Transcript Introduction to Operating Systems (continued)

Introduction to Operating Systems
(continued)
CS-2301, System Programming for
Non-majors
(Slides include materials from The C Programming Language, 2nd ed., by Kernighan and Ritchie and from
C: How to Program, 5th ed., by Deitel and Deitel)
CS-2301 B-term 2008
Introduction to Operating
Systems
1
Review – Four fundamental Abstractions
• Processes & threads
• Multiplexing of processor(s) to create the illusion of
many of them
• Virtual memory
• Multiplexing of physical memory and disk blocks to
create illusion of own memory per process
• Files & persistent storage
• Organizing principles about long-term data storage
• Sockets & connections
• Organizing principles about network communication
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Introduction to Operating
Systems
2
Review – Abstraction
• The distillation of a complex mechanism
into a simple, conceptual model
• User of abstraction does not need to worry
about details
• Implementer of abstraction does not need to
worry about how user will use it (within
limits)
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Introduction to Operating
Systems
3
Definition – Process
• A particular execution of a program
• Different from all other executions of that program
• Different from executions of other programs
• The OS uses one or more CPUs to make it
look like each process has own CPU
• Can execute at same time!
• Uses interrupts to manage and enforce multiplexing
of CPU
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Introduction to Operating
Systems
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Definition – Interrupt
• A mechanism by which the processor
suspends execution of the current, running
program and gives control to the OS
• OS saves the state of the interrupted
program so that it can be restarted later
• OS then takes appropriate action
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Review – Why Processes?
• Independent applications can share a
computing system
• Enables applications to overlap computation
with external activities
• Exploit parallelism in modern processors
• Vehicle for managing resources, security,
etc.
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Resources Assigned to a Process
• Memory
• Virtual or real
• Processor time
• Priorities
• Deadlines for real-time activities
• Privileges
• Security, authentication, etc.
• Files and file space
• For long-term storage, temporary storage
• Devices
• For input and output activity, sensors, etc.
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Note
• Every command on Linux command line
creates a separate process
• May run in foreground or background
• Double-clicking on icon in Windows often
creates a separate process
• May run in same window or another window
• Processes in embedded systems are usually
created at system startup
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Introduction to Operating
Systems
8
Questions about Processes?
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Introduction to Operating
Systems
9
Four fundamental Abstractions
• Processes & threads
• Multiplexing of processor(s) to create the illusion of
many of them
• Virtual memory
• Multiplexing of physical memory and disk blocks to
create illusion of own memory per process
• Files & persistent storage
• Organizing principles about long-term data storage
• Sockets & connections
• Organizing principles about network communication
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Introduction to Operating
Systems
10
Definition – Virtual Memory
• The illusion that each process has its own memory
– Separate from virtual memories of other processes
– (Often) more memory than machine has installed
• Implemented by translating “logical” addresses of program
into “physical” address of RAM (random access memory)
– Every memory reference, both program and data, on-the-fly
• E.g., when the program utters 0x00105C, …
– … the machine accesses 0x61605C
• OS creates illusion by swapping blocks of memory from
RAM to disk and back as needed
– Called pages
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Introduction to Operating
Systems
11
Virtual Memory for Process
(Windows & Linux)
0xFFFFFFFF
stack
(dynamically allocated)
SP
address space
heap
(dynamically allocated)
static data
0x00000000
CS-2301 B-term 2008
program code
(text)
Introduction to Operating
Systems
PC
12
Virtual Memory (continued)
• Typical virtual memory size – 4 GBytes
• Per process — even in a 1 GByte computer!
• “Inactive” pages are not resident in RAM
• Reference results in interrupt called a page fault
• OS brings in page from disk, (maybe) swaps one out
• Unused pages not filled in by OS
• Dereferencing pointer results in Segmentation Fault
• New pages created by OS as needed
• When stack or heap grows or programs are loaded
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Introduction to Operating
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Virtual Memory in Embedded Systems
• Implemented by partitioning RAM
• No swapping in or out
• May not even have a disk!
