Chapter 2 : Lecture 1
Processes and Threads
Outline
Process Concept
Process Scheduling
Operations on Processes
Cooperating Processes
Threads
Interprocess Communication
Process Concept
An operating system executes a variety of
programs
batch systems - jobs
time-shared systems - user programs or tasks
job and program used interchangeably
Process - a program in execution
process execution proceeds in a sequential fashion
A process contains
program counter, stack and data section
Process State
A process changes state as it executes.
new admitted
exit terminated
interrupt
ready running
Scheduler
I/O or dispatch
event I/O or
completion event wait
waiting
Process States
New - The process is being created.
Running - Instructions are being executed.
Waiting - Waiting for some event to occur.
Ready - Waiting to be assigned to a
processor.
Terminated - Process has finished execution.
Process Control Block
Contains information associated with
each process
Process State - e.g. new, ready,
running etc.
Process Number – Process ID
Program Counter - address of
next instruction to be executed
CPU registers - general purpose
registers, stack pointer etc.
CPU scheduling information -
process priority, pointer
Memory Management information
- base/limit information
Accounting information - time Process
limits, process number Control
I/O Status information - list of
Block
I/O devices allocated
Process Scheduling Queues
Job Queue - set of all processes in the system
Ready Queue - set of all processes residing in main
memory, ready and waiting to execute.
Device Queues - set of processes waiting for an I/O
device.
Process migration between the various queues.
Queue Structures - typically linked list, circular list
etc.
Enabling Concurrency and Protection:
Multiplex processes
Only one process (PCB) active at a time
Current state of process held in PCB:
“snapshot” of the execution and protection
environment
Process needs CPU, resources
Give out CPU time to different processes (Scheduling):
Only one process “running” at a time
Give more time to important processes
Give pieces of resources to different processes
(Protection):
Controlled access to non-CPU resources
E.g. Memory Mapping: Give each process their Process
own address space
Control
Block
Enabling Concurrency: Context Switch
Task that switches CPU from one process to
another process
the CPU must save the PCB state of the old process and
load the saved PCB state of the new process.
Context-switch time is overhead
System does no useful work while switching
Overhead sets minimum practical switching time; can
become a bottleneck
Time for context switch is dependent on
hardware support ( 1- 1000 microseconds).
CPU Switch From Process to Process
Code executed in kernel above is overhead
Overhead sets minimum practical switching time
Schedulers
Long-term scheduler (or job scheduler) -
selects which processes should be brought into the ready
queue.
invoked very infrequently (seconds, minutes); may be slow.
controls the degree of multiprogramming
Short term scheduler (or CPU scheduler) -
selects which process should execute next and allocates
CPU.
invoked very frequently (milliseconds) - must be very fast
Medium Term Scheduler
swaps out process temporarily
balances load for better throughput
Medium Term (Time-sharing)
Scheduler
Process Profiles
I/O bound process -
spends more time in I/O, short CPU bursts, CPU
underutilized.
CPU bound process -
spends more time doing computations; few very long CPU
bursts, I/O underutilized.
The right job mix:
Long term scheduler - admits jobs to keep load balanced
between I/O and CPU bound processes
Medium term scheduler – ensures the right mix (by
sometimes swapping out jobs and resuming them later)
Process Creation
Processes are created and deleted
dynamically
Process which creates another process is
called a parent process; the created process
is called a child process.
Result is a tree of processes
e.g. UNIX - processes have dependencies and form a
hierarchy.
Resources required when creating process
CPU time, files, memory, I/O devices etc.
UNIX Process Hierarchy
What does it take to create a process?
Must construct new PCB
Inexpensive
Must set up new page tables for address space
More expensive
Copy data from parent process? (Unix fork() )
Semantics of Unix fork() are that the child process gets a
complete copy of the parent memory and I/O state
Originally very expensive
Much less expensive with “copy on write”
Copy I/O state (file handles, etc)
Medium expense
Process Creation
Resource sharing
Parent and children share all resources.
Children share subset of parent’s resources - prevents
many processes from overloading the system.
