[OS] Thread

Thread model

• process model

  • resource grouping
    • Address space, open files, child processes, accounting information
  • Thread
    • program counter, registers, stack

• Thread model

  • Allows multiple thread of executions to take place in ther same process environment(multithreading)
  • Entity scheduled for execution on the CPU
  • Lightweight process (관련된 정보 적다)

(a) Three processes each with one thread

• The three proccesses are unrelated

(b) One process with three threads (sharing the same address space)

• The three threads are part of the same job and are closely cooperating with each other

• No protection between threads

  • Multiple threads in a process cooperaate.

• Per process items

  • shared by all threads in a process => table entry로 저장

• Per thread items

  • private to each thread => thread마다 수행 위치 다르면 각각 저장한다.

Per process items	Per thread items
Address space	Program counter
Global variable	Register
Open files	Stack
Child Processes	State
pending alarms
Signals and signal handlers
Accounting information

• Multiple threads of execution share a set of resources so they can work together closely to perform some task.
• Like a traditional process, a thread can be in any one of seeveral states :running, blocked, ready
• Each thread has its own stack
• Thread-related library procedures
  • thread_create (name of a procedure for the new thread to run)
  • thread_exit
  • thread_wait
    • blocks the calling thread until a specific thread has exited, 특정 thread exit될 때까지 block
  • thread_yield
    • Allows a thread to voluntarily give up the CPU to let another thread run, CPU에서 자발적 양보, 할일 없을 때, mutex값에서 사용됨, busy waiting x

why use thread?

• In many applications, nultiple activities are going on at once
• Threads are easier to create and destroy than processes.
• Performance gain
• Real parallelism is possible with multiple CPUs.

Thread usage

• A word processor with three threads
  • Interactive thread
  • Reformatting thread
  • Disk-backup thread
• A multithreaded Web server

Implementin Threads in user space

Adv of User-level Thread	Disadv of User-level Thread
Can be implemented on an OS that does not supprot thread	How blocking system calls are implemented
Thread switching is faster than the kernel-level thread	Giving up the CPU
Allows each process to have its own cutomized scheduling algorithm
Scales better

Adv

kernel-level thread은 각 테이블 구성되는데 처음에 kernel에 얼마나 thread table 잡을지? 가 곤란하다. process당 thread table을 가지고 있고 개수도 정해져서 나온다.

Disadv

user level에서는 process 존재만 알아서 같은 프로세스에 속한 thread도 모두 block될 수 있다. read lib sys call x -> thread yield => system lib call 수정해야한다. user level thread의 문제점 중 하나. -> 다른 프로세스가 수행될 기회가 없을 수 있다.

Implementing threads in the kernel

• A threads package managed by kernel : Does not require any new, nonblocking system calls 커널이 thread 존재를 알아서 직접 block시킬 수 있다. 단점은 thread switching is slow...