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Process
A program in execution
An instance of a program
running on a computer
The entity that can be assigned
to and executed on a processor
A unit of activity characterized by
the execution of a sequence of instructions
a current state
an associated set of system resources
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Address Space
PCB
Process in Memory
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Multiprogramming
The interleaved execution of two or more computer
programs by a single processor
An important technique that
enables a
time-sharing system
allows the OS to overlap I/O and computation, creating an efficient system
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Processes
The Process Model
Multiprogramming of four programs
Conceptual model of
4 independent, sequential processes
Only one program active at any
instant
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Cooperating Processes (I)
Sequential programs consist of a single
process
Concurrent applications consist of multiple cooperating processes that execute
concurrently
Advantages
Can exploit multiple CPUs (hardware concurrency) for speeding up application
Application can benefit from software concurrency, e.g., web servers, window systems
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Cooperating Processes (II)
Cooperating processes need to share information
Since each process has its own address space, OS
mechanisms are needed to let process exchange information
Two paradigms for cooperating processes
Shared Memory
OS enables two independent processes to have a shared memory segment in their address spaces
Message-passing
OS provides mechanisms for processes to send and receive messages
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Threads: Motivation
Process created and managed by the OS
kernel
Process creation expensive, e.g., fork system call
Context switching expensive
IPC
requires kernel intervention expensive
Cooperating processes – no need for memory protection, i.e., separate address spaces
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Threads
The Thread Model (1)
(a) Three processes each with
one thread
(b) One process with three threads
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The Thread Model (2)
Items shared by all threads
in a process
Items private to each thread
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The Thread Model (3)
Each thread has its own
stack
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Thread Usage (1)
A word processor with three threads
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Thread Usage (2)
A multithreaded Web server
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Thread Implementation - Packages
Threads are provided as
a package, including operations to create, destroy, and synchronize
them
A package can be implemented as:
User-level threads
Kernel threads
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Implementing Threads in User Space
A user-level threads package
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User-Level Threads
Thread management done by user-level threads library
Examples
POSIX
Pthreads
Mach C-threads
Solaris threads
Java threads
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User-Level Threads
Thread library entirely executed in user mode
Cheap
to manage threads
Create: setup a stack
Destroy: free up memory
Context
switch requires few instructions
Just save CPU registers
Done based on program logic
A blocking system call blocks all peer threads
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Kernel-Level Threads
Kernel is aware of and schedules threads
A
blocking system call, will not block all peer threads
Expensive
to manage threads
Expensive context switch
Kernel Intervention
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Implementing Threads in the Kernel
A threads package managed
by the kernel
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Kernel Threads
Supported by the Kernel
Examples: newer versions of
Windows
UNIX
Linux
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Linux Threads
Linux refers to them as tasks rather
than threads.
Thread creation is done through clone() system call.
Unlike fork(), clone() allows a child task to share the address space of the parent task (process)
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Pthreads
A POSIX standard (IEEE 1003.1c) API for thread
creation and synchronization.
API specifies behavior of the thread library,
implementation is up to development of the library.
POSIX Pthreads - may be provided as either a user or kernel library, as an extension to the POSIX standard.
Common in UNIX operating systems.
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Hybrid Implementations
Multiplexing user-level threads onto kernel-
level threads