Parent and children share no resources.
Execution
Parent and child execute concurrently.
Parent waits until child has terminated.
Address Space
Child process is duplicate of parent process.
Child process has a program loaded into it.
UNIX Process Creation
Fork system call creates new processes
execve system call is used after a fork to
replace the processes memory space with a
new program.
Process Termination
Process executes last statement and asks
the operating system to delete it (exit).
Output data from child to parent (via wait).
Process’ resources are deallocated by operating system.
Parent may terminate execution of child
processes.
Child has exceeded allocated resources.
Task assigned to child is no longer required.
Parent is exiting
OS does not allow child to continue if parent terminates
Cascading termination
Threads
Processes do not share resources well
high context switching overhead
Idea: Separate concurrency from protection
Multithreading: a single program made up of a number of
different concurrent activities
A thread (or lightweight process)
basic unit of CPU utilization; it consists of:
program counter, register set and stack space
A thread shares the following with peer threads:
code section, data section and OS resources (open files, signals)
No protection between threads
Collectively called a task.
Heavyweight process is a task with one thread.
Single and Multithreaded Processes
Threads encapsulate concurrency: “Active” component
Address spaces encapsulate protection: “Passive” part
Keeps buggy program from trashing the system
Benefits
Responsiveness
Resource Sharing
Economy
Utilization of MP Architectures
Threads(Cont.)
In a multiple threaded task, while one server
thread is blocked and waiting, a second
thread in the same task can run.
Cooperation of multiple threads in the same job confers
higher throughput and improved performance.
Applications that require sharing a common buffer (i.e.
producer-consumer) benefit from thread utilization.
Threads provide a mechanism that allows
sequential processes to make blocking
system calls while also achieving parallelism.
Thread State
State shared by all threads in process/addr
space
Contents of memory (global variables, heap)
I/O state (file system, network connections, etc)
State “private” to each thread
Kept in TCB Thread Control Block
CPU registers (including, program counter)
Execution stack
Parameters, Temporary variables
return PCs are kept while called procedures are
executing
Threads (cont.)
Thread context switch still requires a register
set switch, but no memory management
related work!!
Thread states -
ready, blocked, running, terminated
Threads share CPU and only one thread can
run at a time.
No protection among threads.
Examples: Multithreaded programs
Embedded systems
Elevators, Planes, Medical systems, Wristwatches
Single Program, concurrent operations
Most modern OS kernels
Internally concurrent because have to deal with
concurrent requests by multiple users
But no protection needed within kernel
Database Servers
Access to shared data by many concurrent users
Also background utility processing must be done
More Examples: Multithreaded programs
Network Servers
Concurrent requests from network
Again, single program, multiple concurrent operations
File server, Web server, and airline reservation systems
Parallel Programming (More than one physical CPU)
Split program into multiple threads for parallelism
This is called Multiprocessing
# of addr
spaces:
One Many
# threads
Per AS:
MS/DOS, early
One Traditional UNIX
Macintosh
Embedded systems Mach, OS/2, Linux
(Geoworks, VxWorks, Windows 9x???
Many JavaOS,etc)
Win NT to XP, Solaris,
JavaOS, Pilot(PC) HP-UX, OS X
Real operating systems have either
One or many address spaces
One or many threads per address space
Types of Threads
Kernel-supported threads
User-level threads
Hybrid approach implements both user-level
and kernel-supported threads (Solaris 2).
Kernel Threads
Supported by the Kernel
Native threads supported directly by the kernel
Every thread can run or block independently
One process may have several threads waiting on different things
Downside of kernel threads: a bit expensive
Need to make a crossing into kernel mode to schedule
Examples
Windows XP/2000, Solaris, Linux,Tru64 UNIX,
Mac OS X, Mach, OS/2
User Threads
Supported above the kernel, via a set of library calls
at the user level.
Thread management done by user-level threads library
User program provides scheduler and thread package
May have several user threads per kernel thread
User threads may be scheduled non-premptively relative to
each other (only switch on yield())
Advantages
Cheap, Fast
Threads do not need to call OS and cause interrupts to kernel
Disadv: If kernel is single threaded, system call from any
thread can block the entire task.
