Emery Berger

1
Operating SystemsCMPSCI 377Lecture 3 OS
StructuresLecture 4 Processes

• Emery Berger
• University of Massachusetts, Amherst

2
Last TimeOS Computer Architecture

• Modern OS Functionality (brief review)
• Architecture Basics
• Hardware Support for OS Features

3
This TimeOS Structures Processes

• Components
• OS Organizations (kernels)
• Processes

4
Components

• Process
• Synchronization
• Memory management
• File system
• Secondary storage
• I/O systems
• Distributed systems

5
Key Component The Process
Process Memory mgmt. File system Secondary
storage I/O systems Distributed sys.

• OS manages variety of activities
• User programs, batch jobs, command scripts
• Daemons spoolers, name servers, file servers,
  etc.
• Each activity encapsulated in process
• Context (PC, registers, address space, etc.)
  required for activity to run
• Process ! program
• One program may comprise many processes
• Many processes may run same program

6
Processes
Process Memory mgmt. File system Secondary
storage I/O systems Distributed sys.

• OS manages schedules processes
• Creation, deletion
• Resource allocation (e.g., CPU, memory)
• Suspension resumption
• OS supports processes
• Inter-process communication
• Synchronization

7
Process Synchronization Example
Process Memory mgmt. File system Secondary
storage I/O systems Distributed sys.

• Banking transactions
• Cooperating processes operate on single account
• ATM transactions
• Balance computation
• Monthly interest calculation addition
• All may access same account simultaneously
• What could happen?

8
Memory Management
Process Memory mgmt. File system Secondary
storage I/O systems Distributed sys.

• Main memory
• Direct access storage for CPU
• Processes must be in main memory to execute
• OS must
• Maintain page tables (virtual/physical memory)
• Allocate deallocate memory
• Decide how much memory each process gets
• Decide when to remove memory from process

9
File System
Process Memory mgmt. File system Secondary
storage I/O systems Distributed sys.

• Secondary storage devices (e.g., disks)
• Block-level read, write to point on disk
• Too crude to use directly for long-term storage
• File system provides logical objects (files)
  operations on these objects
• Long-term storage entity
• Named collection of persistent information
• Can be read or written to

10
File System Organization
Process Memory mgmt. File system Secondary
storage I/O systems Distributed sys.

• Supports hierarchical organization of files
• Files grouped in directories
• Maintains metadata about files
• Date created
• Last modified date
• Controls access to files
• Who owns can alter files
• Read-only, executable, etc.

11
File System Management
Process Memory mgmt. File system Secondary
storage I/O systems Distributed sys.

• Standard interface to
• Create delete files, directories
• Manipulate files directories
• Read, write, extend, rename, copy, protect
• Map files into memory
• May provide other general services
• Backups
• Quotas

12
Secondary Storage (Disks, etc.)
Process Memory mgmt. File system Secondary
storage I/O systems Distributed sys.

• Persistent memory
• Endures system failures
• Low-level OS routines handle disk functions
• Scheduling disk operations
• Head movement
• Error handling
• May keep track of free space
• Sometimes these routines are in filesystem

13
I/O Systems
Process Memory mgmt. File system Secondary
storage I/O systems Distributed sys.

• Support communication with external devices
• Terminal, keyboard, printer, mouse
• Support buffering spooling of I/O
• Provide general device driver interface
• Hides differences between devices
• Often mimics file system interface
• Provide implementations of device drivers for
  specific devices

14
Distributed Systems
Process Memory mgmt. File system Secondary
storage I/O systems Distributed sys.

• Distributed system collection of processors
  that do not share memory or clock
• Processes must communicate over network
• OS must deal with failures deadlock
• Problems specific to distributed systems
• OS can support distributed file system
• Manages multiple independent storage devices
• All users, servers, storage devices may be
  dispersed

15
OS Structures Processes

• Components
• OS Organizations (kernels)
• Processes

16
Monolithic Kernel

• Classic UNIX approach, Linux
• Everything in kernel
• Fast
• Risky

17
Layered OS Design

• THE operating system
• Dijkstra

• Modular, simple, portable, easy to design/debug
• Communication overhead, copying, bookkeeping

18
The Microkernel Approach

• GoalMinimize contents of kernel
• Why?

