Ch04

Chapter 4: Threads

s Overview
s Multithreading Models
s Threading Issues
s Pthreads
s Windows XP Threads
s Linux Threads
s Java Threads

Operating System Concepts 4.2 Silberschatz, Galvin and Gagne

Thread: Introduction

s Each process has
1. Own Address Space
2. Single thread of control
s A process model has two concepts:
1. Resource grouping
2. Execution
s Sometimes it is useful to separate them

3

Unit of Resource Ownership

s A process has an
q Address space
q Open files
q Child processes
q Accounting information
q Signal handlers
q Etc
s If these are put together in a form of a process, can be managed
more easily

4

Unit of Dispatching
s Path of execution
q Program counter: which instruction is running
q Registers:
 holds current working variables
q Stack:
 Contains the execution history, with one entry for
each procedure called but not yet returned
q State
s Processes are used to group resources together
s Threads are the entities scheduled for execution
on the CPU
s Threads are also called lightweight process
5

Its better to distinguish between the two
concepts

Address
space/Global
Heavy weight process Variables
Open files
Child processes
Accounting info
Signal handlers
Lig
Program counter htIn case of multiple threads per
Registers we
igh
process
Stack tp
ro
ce
Unit of Resource State ss
es
Split Program
Address
space/Global counter Program
Variables Registers
counter
Open files Stack Registers
Program Unit of Dispatch
Child processes State Stack counter
Accounting info Share State Registers
6
Stack
Signal handlers

65

Threads allow you to multiplex 47
of
which resources? 65

149% 0
1. CPU
2. Memory
3. PCBs 81%
60% 62%
4. Open files 38%
5. User authentication structures

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Thread
s A thread is a basic unit of CPU utilization. It consist of
q A thread ID
q A program Counter
q A register Set
q A stack
s Threads share something with its peer threads (all the other5
threads in this particular task) the things that it share are
q Its code section
q Its data section
q Any OS resources, available for the task.
s The first thread starts execution with
int main(int argc, char *argv[])
s The threads appear to the Scheduling part of an OS just like any
other process
s Allow multiple execution paths in the same process environment
8

Threads vs. Processes

Threads Processes
s A thread has no data segment or A process has code/data/heap &
heap other segments
s A thread cannot live on its own, it There must be at least one thread in
must live within a process a process
s There can be more than one thread Threads within a process share
in a process, the first thread calls code/data/heap, share I/O, but each
main & has the process’s stack has its own stack & registers
s Inexpensive creation Expensive creation
s Inexpensive context switching Expensive context switching
s If a thread dies, its stack is If a process dies, its resources are
reclaimed reclaimed & all threads die
s Inter-thread communication via Inter-process communication via
memory. OS and data copying.


Process Vs. Threads

(a) Three threads, each running in (b) Three threads, sharing the
a separate address space same address space

10

Context switch time for which entity
65

is greater?
79%
1. Process
2. 47Thread

21%

d
s
es

a
re
oc

Th
Pr


The Thread Model

Each thread has its own stack
12

Single and Multithreaded Processes

A traditional heavy weight process is same as task
with one thread. It has a single thread of control.
If a process is multi thread, then that means more than
one part of the thread is executing at one time.
Multi threading can be useful in programs such as web
browsers where you can wish to download a file , view an
animation and print something at the same time.


Single and Multithreaded Processes


Implementing Threads
Process’s
s
TCB for address space
Processes define an address
Thread1
space; threads share the address
mapped segments
space
PC
SP DLL’s
s Process Control Block (PCB) State
contains process-specific Registers Heap
information …
q Owner, PID, heap pointer,
priority, active thread, and
pointers to thread information TCB for Stack – thread2
Thread2
s Thread Control Block (TCB) PC Stack – thread1
contains thread-specific information SP
q Stack pointer, PC, thread state State Initialized data
(running, …), register values, a Registers
pointer to PCB, … …
Code


Threads’ Life Cycle
s Threads (just like processes) go through a sequence of start,
ready, running, waiting, and done states

Start Done

Ready Running

Waiting


Benefits Of Multi-Threading

s Responsiveness

Multithreading increase the responsiveness .As the process
consist of more than one thread, if one thread block or busy in
lengthy calculation, some other thread still executing of the
process. So the user get response from executing process.

s Resource Sharing

All threads, which belongs to one process, share the memory and
resources of that process. Secondly it allow the application to have
several different threads within the same address space.


Benefits Of Multi-Threading

s Economy

Allocation of memory and resources or process creation is costly.
All threads of a process share the resources of that process so it is
more economical to create and context switch the thread.

s Utilization of Multi processor Architectures

MP architecture allows the facility of parallel processing, which is
most efficient way of processing. A single process can run on one
CPU even if we have more processors. Multi-threading on MP
system increase the concurrency. If a process is dividing into
multiple threads , these threads can execute simultaneously on
different processors.


