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Slide 6-1 6 Processes Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Announcements • Extension til Friday 11 am for HW #1 • Previous lectures online • Program Assignment #1 online later today, due 2 weeks from today • Homework Set #2 online later today, due a week from today • Read chapter 6 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Slide 6-2 Slide 6-3 What is a Process? • A software program consist of a sequence of code instructions and data – for now, let a simple app = a program – CPU executes the instructions line by line in fetch-execute cycle from RAM – code instructions operate on data – A program is a passive entity Program P1 Code Data • A process is a program actively executing from main memory Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Slide 6-4 Loading and Executing a Program OS Loader Disk P1 binary Code P2 binary Code Process Main Memory Fetch Code and Data Program P1 binary CPU Execution Program Counter (PC) Code Registers ALU Data Data Copyright © 2004 Pearson Education, Inc. Data Write Data Operating Systems: A Modern Perspective, Chapter 6 Slide 6-5 What is a Process? • A process is a program actively Main executing from main Memory memory – has a Program Counter (PC) and execution state associated with it • CPU registers keep state • OS keeps process state in memory • it’s alive! – has an address space associated with it • a limited set of (virtual) addresses that can be accessed by the executing code Copyright © 2004 Pearson Education, Inc. Program P1 binary Process Fetch Code and Data Code CPU Execution Program Counter (PC) Registers ALU Data Write Data Operating Systems: A Modern Perspective, Chapter 6 Slide 6-6 What is a Process? – 2 processes may execute the same program code, but they are considered separate execution sequences Main Memory Program P1 binary Process Fetch Code and Data Code CPU Execution Program Counter (PC) Registers ALU Data Write Data Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 How is a Process Structured in Memory? Main Memory Process P1 Code Data Copyright © 2004 Pearson Education, Inc. Slide 6-7 float f4=3.0; global variables main() { char* ptr; ptr = malloc(); foo1(); } dynamically allocated variables foo1() { int a1; .... } Operating Systems: A Modern Perspective, Chapter 6 functions local variables How is a Process Structured in Memory? Main Memory Process P1 Code Slide 6-8 float f4=3.0; global variables main() { char* ptr; ptr = malloc(); foo1(); } dynamically allocated variables Data Heap Stack Copyright © 2004 Pearson Education, Inc. foo1() { int a1; .... } Operating Systems: A Modern Perspective, Chapter 6 functions local variables How is a Process Structured in Memory? Run-time memory max address User stack • Run-time memory image • Essentially code, data, stack, and heap • Code and data loaded from executable file • Stack grows downward, heap grows upward Unallocated Heap Read/write .data, .bss Read-only .init, .text, .rodata address 0 Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Slide 6-9 Why Allocate Local Variables on a Stack? Slide 6-10 • Strawman approach: pre-allocate all local variables a priori before a process starts executing, just like global variables • What’s wrong with this strawman? – if a function is never called, then you’ve wasted space allocating its local variables – don’t know a priori how many instances of a local variable to allocate if a function calls itself, i.e. recursion Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Why Allocate Local Variables on a Stack? Slide 6-11 • So allocate local variables only on an asneeded basis • A stack provides a simple way to allocate local variables as needed – When a function is called, allocate its local variables on top of the stack - push them on the stack – when a function completes, deallocate these local variables - pop them off the stack Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Why Allocate Dynamic Variables on a Heap in the Process’s Address Space? • Strawman II: could ask the OS to allocate dynamic variables anywhere in memory – very complex keeping tracking of all the different locations in memory • Keeping the dynamic variables in one area (the process’s heap) associated with the process’s address space simplifies memory management Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Slide 6-12 A Process Executes in its Own Address Space Main Memory Process P1 Code • OS tries to provide the illusion or abstraction that the process executes in its own address space, on its own CPU Data Heap Stack Copyright © 2004 Pearson Education, Inc. Slide 6-13 Operating Systems: A Modern Perspective, Chapter 6 Implementing the Process Abstraction Pi CPU Pj CPU Pi Executable Memory Pj Executable Memory Pk CPU … Pk Executable Memory OS Address Space CPU ALU Control Unit Pi Address Space Pk Address Space … Pj Address Space Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Machine Executable Memory OS interface Slide 6-14 Slide 6-15 OS Process Management: External View Application Process Device Mgr UNIX Memory Mgr File Mgr exec() Memory