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Chapter 4Threads

• Threads: Resource ownership and execution

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Processes and Threads

• Processes have two characteristics:– Resource ownership – a process includes

an address space to hold the process image and may be allocated control or ownership of resources

– Scheduling/execution – process execution follows an execution path that may be interleaved with other processes

• These two characteristics are treated independently by the operating system

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Processes and Threads

• The unit of dispatching is referred to as a thread or lightweight process

• The unit of resource ownership is referred to as a process or task

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Multithreading

• The ability of an OS to support multiple, concurrent paths of execution within a single process.

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Single Thread Approaches

• MS-DOS supports a single user process and a single thread

• Some UNIX, support multiple user processes but only support one thread per process

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Multithreading

• Java run-time environment is a single process with multiple threads

• Multiple processes and threads are found in Windows, Solaris, and many modern versions of UNIX

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Processes

• In a multithreaded environment, a process

is defined as

– a unit of resource allocation: a virtual address

space which holds the process image

– a unit of protection: protected access to

processors, other processes (for IPC), files,

I/O resources

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One or More Threads in Process

• Each thread has– An execution state (running, ready, etc.)– Saved thread context when not running– An execution stack– Some per-thread static storage for local

variables– Access to the memory and resources of its

process (all threads of a process share this)

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One view…

• One way to view a thread is as an independent program counter operating within a process.

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Threads vs. processes

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Threads vs. processes

• The thread control block contains register values, priority, and other thread-related state information.

• All threads of a process share the state and resources of that process.– reside in the same address space– have access to the same data

• when one thread alters a data, other threads see the results

• when one thread opens a file, other threads can also access that file

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Benefits of Threads

• Takes less time to create a new thread than a process (can be 10 times faster)

• Less time to terminate a thread than a process

• Switching between two threads takes less time that switching processes

• Threads can communicate with each other – without invoking the kernel

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Benefits of Threads

• If an application is implemented as a set of related units of execution, it is far more efficient to do so as a collection of threads rather than a collection of separate processes

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Example Applications of Threads

• In a file server, a new thread can be spawned for the file management program per each new file request multiple threads within the same process can be executing simultaneously on different processors

• In a spreadsheet program, one thread could read user input while another thread executes user commands

• In a word processor, a thread can be created to do periodic backup asynchronously

• In a multithreaded process, one thread can compute one batch of data while another thread reads the next batch from an I/O device

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Threads vs. processes

• Like processes, threads have execution states – Running, Ready, and Blocked

• Some states are process-level– Suspend: if a process is swapped out, all of

its threads are necessarily swapped out because they all share the address space of the process

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Threads vs. processes

• Like processes, thread may synchronize

with one another.

– Any alteration of a resource by one thread

affects other threads in the same process

• consider two threads each try to add an element to

a linked list at the same time

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Example: Remote Procedure Call

• Consider:– A program that performs two remote

procedure calls (RPCs) • to two different hosts

• to obtain a combined result.

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RPCUsing Single Thread

The program has to wait for a response from each server in turn.

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RPC Using One Thread per Server

The program waits concurrently for the two replies.

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Multithreading on a Uniprocessor

Interleaving of multiple threads within multiple processes on a uniprocessor