Example thread libraries:
POSIX Pthreads, Win32 threads, Java threads
Multithreading Models
Many-to-One
One-to-One
Many-to-Many
Many-to-One
Many user-level threads mapped to single
kernel thread
Examples:
Solaris Green Threads
GNU Portable Threads
One-to-One
Each user-level thread maps to kernel thread
Examples
Windows NT/XP/2000; Linux; Solaris 9 and later
Many-to-Many Model
Allows many user level
threads to be mapped to
many kernel threads
Allows the operating
system to create a
sufficient number of
kernel threads
Solaris prior to version 9
Windows NT/2000 with
the ThreadFiber package
Thread Support in Solaris 2
Solaris 2 is a version of UNIX with support for
kernel and user level threads, symmetric multiprocessing
and real-time scheduling.
Lightweight Processes (LWP)
intermediate between user and kernel level threads
each LWP is connected to exactly one kernel thread
Threads in Solaris 2
Threads in Solaris 2
Resource requirements of thread types
Kernel Thread: small data structure and stack; thread
switching does not require changing memory access
information - relatively fast.
Lightweight Process: PCB with register data, accounting
and memory information - switching between LWP is
relatively slow.
User-level thread: only needs stack and program
counter; no kernel involvement means fast switching.
Kernel only sees the LWPs that support user-level
threads.
Two-level Model
Similar to M:M, except that it allows a user
thread to be bound to kernel thread
Examples
IRIX, HP-UX, Tru64 UNIX, Solaris 8 and earlier
Threading Issues
Semantics of fork() and exec() system
calls
Thread cancellation
Signal handling
Thread pools
Thread specific data
Multi( processing, programming, threading)
Definitions:
Multiprocessing Multiple CPUs
Multiprogramming Multiple Jobs or Processes
Multithreading Multiple threads per Process
What does it mean to run two threads “concurrently”?
Scheduler is free to run threads in any order and interleaving: FIFO, Random,
…
Dispatcher can choose to run each thread to completion or time-slice in big
chunks or small chunks
B
Multiprocessing
C
A B C
Multiprogramming A B C A B C B
Cooperating Processes
Concurrent Processes can be
Independent processes
cannot affect or be affected by the execution of another
process.
Cooperating processes
can affect or be affected by the execution of another process.
Advantages of process cooperation:
Information sharing
Computation speedup
Modularity
Convenience(e.g. editing, printing, compiling)
Concurrent execution requires
process communication and process synchronization
Interprocess Communication (IPC)
Proc 1 Proc 2 Proc 3
Separate address space isolates processes
High Creation/Memory Overhead; (Relatively) High Context-Switch Overhead
Mechanism for processes to communicate and synchronize actions.
Via shared memory - Accomplished by mapping addresses to common DRAM
Read and Write through memory
Via Messaging system - processes communicate without resorting
to shared variables.
send() and receive() messages
Can be used over the network!
Messaging system and shared memory not mutually exclusive
can be used simultaneously within a single OS or a single process.
Shared Memory Communication
Data 2
Code Code
Stack 1 Data
Data
Heap 1 Heap
Heap
Code 1 Stack
Stack
Stack 2 Shared
Shared
Data 1
Prog 1 Prog 2
Heap 2 Virtual
Virtual
Address Code 2 Address
Space 1 Space 2
Shared
Communication occurs by “simply” reading/writing to shared
address page
Really low overhead communication
Introduces complex synchronization problems
Cooperating Processes via Message
Passing
IPC facility provides two operations.
send(message) - message size can be fixed or variable
receive(message)
If processes P and Q wish to communicate, they
need to:
establish a communication link between them
exchange messages via send/receive
Fixed vs. Variable size message
Fixed message size - straightforward physical implementation,
programming task is difficult due to fragmentation
Variable message size - simpler programming, more complex
physical implementation.
Implementation Questions
How are links established?
Can a link be associated with more than 2
processes?
How many links can there be between every pair
of communicating processes?
