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

Context Switch

當CPU 轉換process時，他會將之前的process的(context)狀態存起，load state(context) of new process 

• context switch 沒有生產力，時間的長短取決於hardware
• OS and PCB越複雜，context switch需要時間就越長

Process type

• cooperating: process間會互相影響
  • reason:
    • information sharing
    • computation speedup
    • modularity
    • convenience
  • need interprocess communication(IPC)
    • shared memory
    • message passing
      • 建立communication link
      • exchange messages via send/receive
      • release the link
• independent

indirect communication: messages are directed and received from mailboxes

message passing may be either blocking or non-blocking

• blocking send/receive: the sender/receiver block until the message is received/available
• non-blocking send/receive