1 Pertemuan 5 Komunikasi antar Proses / Interprocess Communication (IPC) Matakuliah: T0316/sistem Operasi Tahun: 2005 Versi/Revisi: 5 OFFCLASS01

2 Learning Outcomes Pada akhir pertemuan ini, diharapkan mahasiswa akan mampu : menjelaskan perlunya komunikasi antar process, dan menerangkan beberapa mekanisme untuk berkomunikasi (C2)

3 Outline Materi Race Condition Critical Region Mutual Exclusion with busy waiting –Disabling interrupts –Lock variables –Strict Alternation –Peterson’s solution –TSL Instruction Sleep and wakeup

4 Problems - Passing information between process - Making sure two or more processes do not get into each other’s way - Proper sequencing when dependencies are present Interprocess Communication RACE CONDITION Situation where two or more processes are reading or writing some shared data and the final result depends on who runs precisely when

5 Examples : spooler directory for printer Two processes want to access shared memory at same time

6 spooler (2) Process A Reads in = 7 Store local_next_slot = 7 Reads local_next_slot = 7 Store buffer[7] = A Update in = 8 Process B Reads in = 7 Store local_next_slot = 7 Store buffer[7] = B Update in = 8 B will never receive any output

7 Critical Regions Four conditions to provide mutual exclusion 1.No two processes simultaneously in critical region 2.No assumptions made about speeds or numbers of CPUs 3.No process running outside its critical region may block another process 4.No process must wait forever to enter its critical region Mutual Exclusion:  mechanism to prevent process accessing resource used by another process Part of the programs in which the shared memory is accessed

8 Mutual exclusion using critical regions

9 1. Disabling Interrupt  Each process disable all interrupts just after entering its critical regions and re-enable them just before leaving it but, disabling interrupt shall not be carried out by user process Mutual Exclusion with Busy Waiting

10 2. Lock Variables  Shared (lock) variables, initially set to 0  If lock = 0, lock is set to = 1, then enters the critial region  If lock = 1, wait until lock = 0 problem: race condition

11 3. Strict Alternation (a) Process 0. (b) Process 1.

12 4. Peterson’s Solution

13 5. TSL Instruction (Test and Set Lock) Note: Peterson and TSL are correct bur requires busy waiting Initially lock = 0  JNE = false  just RET

14 Sleep:system call yang menyebabkan proses yang memanggil diblock, atau ditunda (suspended) Wakup:system call yang menyebabkan proses dibangunkan, atau menjadi ready Sleep and Wakeup Case: Producer-Consumer Problem (Bounded-Buffer Problem)

15 Example of producer- consumer problem with fatal race condition