What is the capacity of a link?
Fixed or variable size messages?
Unidirectional or bidirectional links?
…….
Direct Communication
Sender and Receiver processes must name
each other explicitly:
send(P, message) - send a message to process P
receive(Q, message) - receive a message from process Q
Properties of communication link:
Links are established automatically.
A link is associated with exactly one pair of communicating
processes.
Exactly one link between each pair.
Link may be unidirectional, usually bidirectional.
Indirect Communication
Messages are directed to and received from
mailboxes (also called ports)
Unique ID for every mailbox.
Processes can communicate only if they share a mailbox.
Send(A, message) /* send message to mailbox A */
Receive(A, message) /* receive message from mailbox A */
Properties of communication link
Link established only if processes share a common mailbox.
Link can be associated with many processes.
Pair of processes may share several communication links
Links may be unidirectional or bidirectional
Indirect Communication using
mailboxes
Mailboxes (cont.)
Operations
create a new mailbox
send/receive messages through mailbox
destroy a mailbox
Issue: Mailbox sharing
P1, P2 and P3 share mailbox A.
P1 sends message, P2 and P3 receive… who gets
message??
Possible Solutions
disallow links between more than 2 processes
allow only one process at a time to execute receive operation
allow system to arbitrarily select receiver and then notify
sender.
Message Buffering
Link has some capacity - determine the
number of messages that can reside
temporarily in it.
Queue of messages attached to link
Zero-capacity Queues: 0 messages
sender waits for receiver (synchronization is called
rendezvous)
Bounded capacity Queues: Finite length of n messages
sender waits if link is full
Unbounded capacity Queues: Infinite queue length
sender never waits
Message Problems - Exception
Conditions
Process Termination
Problem: P(sender) terminates, Q(receiver) blocks forever.
Solutions:
System terminates Q.
System notifies Q that P has terminated.
Q has an internal mechanism(timer) that determines how long to wait
for a message from P.
Problem: P(sender) sends message, Q(receiver) terminates.
In automatic buffering, P sends message until buffer is full or
forever. In no-buffering scheme, P blocks forever.
Solutions:
System notifies P
System terminates P
P and Q use acknowledgement with timeout
Message Problems - Exception
Conditions
Lost Messages
OS guarantees retransmission
sender is responsible for detecting it using timeouts
sender gets an exception
Scrambled Messages
Message arrives from sender P to receiver Q, but
information in message is corrupted due to noise in
communication channel.
Solution
need error detection mechanism, e.g. CHECKSUM
need error correction mechanism, e.g. retransmission
Producer-Consumer Problem
Paradigm for cooperating processes;
producer process produces information that is
consumed by a consumer process.
We need buffer of items that can be filled by
producer and emptied by consumer.
Unbounded-buffer places no practical limit on the size of
the buffer. Consumer may wait, producer never waits.
Bounded-buffer assumes that there is a fixed buffer size.
Consumer waits for new item, producer waits if buffer is full.
Producer and Consumer must synchronize.
Producer-Consumer Problem
Bounded Buffer using IPC
(messaging)
Producer
repeat
…
produce an item in nextp;
…
send(consumer, nextp);
until false;
Consumer
repeat
receive(producer, nextc);
…
consume item from nextc;
…
until false;
Bounded-buffer - Shared Memory
Solution
Shared data
var n;
type item = ….;
var buffer: array[0..n-1] of item;
in, out: 0..n-1;
in :=0; out:= 0; /* shared buffer = circular array */
/* Buffer empty if in == out */
/* Buffer full if (in+1) mod n == out */
/* noop means ‘do nothing’ */
Bounded Buffer - Shared Memory
Solution
Producer process - creates filled buffers
repeat
…
produce an item in nextp
…
while in+1 mod n = out do noop;
buffer[in] := nextp;
in := in+1 mod n;
until false;
Bounded Buffer - Shared Memory
Solution
Consumer process - Empties filled buffers
repeat
while in = out do noop;
nextc := buffer[out] ;
out:= out+1 mod n;
…
consume the next item in nextc
…
until false
