std::fstream buffering vs manual buffering (why 10x gain with manual buffering)?

Question

I have tested two writing configurations:

1. Fstream buffering:

   // Initialization
   const unsigned int length = 8192;
   char buffer[length];
   std::ofstream stream;
   stream.rdbuf()->pubsetbuf(buffer, length);
   stream.open("test.dat", std::ios::binary | std::ios::trunc)
   
   // To write I use :
   stream.write(reinterpret_cast<char*>(&x), sizeof(x));
   

2. Manual buffering:

   // Initialization
   const unsigned int length = 8192;
   char buffer[length];
   std::ofstream stream("test.dat", std::ios::binary | std::ios::trunc);
   
   // Then I put manually the data in the buffer
   
   // To write I use :
   stream.write(buffer, length);
   

I expected the same result...

But my manual buffering improve performance by a factor of 10 to write a file of 100MB, and the fstream buffering does not change anything compared to the normal situation (without redefining a buffer).

Does someone has an explanation of this situation ?

EDIT : Here are the news : a benchmark just done on a supercomputer (linux 64-bit architecture, lasts intel Xeon 8-core, Lustre filesystem and ... hopefully well configured compilers) (and I don't explain the reason of the "resonance" for a 1kB manual buffer...)

EDIT 2 : And the resonance at 1024 B (if someone has an idea about that, I'm interested) :

Answer 1

This is basically due to function call overhead and indirection. The ofstream::write() method is inherited from ostream. That function is not inlined in libstdc++, which is the first source of overhead. Then ostream::write() has to call rdbuf()->sputn() to do the actual writing, which is a virtual function call.

On top of that, libstdc++ redirects sputn() to another virtual function xsputn() which adds another virtual function call.

If you put the characters into the buffer yourself, you can avoid that overhead.