Operating System - CH4 Threads

Process creation is heavy-weight

Thread creation is light-weight

multi-thread優點

• responsiveness
• resource sharing
• economy
• scalability 同一個process內的不同thread可以平行在不同CPUs上執行

multicore programming challenge

• divide activities
• balance
• data splitting
• data dependency
• test and debugging

User Threads

Kernel Threads: kernel 知道每條thread

Operating System - CH3 Process

Process: a program in execution

• program counter 
• stack : temporary data
• data section: global var
• heap: containing memory allocated dynamicalling during run time

Process Control Block(PCB): kernel 為管理眾多processes, 會在kernel memory area中，針對每個process各自建立PCB, 紀錄process之related info.

Process Scheduling: 最大化CPU的使用，quickly switch process onto CPU from time sharing

• job queue: all processes in system
• ready queue: process in main memory, ready to execute
• device queue: process waiting for an I/O devices

Process scheduler

• long-term: 從 job queue 挑出進入ready queue
  • control the degree of multiprogramming
• short-term: 從 ready queue挑出 準備進入ＣＰＵ執行
• medium term： 適合time-sharing, 可以調解CPU-bound and I/O-bound( spending more time doing I/O than computations, many short CPU bursts)

• 當記憶體空間不足時，此時又有較高priority的process需要記憶體，scheduler就會將某些process swap out，等記憶體空間足夠時，才將之swap in 到ready queue

Operating System - CH2 Operating-System Structures

Command line interface or Command Interpreter allow direct command entry

System calls: programming interface to the services provide by the OS. 作為user processes與kernel之間的溝通介面，當執行中的user process需要OS提供的服務時，先發Trap通知OS, 並告訴OS要執行的服務項目, OS才執行一系列相關之system calls，完成該服務請求，並傳回結果給user process。

shells: 提供存取kernel的程式

OS design and implementation: 

• user goals: convenient to use, learn, safe, fast
• system goals: easy to design, implement and maintain

• mechanism: how to do it
• policy: what will be done?

Layered approach

• 上層可以使用下層的服務，但下層不能使用上層
• 上層為interface，下層為hardware
• 優點: 模組化，可以分工加速系統開發，易測試與維護
• 缺點: 難以界定層次的劃分

Microkernel system structure

• 將OS kernel眾多模組中移除一些較非必要的模組，改成在user mode下 以system library(software)或user program來執行
• use message passing
• 優點
  • 容易extend
  • 移植到另一平台會更方便
  • more reliable(安全性提升)

Virtual Machines

• OS 利用software技術, 創造出一份與底層硬體一模一樣的功能介面，此一abstract machine，稱為VM。
• 很難去implement，must provide 底層硬體的複製，typically runs in user mode

core dump: application error log file

crash dump: os error log file