内核函数get_free_pages, kmalloc, vmalloc比较（转载）

(1) 請比較與說明幾個 memory management 相關 kernel functions: --get_free_pages(), kmalloc(), vmalloc(), brk(), 其管理機制、使用時機、特性比較。

[Ans]

_ _ get_free_pages	Page frame management	Kernel	Physical
kmalloc	Memory area management	Kernel	Physical
vmalloc	Non-contiguous memory area management	Kernel	Physical/ virtual
brk	Process address space management	User process	viirtual

 

a. __get_free_pages() : 

     對象: Contiguous Page frame

     Allocate方式:  Buddy System Algorithm

b. kmalloc():

     對象:  Contiguous Memory Area

     Allocate方式: Slab Allocator

c. vmalloc() :

   對象: Non-Contiguous Memory Area

    Allocate方式: Slab Allocator

d. brk() :

 對象: Memory Region (User Process Address Space)

 作用: Change heap size of the process

 

[Notes]

     (1) 在Linux上, Physical memory 依Size固定與否,分成兩種概念: 

           Page Frame:         Fixed Length (4KB)

           Memory Area:    Arbitrary Length

      相對應這兩種, 各用不同的實體記憶體管理方式(allocate/release)

　　　Buddy System Algorithm --> Page Frame

                 Slab Allocator --> Memory Area

[Notice]

不管 page frame 或 memory area都是指"contiguous physical address",

   只是有分固定或任意長度的差別.

 

      (2) __get_free_page: “  Used to request 2 ^order contiguous page frame “

             kmalloc(): “ Used to allocate contiguous general objects .(memory area) “

       

[Notice]

       -因用Buddy System Algorithm,故是2^order Size.

       -在Slab Allocator把每個memory area視為 Object. 雖然 Memory area 可任意長度,但在Linux Implement 作法是把Object的Size 定為geometrically distributed size (ranging from 32 ~1331072).

 

     (3) kmalloc() 與 vmalloc() 主要差別是, vmalloc()所要到的 Memory area 是非連續的.

          即 “ linear address is assigned to noncontiguous memory area ”, 每個 Memory area是連續的

         physical address , 但個別 memory area是不相連的.

 

(4) Memory Region 是指 “ interval of  user process address space “, 通常起始位址與長度是4096的倍數, 故 process 的 address space 就由 a set of memory region list來代表在Linear address space上的位置. brk() 這個System Call即是在User Level 向Kernel要求調整Process 的heap Size.

 

原文地址：http://blog.chinaunix.net/u1/35281/showart_279244.html