Mgr File Mgr Process Mgr Device Mgr wait() CreateThread() CloseHandle() CreateProcess() WaitForSingleObject() Process Mgr fork() Windows Hardware Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Process Manager Responsibilities Slide 6-16 • Define & implement the essential characteristics of a process and thread – Algorithms to define the behavior – Data structures to preserve the state of the execution • Define what “things” threads in the process can reference – the address space (most of the “things” are memory locations) • Manage the resources used by the processes/threads • Tools to create/destroy/manipulate processes & threads • Tools to time-multiplex the CPU – Scheduling the (Chapter 7) • Tools to allow threads to synchronization the operation with one another (Chapters 8-9) • Mechanisms to handle deadlock (Chapter 10) • Mechanisms to handle protection (Chapter 14) Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Slide 6-17 Process Manager Overview Program Process Abstract Computing Environment File Manager Process Deadlock Description Protection Synchronization Device Manager Devices Copyright © 2004 Pearson Education, Inc. Memory Manager Memory Scheduler CPU Operating Systems: A Modern Perspective, Chapter 6 Resource Resource Resource Manager Manager Manager Other H/W Slide 6-18 OS Process Management Main Memory Process P1 Code Data Heap Stack Copyright © 2004 Pearson Education, Inc. • OS keeps a Process Control Block (PCB) for each process: – Process state: new, running, waiting, ready, terminated – Program counter – CPU registers – CPU-scheduling information, e.g. priority – memory management info: value of base and limit registers, page tables, segment tables – I/O info: open files, etc. Operating Systems: A Modern Perspective, Chapter 6 Multiple Processes Slide 6-19 Main Memory Process P1 Process P2 Code Code Data Heap Data Stack Heap Stack Copyright © 2004 Pearson Education, Inc. • Each process is in memory • Only one process at a time executes on the CPU • OS provides the mechanisms to switch between processes – this is called a context switch Operating Systems: A Modern Perspective, Chapter 6 Context Switching Slide 6-20 Executable Memory Initialization Interrupt 1 Process Manager 7 8 Interrupt Handler 2 4 3 P1 9 5 P2 6 Pn Copyright © 2004 Pearson Education, Inc. • Each time a process is switched out, its context must be saved, e.g. in the PCB • Each time a process is switched in, its context is restored • This usually requires copying of registers Operating Systems: A Modern Perspective, Chapter 6 Context Switches • A context switch can occur because of – a system call – an I/O interrupt, e.g. disk has finished reading – a timer interrupt • this is how you implement multitasking • Set a timer in the CPU for periodic interrupt, say every 1 ms • On an interrupt, go to the timer interrupt handler, e.g. the scheduler, and switch to another process in the ready queue • Context switch time is pure overhead – it is the price you pay for multiprogramming – typically a few milliseconds Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Slide 6-21 Multiple Processes: State Diagram Request Done Running Request Schedule Start Allocate Blocked Copyright © 2004 Pearson Education, Inc. Ready Operating Systems: A Modern Perspective, Chapter 6 Slide 6-22 Communicating Between Processes Slide 6-23 • Inter-Process Communication or IPC – would like two processes to share information between them • shared file • split a single application into multiple processes to speed up execution by allowing overlapped I/O • split an application into multiple processes for modularity of coding – if address spaces are completely isolated from one another, then how do we share data? Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Communicating Between Processes Slide 6-24 • Two types of IPC – shared memory - OS provides mechanisms that allow creation of a shared memory buffer between processes • shmget() creates a shared memory segment, using a name (key ID) • shmctl() to modify control information and permissions related to a shared memory segment • shmat() to attach a shared memory segment to a process’s address space • allows fast and high volume reading/writing of buffer in memory • applies to processes on the same machine – message passing - OS provides constructs that allow communication via buffers • typically implemented via system calls, and is hence slower than shared memory • sending process has a buffer to send/receive messages, as does the receiving process • used to pass small messages • extends to communicating between processes on different machines Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Communicating Between Processes • Shared access to the same memory introduces complexity – need to synchronize access – Producer-Consumer example • if two producers write at the same time to shared memory, then they can overwrite each other’s data • if a producer writes while a consumer is reading, then the consumer may read inconsistent data Copyright © 2004 Pearson Education, Inc. Operating Systems: A Modern Perspective, Chapter 6 Slide 6-25