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Operating Systems
CMPSCI 377
Lecture 3: OS Structures
Lecture 4: Processes
Emery Berger
University of Massachusetts, Amherst
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
Last Time:
OS & Computer Architecture
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Modern OS Functionality (brief review)
Architecture Basics
Hardware Support for OS Features
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
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This Time:
OS Structures & Processes
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Components
OS Organizations (kernels)
Processes
UNIVERSITY OF MASSACHUSETTS, AMHERST • Department of Computer Science
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Components
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Process
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Synchronization
Memory management
File system
Secondary storage
I/O systems
Distributed systems
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Process
Memory mgmt.
File system
Secondary storage
I/O systems
Distributed sys.
Key Component:The Process
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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
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One program may comprise many processes
Many processes may run same program
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Process
Memory mgmt.
File system
Secondary storage
I/O systems
Distributed sys.
Processes
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OS manages & schedules processes
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Creation, deletion
Resource allocation (e.g., CPU, memory)
Suspension & resumption
OS supports processes
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Inter-process communication
Synchronization
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Process
Memory mgmt.
File system
Secondary storage
I/O systems
Distributed sys.
Process Synchronization Example
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Banking transactions
Cooperating processes operate on single
account
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ATM transactions
Balance computation
Monthly interest calculation & addition
All may access same account simultaneously
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What could happen?
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Process
Memory mgmt.
File system
Secondary storage
I/O systems
Distributed sys.
Memory Management
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Main memory:
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Direct access storage for CPU
Processes: must be in main memory to execute
OS must:
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Maintain page tables (virtual/physical
memory)
Allocate & deallocate memory
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Decide how much memory each process gets
Decide when to remove memory from process
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Process
Memory mgmt.
File system
Secondary storage
I/O systems
Distributed sys.
File System
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Secondary storage devices (e.g., disks)
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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
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Long-term storage entity
Named collection of persistent information
Can be read or written to
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Process
Memory mgmt.
File system
Secondary storage
I/O systems
Distributed sys.
File System Organization
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Supports hierarchical organization of files
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Maintains metadata about files:
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Files grouped in directories
Date created
Last modified date
Controls access to files
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Who owns & can alter files
Read-only, executable, etc.
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Process
Memory mgmt.
File system
Secondary storage
I/O systems
Distributed sys.
File System Management
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Standard interface to:
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Create & delete files, directories
Manipulate files & directories
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Read, write, extend, rename, copy, protect
Map files into memory
May provide other general services:
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Backups
Quotas
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Process
Memory mgmt.
File system
Secondary storage
I/O systems
Distributed sys.
Secondary Storage (Disks, etc.)
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Persistent memory
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Endures system failures
Low-level OS routines handle disk functions:
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Scheduling disk operations
Head movement
Error handling
May keep track of free space
Sometimes these routines are in filesystem
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Process
Memory mgmt.
File system
Secondary storage
I/O systems
Distributed sys.
I/O Systems
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Support communication with external
devices:
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Support buffering & spooling of I/O
Provide general device driver interface
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Terminal, keyboard, printer, mouse
Hides differences between devices
Often mimics file system interface
Provide implementations of device drivers
for specific devices
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Process
Memory mgmt.
File system
Secondary storage
I/O systems
Distributed sys.
Distributed Systems
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Distributed system = collection of processors that
do not share memory or clock
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Processes must communicate over network
OS must deal with failures & deadlock
 Problems specific to distributed systems
OS can support distributed file system
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Manages multiple independent storage devices
All users, servers, storage devices may be dispersed
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OS Structures & Processes
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Components
OS Organizations (kernels)
Processes
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Monolithic Kernel
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Classic
UNIX
approach,
Linux
Everything
in kernel
+ Fast
- Risky
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Layered OS Design
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“THE” operating system
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Dijkstra
+ Modular, simple,
portable, easy to
design/debug
- Communication
overhead, copying,
bookkeeping
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The Microkernel Approach
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Goal:
Minimize
contents of
kernel
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Why?
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Microkernel: Motivation
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Minimize contents of kernel:
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Reduces risk of crashing OS
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Put functionality in user-level processes
Simplifies extension & customization
First μ-kernel: Hydra (CMU)
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Current systems: Mach (also CMU), by Rick
Rashid et al. (now head of Microsoft Research)
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μ-kernels vs. Monolithic Kernels
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Past conventional wisdom: (1990’s)
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Today: much faster computers
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Mach – beautiful research idea but “failed” in practice
 Too slow!
Linux – ugly, monolithic, but fast
Mach: fast enough (Mac OS X)
Reliability, simplicity, robustness now more
important than performance
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OS Structures & Processes
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Components
OS Organizations (kernels)
Processes
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Processes
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Process Concept
Process States
Process Scheduling
Process Management
Interprocess Communication
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Process Concept
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OS executes variety of programs:
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Process – program in execution
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Batch system – jobs
Time-shared systems – user programs or tasks
process execution sequential (kind of)
Process includes:
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program counter
stack
data section
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Process States
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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)
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Process State Diagram
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Transitions:
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Program
actions
(system calls)
OS actions
(scheduling)
External
events
(interrupts)
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Process Execution Example
void main() {
printf (“Hello world\n”);
}
1.
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6.
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New
Ready
Running
Waiting for I/O
Ready
Running
Terminated
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Process Data Structures
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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
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Process Scheduling Queues
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Job queue
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Ready queue
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Set of processes residing in main memory ready &
waiting to execute
Device queues
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Set of all processes in system
Set of processes waiting for I/O device
One per device
Process migration between the various queues.
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Process Queues Example
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CPU Scheduling
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Process Scheduling
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PCBs and Hardware State
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Switching processes: context switch
Relatively expensive
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Start:
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Time between switches (quantum) must be
long enough to amortize this cost
OS picks ready process
Loads register values from PCB
Stop:
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OS saves registers into PCB
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Process Creation
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One process can create other processes
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Creator = parent, new processes = children
Parent can wait for child to complete,
or continue in parallel
UNIX: fork() used to create child processes
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Copies variables & registers from parent to child
Memory lazily copied (copy-on-write)
Only difference between parent & child: return value
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Parent: returns process id of child
Child: returns 0
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Process Creation Example
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Logging into UNIX creates shell process
Every command typed into shell:
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Child of shell process (spawned by fork)
Executes command via exec
Example:
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Type “emacs”
OS forks new process
exec executes emacs
If followed by “&”, runs in parallel;
otherwise, waits until done
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Example UNIX Program: Fork
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Process Termination
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On termination, OS reclaims all resources
assigned to process
UNIX processes:
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Can terminate self via exit system call
Can terminate child via kill system call
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Example: Process Termination
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Cooperating Processes
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Cooperating processes work together to
accomplish a task
Advantages:
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Can improve performance by overlapping
activities or performing work in parallel
Can enable simpler program design
Simplifies distribution
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Processes can live on different machines
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Interprocess Communication
Processes communicate in one of two ways:
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Message passing:
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Send and receive information
Numerous means: sockets, pipes, etc.
Shared memory:
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Establish mapping to named memory object
Use mmap
Fork processes that share this structure
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Process Summary
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Process = unit of execution
Represented by Process Control Blocks
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Process state:
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New, Ready, Waiting, Running, or Terminated
One running process at a time (on a uniprocessor)
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Contain process state, scheduling info, etc.
Context switch when changing process executing on
CPU
Communicate by message passing or shared
memory
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The End
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