[Bug libstdc++/63746] New: i/o fragmentation inside basic_filebuf::xsputn

["pluto at agmk dot net" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=63746

            Bug ID: 63746
           Summary: i/o fragmentation inside basic_filebuf::xsputn
           Product: gcc
           Version: 4.9.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: pluto at agmk dot net

Hi,
i'm trying to minimize small i/o operations for better overal
application perfromance over networked filesystems.
setting quite large stream buffer doesn't work as axpected
in some cases.

#include <iostream>
#include <fstream>
#include <memory>
#include <sstream>
#include <string>

int main()
{
    std::size_t bufferSize = 1024*1024;
    std::unique_ptr< char > buffer( new char[ bufferSize ] );
    std::ofstream f;
    f.rdbuf()->pubsetbuf( buffer.get(), bufferSize );
    f.open( "s.txt", std::ios_base::out | std::ios_base::binary );

    std::stringstream ss;

    std::string s1( 10240, 'x' );

    ss.str( std::string() );
    ss << s1;
    f << ss.rdbuf();

    ss.str( std::string() );
    ss << s1;
    f << ss.rdbuf();

    f.close();
}



% g++ s.cpp -o s -Wall -g2 -std=c++11
% LANG=C strace ./s
(...)
open("s.txt", O_WRONLY|O_CREAT|O_TRUNC, 0666) = 3
writev(3, [{NULL, 0}, {"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"..., 8136}], 2) = 8136
writev(3, [{"", 0}, {"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"..., 2104}], 2) = 2104
writev(3, [{"", 0}, {"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx"..., 10240}], 2) = 10240
close(3)


i've expected that 20kB i/o from testcase will fit in the 1MB buffer
but the fstream.tcc contains magic const __chunk = 1k and logic that
not use the buffer in the optimal way.