Linux内存管理

首先提供Gustavo Duarte写的三篇非常精彩的文章：

1. Anatomy of a Program in Memory

2. How The Kernel Manages Your

3. Page Cache, the Affair Between Memory and Files

 

然后看看Linux下malloc的实现原理：

1. Doug Lea. A Memory Allocator

2. ptmalloc原理分析

 

看完上面的文章，我们对Linux的内存管理就有了大概的印象，也知道了malloc和mmap对于OS而言意味着什么。剩下的有几个问题：

 

1. 在申请大于一定阈值（128K）时，malloc为什么要使用mmap

Efficiency Considerations for malloc 写道

As opposed to other versions, the malloc in the GNU C Library does not round up block sizes to powers of two, neither for large nor for small sizes. Neighboring chunks can be coalesced on a free no matter what their size is. This makes the implementation suitable for all kinds of allocation patterns without generally incurring high memory waste through fragmentation.

Very large blocks (much larger than a page) are allocated with mmap (anonymous or via /dev/zero) by this implementation. This has the great advantage that these chunks are returned to the system immediately when they are freed. Therefore, it cannot happen that a large chunk becomes "locked" in between smaller ones and even after calling free wastes memory. The size threshold for mmap to be used can be adjusted with mallopt. The use of mmap can also be disabled completely

 

2. 处理文件时，是不是使用memory-mapped-file都要优于标准IO

    关于这个问题，可以先了解一下memory mapped file 的优缺点。一个明显的问题是当文件大于虚拟地址空间时，将无法一次性将文件映射到虚拟地址空间里面。此外，也有报告支持在特定的场景下，标准IO要优于memory mapped file。

 

3. 对于非匿名memory mapping，如果文件被修改了，内存中的相应内容会被修改吗？

    不会。

 

4. 应用中如果需要大块的内存，使用malloc还是mmap？各有什么优缺点？