Buffering in libstc++ 3.0

[Peter Simons <simons at cryp dot to>]

Hi everybody,

I have experienced some weird effects with g++ 3.0 and hope that you
guys can explain to me what I am doing wrong. I compiled the program

 | #include <iostream>
 | using namespace std;
 | 
 | int main(int argc, char** argv)
 |     {
 | #ifdef NO_STDIO_SYNC
 |     ios::sync_with_stdio(false);
 | #endif
 | #ifdef USE_MY_BUFFER
 |     char buf[1*1024];
 |     cin.rdbuf()->pubsetbuf(buf, sizeof(buf));
 | #endif
 | 
 |     string line;
 |     for (getline(cin, line); cin; getline(cin, line))
 |      ;
 | 
 |     return 0;
 |     }

in three versions: With no defines, with NO_STDIO_SYNC defined, with
USE_MY_BUFFER defined, and with both labels defined. Then I time(1)ed
the program reading a 50000-line file, getting the following results:

 | simons@peti:/tmp$ time ./test-plain <input 
 | real    0m18.941s
 | user    0m13.350s
 | sys     0m2.880s
 | 
 | simons@peti:/tmp$ time ./test-no_stdio_sync <input 
 | real    0m18.187s
 | user    0m13.300s
 | sys     0m2.590s
 | 
 | simons@peti:/tmp$ time ./test-my_buffer <input 
 | real    0m3.795s
 | user    0m3.340s
 | sys     0m0.030s
 | 
 | simons@peti:/tmp$ time ./test-my_buffer-no_stdio_sync <input 
 | real    0m4.862s
 | user    0m3.300s
 | sys     0m0.050s
 | 
 | simons@peti:/tmp$ time ./test-my_buffer-no_stdio_sync <input 
 | real    0m4.054s
 | user    0m3.350s
 | sys     0m0.020s

It seems that switching the synchronization between std::cin and stdio
off doesn't have hardly any effect at all -- contrary to what I read
in the FAQ. Furthermore, it seems as if the default instance of
std::cin does not have buffering enabled at all! Why is this so?

Does anyone know any other mechanisms that would further speed up I/O
with streams? I noticed that compiling with -O3 works wonders, but I
am sure there are more things one could do to increase the
performance.

	-peter