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 ordercontiguous page frame “
             kmalloc(): “ Used to allocate contiguous general objects .(memory area) “
       
[Notice]
       -因用Buddy System Algorithm,故是2order 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