CS:APP 第 9 章重點提示和練習

導讀

• Linux 核心設計: 不僅是個執行單元的 Process
• Linux 核心設計: 記憶體管理
• Introduction to Memory Management in Linux / video
• mmap 系統呼叫
  

出處: Memory mapping

Virtual Memory

• Virtual Memory: Concepts / 錄影

Why Virtual Memory (VM)?

• Uses main memory efficiently
  • Use DRAM as a cache for parts of a virtual address space
• Simplifies memory management
  • Each process gets the same uniform linear address space
• Isolates address spaces
  • One process can’t interfere with another’s memory
  • User program cannot access privileged kernel information and code

Page 561 ~ 563

MMU 的 page size 選擇是有限的，而在 x86 32-bit 保護模式有以下兩種 page 設定:

• normal ones, 4 KiB
• huge ones, 4 MiB

不是每種 x86 處理器都支援 large page，需要具備 Page Size Extension (PSE)

4 KiB 不僅在 x86 架構出現，其他架構也常見，計算方式:
So assume that both page directory and page table contain 2^x entries, and the page size is 2^y bytes. To make full use of the 2³² address, we have:

2^x * 2 ^x * 2^y = 2³²

$\to 2x+y=32$

Each entry in page directory/table consumes 4 bytes (32-bit), therefore:

2^y / 4 = 2^x

$\to y-2=x$

Thus y = 12, and page size in bytes will be 2^y = 2¹² = 4 KiB.

注意: 4M hugepages are only for 32-bit mode x86. 64-bit x86 uses 2M or 1G hugepages, because the 4-level page-table format uses 9 bits per level.

Page size trade-off

#include <stdio.h>
#include <unistd.h> /* sysconf(3) */
int main(void) {
    printf("The page size for this system is %ld bytes.\n",
           sysconf(_SC_PAGESIZE)); /* _SC_PAGE_SIZE is OK too. */
    return 0;
}

參考輸出: (x86_64/Linux)

The page size for this system is 4096 bytes.

往後到 p.19

Page 565

• Virtual memory seems terribly inefficient, but it works
  because of locality.
  • At any point in time, programs tend to access a set of active
    virtual pages called the working set
• Programs with better temporal locality will have smaller working sets
• If (working set size < main memory size)
  • Good performance for one process (after cold misses)
  • If (working set size > main memory size )
• Thrashing: Performance meltdown where pages are swapped (copied)
  in and out continuously
• If multiple processes run at the same time, thrashing occurs if
  their total working set size > main memory size

Page 565 ~ 567

• 反璞歸真系列之探究 UNIX v6 作業系統設計
  • 在 1970 年 1 月份發佈的 16-bit PDP-11 即具備 KL-11 MMU

VM Address Translation
p.29 到 p.42
Page 568 ~ 575

• Virtual Memory: Systems / 錄影
  
  
  
  
  Page 579

Copy-on-write (COW): p.31 ~ p.42
Page 582 ~ 586

搭配參照:

• The Linux Virtual Memory System
• 20 years of Linux Virtual Memory: from simple server workloads to cloud virtualization