dlopen vs链接开销 [英] dlopen vs linking overhead

问题描述

假设我有一个库 - foo.so。当构建我的二进制（需要这个库）时，我可以（1）链接foo.so，或者（2）在程序源代码中，dlopen这个库，然后调用这个库提供的函数

当我从图书馆调用函数时，（1）和（2）之间是否有性能差异？请注意，我知道会有不同的初始化特征（如dlopen的成本，首次使用符号等的开销），但是在稳定状态下，两者的选择是否同样快或是一个更快？

谢谢。

解决方案

如果库是共享对象（即某些 lib * .so 文件）使用 gcc -Wall -fPIC -O2 编译，并链接到 gcc -shared 那么它是一个 ELF 独立代码共享库。

^{PIC有一点比起64位x86-64的32位x86（其寄存器少）的成本更高，这有一些寻址模式便于PIC}

它是一样的（稳定状态）性能明智的如果它是 dlopen -ed或如果它是动态链接。因为在这两种情况下，真正的链接都是由动态链接器完成的（例如 ld-linux.so ）sine libdl.so 基本上是动态链接器的包装器。

^{当调用的是 lib * .so 中的代码，而且如果您 dlopen 它或如果您链接。}

如果图书馆静态链接到图书馆，事情可能会有所不同 LIB * .A 。您甚至可以使用链接时间优化链接图库和程序 / a>最近的GCC编译器的能力（编译和链接与 gcc -flto -Wall -O2 ）

阅读Drepper的如何编写共享库论文和程序库HowTo 和 Levine's Linkers&装载机。

Suppose I have a library - foo.so . When building my binary (which needs this library), I can either (1) link foo.so , or, (2) within the program source code, dlopen this library and then call functions provided by this library

Is there any performance difference between (1) and (2) when I call a function from the library? Note that I am aware that there will be different initialization characteristics (like the cost of the dlopen , overhead for first usage of a symbol etc) but in the steady state, are both alternatives equally fast or is one faster?

Thanks.

解决方案

If the library is a shared object (ie some lib*.so file) compiled with gcc -Wall -fPIC -O2 and linked with gcc -shared then it is an ELF Position Independent Code shared library.

^{PIC is a bit more costly on 32 bits x86 -which has few registers- than on 64 bits x86-64 -which has some addressing mode facilitating PIC}

It is the same (in steady state) performance wise if it is dlopen-ed or if it is dynamically linked. Because in both cases the real linking is done by the dynamic linker (e.g. ld-linux.so) sine libdl.so is basically a wrapper to the dynamic linker.

^{What matters performance wise when called is the code inside the lib*.so and it does not change if you dlopen it or if you link it.}

Things could be slightly different if the library is statically linked lib*.a. You could even compile and link both the library and the program with the link time optimization ability of recent GCC compilers (compile and link with gcc -flto -Wall -O2)

Read Drepper's How to Write Shared Library paper and the Program Library HowTo and Levine's Linkers & Loaders book.

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