This is the mail archive of the
gcc-bugs@gcc.gnu.org
mailing list for the GCC project.
[Bug libstdc++/63746] New: i/o fragmentation inside basic_filebuf::xsputn
- From: "pluto at agmk dot net" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Wed, 05 Nov 2014 09:05:30 +0000
- Subject: [Bug libstdc++/63746] New: i/o fragmentation inside basic_filebuf::xsputn
- Auto-submitted: auto-generated
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.