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Systems
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Questions about Processes
or Virtual Memory?
Not in Kernighan & Ritchie
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Introduction to Operating
Systems
16
Threads
• A refinement of concept of process
• Short for “thread of control”
• Concurrent execution of a function within
the context of a process
• Needs own stack in order to call other functions
• Shares heap, static data, and code with other threads
of same process
• Reason
• Application may need to manage concurrency of its
own computation with external events
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Systems
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Thread Interface – POSIX standard
#include <pthread.h>
• int pthread_create(pthread_t *thread, const
pthread_attr_t *attr, void*(*start_routine) (void),
void *arg)
– creates a new thread of control
– new thread begins executing at start_routine
• pthread_exit(void *value_ptr)
– terminates the calling thread
• pthread_join(pthread_t thread, void **value_ptr)
– blocks the calling thread until the thread specified terminates
• pthread_t pthread_self()
– Returns the calling thread's identifier
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Introduction to Operating
Systems
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Thread Interface – POSIX standard
#include <pthread.h>
• int pthread_create(pthread_t *thread, const
pthread_attr_t *attr, void*(*start_routine) (void),
void *arg)
– creates a new thread of control
– new thread begins executing at start_routine
• pthread_exit(void *value_ptr)
– terminates the calling thread
• pthread_join(pthread_t thread, void **value_ptr)
– blocks the calling thread until the thread specified terminates
• pthread_t pthread_self()
– Returns the calling thread's identifier
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Introduction to Operating
Systems
19
Thread Interface – POSIX standard
#include <pthread.h>
• int pthread_create(pthread_t *thread, const
pthread_attr_t *attr, void*(*start_routine) (void),
void *arg)
– creates a new thread of control
– new thread begins executing at start_routine
• pthread_exit(void *value_ptr)
– terminates the calling thread
• pthread_join(pthread_t thread, void **value_ptr)
– blocks the calling thread until the thread specified terminates
• pthread_t pthread_self()
– Returns the calling thread's identifier
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Introduction to Operating
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Thread Example
pthread_create(tp, &f, &args)
f
main function
g
main function
f
h
main function
f
pthread_join(tp)
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Introduction to Operating
Systems
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Thread Example
pthread_create(tp, &f, &args)
Two threads run
at the same time
f
main function
g
main function
f
h
main function
f
pthread_join(tp)
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Introduction to Operating
Systems
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Virtual Memory for Multiple Threads
0xFFFFFFFF
thread 1 stack
SP (T1)
thread 2 stack
SP (T2)
thread 3 stack
SP (T3)
Virtual
address space
heap
static data
0x00000000
CS-2301 B-term 2008
PC (T2)
PC (T1)
PC (T3)
code
(text)
Introduction to Operating
Systems
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Why Threads?
• To enable development of applications with
concurrent activities inside them
• Need to share data (difficult with separate virtual
memories)
• Examples
•
•
•
•
•
Web server over common data pages
Transaction processor over common data base
Applications within a mobile phone or PDA
Applications with different speeds of devices
…
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Example Thread Application
• One or more threads to get data from sensor
• 1000 times per second; cannot afford to miss a
reading
• Places data on queue
• Another thread processes and displays data
• Removes and processes each data item from queue
• Displays system state on control panel
• User thread
• Allows operator to adjust system parameters
• While other threads are still running
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Systems
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Additional Functions & Data Required
• pthread_mutex_t — mutual exclusion lock
– Enables a thread to “lock” some data so that
other threads do not touch in mid-operation
• pthread_cond_t — condition variable
– Enables a thread to wait for an event to happen
– Signaled by another thread
• See man pages for details
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Systems
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Questions about Threads?
