 
> I seem to remember a trick that puts some more wind into iostream's
> sails, so I tried that as well:
> 
> #include <string>
> #include <iostream>
> using namespace std;
> 
> int main() {
>   cin.sync_with_stdio(false);
>   cout.sync_with_stdio(false);
>   string s;
>   while (getline(std::cin, s)) {
>     cout << s << '\n';
>   }
> }
> 
> Result:
> 
> 13.3s        cppcat

Try the way IOStreams would be used if you didn't want
it to go slowly:

#include <string>
#include <iostream>

int main() {
    std::ios_base::sync_with_stdio(false);
    std::cin.tie(NULL);
    std::string s;
    while (std::getline(std::cin, s)) {
        std::cout << s << '\n';
    }
}

(Excuse the lack of a using directive there; I find the
code more readable without them.  YMMV.)

I don't have your sample file or your machine, but for
the quick tests I just ran on this one machine, the code
above runs move than 60% faster.  Without using tie(),
each read from standard input causes a flush of standard
output (so that, by default, they work appropriately for
console I/O).

It's certainly true that making efficient use of IOStreams
needs some specific knowledge, and that writing an
efficient implementation of IOStreams is far from trivial.
But if we're comparing to C++, we should probably compare
to some reasonably efficient idiomatic C++.

-- James