Chapter 2 (PART 1) Light-Weight Process (Threads) Department of Computer Science Southern Illinois University Edwardsville Summer, 2004 Dr. Hiroshi Fujinoki CS 414 Operating Systems Threads/001

CS 414 Operating Systems Threads/002 Presentation Agenda Two inefficiency problems in “process” The two problems motivate “light-weigh process” (“Light-weight process” = “Thread”) Introduce “thread” as a solution for the two problems in “process” How thread works?

CS 414 Operating Systems Threads/003 Structure of process PCB Address Space  Stack  Code  Heap For some applications, this structure gives problems Assume an application program that (1) Requires multiple processes to execute an application program (2) Requires inter-process communications

BBS Server Host Implementation CS 414 Operating Systems Threads/004 Example Network BBS Server BBS DB Client A Client B Client C Client D BBS DB Client A Client B Client C Process A Process B Process C Multiprogramming

CS 414 Operating Systems Threads/005 Two inefficiency problems using processes in this BBS Server  Context-switching overhead - When a context-switch happens, an entire PCB of the running process has to be saved and the one for the next has to be loaded.  System call overhead in accessing shared information - In order for the BBS information to be shared, the shared information needs to be in a different address space - To access data in a different address space, each process needs to call a system call

CS 414 Operating Systems Threads/006 Problem 1: Overhead for context switching Process A Process B Process C Client B arrives Client A arrives Running Ready What if the number of concurrent clients increased? Running Client C arrives Ready Running Context-switching overhead

CS 414 Operating Systems Threads/007 Problem 1: Overhead for context switching (continued) If we just increase the number of clients, what will happen? Process A Process B Process C Process D Process E Running Ready Running Ready Running Ready Running Ready Running Ready Non-responding interval

CS 414 Operating Systems Threads/008 Problem 1: Overhead for context switching (continued) What could we do to reduce non-responding interval? Process A Process B Process C Process D Process E Running Ready Running Ready Running Ready Running This causes another problem Effective CPU goes down Non-responding interval

CS 414 Operating Systems Threads/009 Problem 2: System call overhead in accessing shared information BBS Server Host BBS DB Client A Client B Client C Process A Process B Process C Multiprogramming Process A Process C Process B BBS DB (Shared data) OS

CS 414 Operating Systems Threads/010 Two inefficiency problems using processes in this BBS Server - If number of clients is increased  Context-switching overhead  System call overhead in accessing shared information Either response time or effective CPU utilization is sacrificed - Shared data is implemented with another address space Slower program execution Light-weight process (thread) is a solution for the two problems

One Address Space CS 414 Operating Systems Threads/011 Concepts of light-weight process Multiple execution paths in a process Thread PCB A Address Space Process A PCB B Address Space Process B PCB C Address Space Process C PCB X Global Stack Global Heap Global PCB

Process CS 414 Operating Systems Threads/012 Concepts of light-weight process PCB A Address Space Process A PCB B Address Space Process B PCB C Address Space Process C PCB X Global Stack Global Heap Global PCB Private Stack Code Private Heap Private PCB

CS 414 Operating Systems Threads/013 Concepts of light-weight process PCB A Address Space Process A PCB B Address Space Process B PCB C Address Space Process C Process PCB X Global Stack Global Heap Private Stack Code Private Heap Private PCB Global PCB PCB N = Global PCB + Private PCB

CS 414 Operating Systems Threads/014 Concepts of light-weight process PCB w/out thread support Global PCB Private PCB When a CPU needs to be switched from a thread to another, only this part should be swapped Remaining Question: How context-switching overhead can be reduced?

CS 414 Operating Systems Threads/015 Concepts of light-weight process PCB w/out thread support Global PCB Private PCB When a CPU needs to be switched from a thread to another, only this part should be swapped For CPU switching between threads, global PCB remains remains same

CS 414 Operating Systems Threads/016 Definition of Threads Thread = Multi-threading - Making multiple threads in ready state - Similar to “multi-programming” - Much less overhead for context-switching and communication Multiple execution paths within a process “light-weight process”

CS 414 Operating Systems Threads/017 Major Advantages in threads Thread reduces context-switching and inter-process communication overhead (thus “light-weight”). The general advantages in threads are: - Share all resources assigned to a “process” Files opened, I/O devices assigned, global memory allocated, etc. - Ability to execute multiple different tasks in asynchronous way With easy and fast inter-thread communication

CS 414 Operating Systems Threads/018 Major Disadvantages in threads No memory protection between threads Managing assigned I/O resources could be very complex - Any thread in a process can use any resources assigned to a process - Multiple threads can be executed in any order Example Implementation of threads is not standardized - Each major OS (Solaris, LINUX, BSD, Windows) has its own implementation of threads and all different Programmers are responsible for avoiding “illegal access”