Not in Kernighan & Ritchie
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Systems
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Four fundamental Abstractions
• Processes & threads
• Multiplexing of processor(s) to create the illusion of
many of them
• Virtual memory
• Multiplexing of physical memory and disk blocks to
create illusion of own memory per process
• Files & persistent storage
• Organizing principles about long-term data storage
• Sockets & connections
• Organizing principles about network communication
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Introduction to Operating
Systems
28
Definition – File
• A (potentially) large amount of information or data
that lives a (potentially) very long time
• Often much larger than the memory of the computer
• Often much longer than any computation
• Sometimes longer than life of machine itself
• (Usually) organized as a linear array of bytes or
blocks
• Internal structure is imposed by application
• (Occasionally) blocks may be variable length
• (Often) requiring concurrent access by multiple
threads or processes
• Even by processes on different machines!
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Introduction to Operating
Systems
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Implementations of Files
• Usually on disks (or devices that mimic disks)
• Magnetic – hard drive or floppy
• Optical – CD, DVD
• Flash drives – electronic memory, organized as disks
• Requirement
• Preserve data contents during power-off or disasters
• Directory / Folder
• Special kind of file that points to other files
• Associates names with files
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Introduction to Operating
Systems
30
Implementations of Files
• Usually on disks (or devices that mimic disks)
• Magnetic – hard drive or floppy
• Optical – CD, DVD
• Flash drives – electronic memory, organized as disks
• Requirement
• Preserve data contents during power-off or disasters
• Directory / Folder
• Special kind of file that points to other files
• Associates names with files
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Introduction to Operating
Systems
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File Access in C
• See Kernighan & Ritchie, Chapter 8
• Raw file access
• Without simplifying stream functions – e.g.,
– scanf, fscanf, printf, fprintf, fgetc, etc.
• read and write raw disk blocks
• Seek to a file position
– lseek, fseek — sets file pointer to specified
location
– Subsequent read, write, etc., start there
– ftell – returns file pointer
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Introduction to Operating
Systems
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Organizations of Files
• Contiguous
• Blocks stored contiguously on storage medium
• E.g., CD, DVD, some large database systems
• Access time to any block is O(1)
• Linked
• Blocks linked together – File Allocation Table (FAT)
• Access time is O(n)
• Indexed
• Blocks accessed via tree of index blocks (i-nodes)
• Access time is O(log n)
• However, base of logarithm may be very large (>100)
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Introduction to Operating
Systems
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Organizations of Files
• Contiguous
• Blocks stored contiguously on storage medium
• E.g., CD, DVD, some large database systems
• Access time to any block is O(1)
• Linked
• Blocks linked together – File Allocation Table (FAT)
• Access time is O(n)
• Indexed
• Blocks accessed via tree of index blocks (i-nodes)
• Access time is O(log n)
• However, base of logarithm may be very large (>100)
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Introduction to Operating
Systems
34
Organizations of Files
• Contiguous
• Blocks stored contiguously on storage medium
• E.g., CD, DVD, some large database systems
• Access time to any block is O(1)
• Linked
• Blocks linked together – File Allocation Table (FAT)
• Access time is O(n)
• Indexed
• Blocks accessed via tree of index blocks (i-nodes)
• Access time is O(log n)
• However, base of logarithm may be very large (>100)
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Introduction to Operating
Systems
35
Questions about Files?
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Introduction to Operating
Systems
36
Four fundamental Abstractions
• Processes & threads
• Multiplexing of processor(s) to create the illusion of
many of them
• Virtual memory
• Multiplexing of physical memory and disk blocks to
create illusion of own memory per process
• Files & persistent storage
• Organizing principles about long-term data storage
• Sockets & connections
• Organizing principles about network communication
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Introduction to Operating
Systems
37
Sockets and Connections
• Connection:
– a serial conversation over a network between two end
points
• e.g., processes, threads, tasks on different computers
– organized as a sequence of messages or datagrams
– distinct from all other connections
• Socket:
– An end point of a connection
– An abstraction that allows a process to send or receive
only the information of that connection
– Multiplexed on network with all other connections
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Introduction to Operating
Systems
38
Sockets and Connections
• Defer to another course
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Introduction to Operating
Systems
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General Questions?
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Introduction to Operating
Systems
40