19
Microkernel Motivation

• Minimize contents of kernel
• Reduces risk of crashing OS
• Put functionality in user-level processes
• Simplifies extension customization
• First µ-kernel Hydra (CMU)
• Current systems Mach (also CMU), by Rick Rashid
  et al. (now head of Microsoft Research)

20
µ-kernels vs. Monolithic Kernels

• Past conventional wisdom (1990s)
• Mach beautiful research idea but failed in
  practice
• Too slow!
• Linux ugly, monolithic, but fast
• Today much faster computers
• Mach fast enough (Mac OS X)
• Reliability, simplicity, robustness now more
  important than performance

21
OS Structures Processes

• Components
• OS Organizations (kernels)
• Processes

22
Processes

• Process Concept
• Process States
• Process Scheduling
• Process Management
• Interprocess Communication

23
Process Concept

• OS executes variety of programs
• Batch system jobs
• Time-shared systems user programs or tasks
• Process program in execution
• process execution sequential (kind of)
• Process includes
• program counter
• stack
• data section

24
Process States

• New
• Process being created
• Running
• Instructions being executed
• Waiting
• Process waiting for some event to occur
• Ready
• Process waiting to be assigned to a processor
• Terminated
• (duh)

25
Process State Diagram

• Transitions
• Program actions (system calls)
• OS actions (scheduling)
• External events (interrupts)

26
Process Execution Example

1. New
2. Ready
3. Running
4. Waiting for I/O
5. Ready
6. Running
7. Terminated

• void main() printf (Hello world\n)

27
Process Data Structures

• Process Control Block
• Tracks state
• OS allocates, places on queue
• OS deallocates on termination
• Lots of info
• Process state
• Program counter
• CPU registers
• CPU scheduling information
• Memory-management information
• Accounting information
• I/O status information

28
Process Scheduling Queues

• Job queue
• Set of all processes in system
• Ready queue
• Set of processes residing in main memory ready
  waiting to execute
• Device queues
• Set of processes waiting for I/O device
• One per device
• Process migration between the various queues.

29
Process Queues Example
30
CPU Scheduling
31
Process Scheduling
32
PCBs and Hardware State

• Switching processes context switch
• Relatively expensive
• Time between switches (quantum) must be long
  enough to amortize this cost
• Start
• OS picks ready process
• Loads register values from PCB
• Stop
• OS saves registers into PCB

33
Process Creation

• One process can create other processes
• Creator parent, new processes children
• Parent can wait for child to complete,or
  continue in parallel
• UNIX fork() used to create child processes
• Copies variables registers from parent to child
• Memory lazily copied (copy-on-write)
• Only difference between parent child return
  value
• Parent returns process id of child
• Child returns 0

34
Process Creation Example

• Logging into UNIX creates shell process
• Every command typed into shell
• Child of shell process (spawned by fork)
• Executes command via exec
• Example
• Type emacs
• OS forks new process
• exec executes emacs
• If followed by , runs in parallelotherwise,
  waits until done

35
Example UNIX Program Fork
36
Process Termination

• On termination, OS reclaims all resources
  assigned to process
• UNIX processes
• Can terminate self via exit system call
• Can terminate child via kill system call

37
Example Process Termination
38
Cooperating Processes

• Cooperating processes work together to accomplish
  a task
• Advantages
• Can improve performance by overlapping activities
  or performing work in parallel
• Can enable simpler program design
• Simplifies distribution
• Processes can live on different machines

39
Interprocess Communication

• Processes communicate in one of two ways
• Message passing
• Send and receive information
• Numerous means sockets, pipes, etc.
• Shared memory
• Establish mapping to named memory object
• Use mmap
• Fork processes that share this structure

40
Process Summary

• Process unit of execution
• Represented by Process Control Blocks
• Contain process state, scheduling info, etc.
• Process state
• New, Ready, Waiting, Running, or Terminated
• One running process at a time (on a uniprocessor)
• Context switch when changing process executing on
  CPU
• Communicate by message passing or shared memory

41
The End