std :: fstream缓冲vs手动缓冲（为什么10倍增益与手动缓冲）？ [英] std::fstream buffering vs manual buffering (why 10x gain with manual buffering)?

问题描述

我已测试过两种写入配置：

I have tested two writing configurations :

1）Fstream缓冲：

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）手动缓冲：

// 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);

我预期同样的结果...

I expected the same result...

但是我的手动缓冲将性能提高了10倍，写入一个100MB的文件，与正常情况（没有重新定义缓冲区）相比，fstream缓冲没有任何改变。

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).

有人有这种情况的解释吗？

Does someone has an explanation of this situation ?

编辑：
这是新闻：一个基准只是在超级计算机位结构，持续intel Xeon 8核，Lustre文件系统和希望配置良好的编译器）

（我不解释1kB手动缓冲区的共振原因...）

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...)

编辑2：
和在1024 B的共鸣（如果有人有一个想法，我感兴趣）：

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

推荐答案

这基本上是由于函数调用开销和间接。 ofstream :: write（）方法继承自ostream。这个函数没有内联在libstdc ++中，这是第一个开销源。然后ostream :: write（）必须调用rdbuf（） - > sputn（）来做实际写作，这是一个虚函数调用。

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.

，libstdc ++将sputn（）重定向到另一个虚函数xsputn（），它添加了另一个虚函数调用。

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

如果你自己把字符放入缓冲区， 。

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

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