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Why Threads : Issues with Processes

What are threads?

Threading models

Threading issues

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The fork() system call is expensive

Copying the entire parent process as child

process. It may not always be required to copy

entire parent process to child process as the child

is going to call exec()system call immediately in

most of the cases.

IPC is required to pass information betweena parent and its child processes.

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A thread is a lightweight process which

executes within the address space of a process.

A thread is an execution context that is

independently scheduled, but shares single

addresses space with other threads of the same

process.

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Thread

Process

Terminated

f2

f1

A traditional process can be viewed as:

An address space with a single thread!

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Suppose we want that f1 and f2 should be executed

by separate threads, while main function is executed

concurrently by another thread.

Multi threading refers to the ability of an OS tosupport multiple threads of execution with in a

single process. A single program made up of a

number ofdifferent concurrent activities; sometimes

called multitasking.

Multithreading works similar to multiprogramming.

The CPU switches rapidly back and forth among

threads providing the illusion that threads are

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main()

{

thread(t1,f1);

thread(t2,f2);

}

f1(){ }

f2()

{ }

main t2t1

Process Address Space

PC

PC

PC

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A thread is a lightweight process which executes within the

address space of a process.

A thread can be scheduled to run on a CPU as an independent unit

and terminate.

Multiple threads can run simultaneously.

Threads may be managed by the operating system or by a user

application

Examples: Win32 threads, C-threads, Pthreads

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MS-DOS supports a single user

process and a single thread.

Some UNIX, support multipleuser processes but only supportone thread per process

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Java run-time environment is a

single process with multiple

threads

Multiple processes andthreads

are found in Windows, Solaris,

and many modern versions of

UNIX

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Each thread has

An execution state (running, ready, etc.) & An execution stack

Saved thread context when not running

Some per-thread static storage for local variables

Access to the memory and resources of its process (all threads of a process share this)

They reside in the same address space and have access to the same data.

When one thread alters an item of data in memory, other threads see the results if andwhen they access that item.

If one thread opens a file with read privileges, other threads in the same process can

also read from that file.

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AddressSpace

T1

Stack

State

Pro

gram

Staticdata

Resources

Stack

State

T2

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A thread can be in states similar to a process (new, ready,

running, blocked, terminated)

A thread can create another thread

Multiple threads can operate within the same address

space

No automatic protection mechanism is in place for

threadsthey are meant to help each other

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Processors were originally developed with only one

core. The core is the part of the processor that

actually performs the reading and executing of

instructions. A multi-core processor is a singlecomponent with two or more independent actual

processors (called "cores").

Multithread programming make more efficient use

of multiple cores.

It allows separate concurrent tasks to run parallel on

individual cores.

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Three most popular threading models

User-level threads

Kernel-level threads

Combination of user- and kernel-level threads

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By default an application begins with a single

thread within a process managed by Kernel.

Later the application may spawn a new thread

within the same process by invoking spawn

utility in the thread library.

User-level threads perform threadingoperations in user space

Threads are created by runtimelibraries.

When ever ULT makes a system call, Kernel

takes it as the

system call has been made by the process, so

all the threads of

the process are blocked.

Thread switching is fast and CPU is

not interrupted during

thread switching (No mode switch).

Kernel is scheduling the

processes while user level library will

be scheduling the thread SPRING 2011 - 102

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Thread management done by kernel

and Kernel is aware of threads

When ever a KLT makes a system call, only that

thread

is blocked and the Kernel can schedule another

thread

of same process. So all the threads of the

process are

NOT blocked

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Operating system maps all threads in amultithreaded process to single executioncontext

Many user threads per kernel thread; i.e. Kernel

sees just one thread.

Advantages

Thread management is done in user space, so it

is efficient.Disadvantages

When a thread within a process makes a system

call the entire process blocks.

Because only one thread can access kernel at a

time, no parallelism on multiprocessors is possible.

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User

level

Threads

Kernel

level

Threads

Multiple user threads are multiplexed

over a smaller or equal number of

kernel threads

Advantage

Application can create as many user

threads as needed.

Kernel threads run in parallel onmultiprocessors

When a thread blocks, another thread can

still run

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Semantics offork() and exec() system calls

Thread cancellation

Signal handling

Thread pools

Thread specific data

Scheduler activations

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Does fork() duplicate only the

calling thread

or

all threads?

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Terminating a thread before it has finished

Two general approaches:

Asynchronous cancellation

terminates the target thread

immediately.

Prematurely terminating a

thread can cause subtle errors in

processes because multiple

threads share the same address

space

Deferred cancellation allows the

target thread to periodically

check if it should be cancelled

Some thread implementations

allow a thread to determine

when it can be terminated to

prevent process from entering

inconsistent state

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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

Options:

Deliver the signal to the thread to which the signal applies

Deliver the signal to every thread in the process

Deliver the signal to certain threads in the process

Assign a specific thread to receive all signals for the process

SignalsareusedinUNIXsystemst

onotifyaprocess

thatapa

rticulareventhasoccurred

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Create a number of threads in a pool where

they await work

Advantages:

Usually slightly faster to service a request with

an existing thread than create a new thread

Allows the number of threads in the

application(s) to be bound to the size of the

pool

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Allows each thread to have its own copy of data

Useful when you do not have control over thethread creation process (i.e., when using a thread

pool)

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