Transcript Figure 5.01

Multithreading
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Allows application to split itself into multiple “threads”
of execution (“threads of execution”).
OS support for creating threads, terminating threads,
and preemptively switches control among (kernellevel) threads.
A thread is simply a function that can call other
functions.
Multithreading
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Threads are part of same process and share all
process resources:
Memory
Open Files
Global Variables
Static Variables
Each thread has its own:
processor (and math coprocessor) state
stack
Single and Multithreaded
Processes
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Lightweight process.
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State:
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Threads share all process resources.
Thread ID, program counter, register set, and stack.
User-level threads and kernel-level threads.
Benefits
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Performance: Overlap communication/computation
Responsiveness: Separate thread to handle user
input.
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Web server spawns separate thread to handle incoming
request.
Resource Sharing: e.g., one code, data segment.
Economy: Much cheaper to create and switch than
processes.
Utilization of MP Architectures: Assign each thread to
a separate processor if available.
User Threads
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Thread management done by user-level threads
library.
Advantages:
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Very fast: Does not involve kernel in creation, scheduling, or
switching.
Disadvantages:
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When a thread blocks, whole process blocks.
Kernel Threads
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Supported by the Kernel.
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Advantage:
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Kernel creates, schedules, and switches threads.
When one thread blocks, the whole process does not have
to block.
Thus can overlap I/O and computation.
Disadvantage:
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Slower since kernel is involved.
Multithreading Models
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Many-to-One
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One-to-One
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Many-to-Many
Many-to-One
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Many user-level threads mapped to single kernel
thread.
Used on systems that do not support kernel threads.
Many-to-One Model
One-to-One
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Each user-level thread maps to kernel thread.
Examples
- Windows 95/98/NT/2000
- OS/2
One-to-one Model
Many-to-Many Model
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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 2
Windows NT/2000 with the ThreadFiber package
Many-to-Many Model
Pthreads
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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.
Common in UNIX operating systems.
Solaris 2 Threads
Solaris Process
Windows 2000 Threads
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Implements the one-to-one mapping.
Each thread contains
- a thread id
- register set
- separate user and kernel stacks
- private data storage area