Types of Threads

s There are two types of threads

q Kernel Threads

q User Threads


User Threads
s User level threads are not seen by operating system and also very fast

(switching from one thread to another thread in a single process does
not require context switch since same process is still executing).
However, if the thread that is currently executing blocks, the rest of the
process may also blocked( if OS is using only one single kernel thread
for this process i.e. the thread that kernel sees is same as blocked
thread , hence kernel assume that whole process is blocked).
s Thread management done by user-level threads library
s Three primary thread libraries:
q POSIX Pthreads
q Win32 threads
q Java threads


Kernel Threads

s Kernel Supported threads are seen by operating system and must be
scheduled by the operating system. One multi thread may have multiple
kernel threads.

s Examples
q Windows XP/2000
q Solaris
q Linux
q Tru64 UNIX
q Mac OS X


User-Level vs. Kernel Threads

User-Level Kernel-Level
s Managed by application s Managed by kernel
s Kernel not aware of thread s Consumes kernel resources
s Context switching cheap s Context switching expensive
s Create as many as needed s Number limited by kernel resources
s Must be used with care s Simpler to use

Key issue: kernel threads provide virtual processors to user-level threads,
but if all of kthreads block, then all user-level threads will block
even if the program logic allows them to proceed

Thread Libraries

s Thread library provides programmer with API for
creating and managing threads
s Two primary ways of implementing
q Library entirely in user space with no kernel support. All the
code and data structure exists in user space. This means that
invoking of a function in a the library results in a local function
call in user space and not a system call.
q Kernel-level library supported by the OS. In this case code and
data structures for the library exits in kernel space. Invoking a
function in the API of library typically results in a system call to
a kernel.


Thread Libraries

s Three main thread libraries are used
in today
q POSIX Pthreads
q Win32

q Java


Multithreading Models

s Many-to-One

s One-to-One

s Many-to-Many


Many-to-One

s Many user-level threads mapped to single kernel thread.

s It is efficient because it is implemented in user space. A process

using this model blocked entirely if a thread makes a blocking
system call.

s Only one thread can access the kernel at a time so it can not be

run in parallel on multiprocessor.

s Examples:

q Solaris Green Threads

q GNU Portable Threads("Genuinely Not Unix" ; GNU is an
operating system composed of free software)


Many-to-One Model


One-to-One

s Each user-level thread maps to kernel thread.

s It provides more concurrency because it allows another thread to
execute when threads invoke the blocking system call.

s It facilitates the parallelism in multiprocessor systems.

s Each user thread requires a kernel thread, which may affect the

performance of the system.

s Creation of threads in this model is restricted to certain number.
s Examples
q Windows NT/XP/2000
q Linux
q Solaris 9 and later

One-to-one Model


Many-to-Many Model
s Allows many user level threads to be mapped to many kernel threads

s Allows the operating system to create a sufficient number of kernel

threads

s Number of kernel threads may be specific to a either a particular

application or a particular machine.

s The user can create any number of threads and corresponding kernel

level threads can run in parallel on multiprocessor.

s When a thread makes a blocking system call, the kernel can execute

another thread.
s Solaris prior to version 9
s Windows NT/2000 with the ThreadFiber package


Many-to-Many Model


Multithreading Models: Comparison

s The many-to-one model allows the developer to
create as many user threads as he/she wishes,
but true concurrency can not be achieved
because only one kernel thread can be scheduled
for execution at a time
s The one-to-one model allows more concurrence,
but the developer has to be careful not to create
too many threads within an application
s The many-to-many model does not have these
disadvantages and limitations: developers can
create as many user threads as necessary, and
the corresponding kernel threads can run in
parallel on a multiprocessor

Two-level Model

s Similar to M:M, except that it allows a user thread to be bound
to kernel thread
s Examples
q IRIX
q HP-UX
q Tru64 UNIX
q Solaris 8 and earlier


Two-level Model
user-level
threads

LWP LWP LWP LWP

• Combination of one-to-one + “strict” many-to-many models
• Supports both bound and unbound threads
– Bound threads - permanently mapped to a single, dedicated LWP
– Unbound threads - may move among LWPs in set
• Thread creation, scheduling, synchronization done in user space
• Flexible approach, “best of both worlds”
• Used in Solaris implementation of Pthreads and several other Unix
implementations (IRIX, HP-UX)

Two-level Model


Pthreads

s A POSIX standard (IEEE 1003.1c) API for thread
creation and synchronization
s API specifies behavior of the thread library,
implementation is up to development of the library
s Common in UNIX operating systems (Solaris, Linux,
Mac OS X)


Java Threads

s Java threads are managed by the JVM

s Java threads may be created by:

q Extending Thread class
q Implementing the Runnable interface


Java Thread States


Threading Issues

s Semantics of fork() and exec() system calls
s Thread cancellation
s Signal handling
s Thread pools
s Thread specific data
s Scheduler activations


Semantics of fork() and exec()

s As the fork() system call is used to create a
separate, duplicate process.
s The semantics of the fork() and exec() system
calls change in a multithreaded program.
s If one thread in a program calls the fork(), does
the new process duplicate all the threads or is the
new process single threaded?


Semantics of fork() and exec()

s Does fork() duplicate only the calling thread or all threads?

• Two versions of fork() in UNIX:, one that duplicates all threads and another that
duplicates only the thread that invoked the fork() system call.

• If a thread invokes the exec() system call, the program specified in the parameter
to exec() will replace the entire process – including all threads.

• If exec() is called immediately after forking, then duplicating all threads is
unnecessary, as the program specified in the parameters to exec() will replace
the process. In this instance, duplicating only the calling thread is appropriate.

• If the separate process does not call exec() after forking, the separate process
should duplicate all threads.


Thread Cancellation

s Thread cancellation is the task of terminating a thread before it
has finished. For example, if multiple threads are concurrently
searching through a database and one thread return the result,
the remaining threads might be canceled.
s A thread that is to be often canceled is referred as the target
thread.
s Two general approaches:
q Asynchronous cancellation terminates the target thread
immediately
q Deferred cancellation allows the target thread to
periodically check if it should be cancelled, allowing it an
opportunity to terminate itself in an orderly fashion.


Problem in Thread Cancellation

s The difficulty with cancellation occurs in a situations
q where resources have been allocated to a cancel thread
q Where thread has been cancelled in the midst of updating
data it share with other threads.
s This becomes especially difficult with asynchronous
cancellation. Often operating system will reclaim
system resources from the canceled thread but will
not reclaim all the resources. Therefore, canceling a
thread asynchronously may not free a necessary
system-wide resource.


Signal Handling

s A signal is used in a UNIX system to notify a process
that a particular event has occurred. Signal may be
received either asynchronously or synchronously. All
Signal follow the same pattern.
s A signal handler is used to process signals
1. Signal is generated by particular event
2. Signal is delivered to a process
3. Signal is handled


Signal Handling

s Examples of synchronous signals include illegal memory

access and division by 0. if a running program perform either
of these operations a signal is generated. Synchronous signals
are delivered to the same process that performed the
operation that caused the signals.

s When a signal is generated by some event external to a

running process, that process receive the signal
asynchronously. Examples of such signals include
terminating a process with specific key strokes (such as
<control><C>) and having a timer expire. Typically ,
asynchronous signal is sent to another process.

Signal Handling in UNIX

s Every signal may be handled by one of two possible
handlers:

 A default signal handler that is run by the kernel when
handling that signal.
 A user-defined signal handler that is called to handle
a signal.


Signal Handling

s Delivering the signal in multithreaded programs is
more complicated, where a process may have
several threads. Following Options exist:
q Deliver the signal to the thread to which the
signal applies
q Deliver the signal to every thread in the
process
q Deliver the signal to certain threads in the
process
q Assign a specific thread to receive all signals
for the process.


Thread Pools

s The general idea is to create a number of threads at process

startup and place them into a pool where they sit and wait for
work

s Advantages:

q Usually slightly faster to service a request with an existing
thread than create a new thread

q Allows the number of threads in the application(s) to be
bound to the size of the pool


Thread Specific Data

s Allows each thread to have its own copy of data
s Useful when you do not have control over the thread
creation process (i.e., when using a thread pool)


Scheduler Activations

s A final issue to be considered with multithreaded programs

concerns with communication between kernel level and user
level thread library.

s Both M:M and Two-level models require communication to

maintain the appropriate number of kernel threads allocated to
the application.

s Many system implementing either M:M or Two level model place

an intermediate data structure between user and kernel threads.
This data structure is typically known as light weight process
or (LWP).


Scheduler Activations
s One scheme for communication between user thread library and kernel

thread library is known as Scheduler Activation. It works as follow:
q The kernel provides an application with a set of virtual processor
(LWPs), and the application can schedule user threads onto an
available virtual processor.

s The kernel must inform an application about certain events. This

procedure is known as upcalls - a communication mechanism from the
kernel to the thread library.

s Upcalls are handled by thread library with an upcall handler.

s Upcall handler must run on virtual processor.

s This communication allows an application to maintain the correct number

of kernel threads.

Operating System Examples

s Windows XP Threads
s Linux Thread


Windows XP Threads

s Implements the one-to-one mapping, kernel-level
s Each thread contains
q A thread id
q Register set
q Separate user and kernel stacks
q Private data storage area
s The register set, stacks, and private storage area are
known as the context of the threads
s The primary data structures of a thread include:
q ETHREAD (executive thread block)
q KTHREAD (kernel thread block)
q TEB (thread environment block)


Windows XP Threads


Linux Threads

s Linux refers to them as tasks rather than threads
s Thread creation is done through clone() system call
s clone() allows a child task to share the address space
of the parent task (process)


Linux Threads


Thread Usage
s Less time to create a new thread than a process
 the newly created thread uses the current process address
space
 no resources attached to them
s Less time to terminate a thread than a process.
s Less time to switch between two threads within the same
process, because the newly created thread uses the
current process address space.
s Less communication overheads
 threads share everything: address space, in particular. So, data
produced by one thread is immediately available to all the other
threads
s Performance gain
 Substantial Computing and Substantial Input/output
s Useful on systems with multiple processors57

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Editor's Notes