CGS 3763 Operating Systems Concepts Spring 2013

Dan C. MarinescuOffice: HEC 304Office hours: M-Wd 11:30 - 12:30 AM

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Reading assignments Chapter 4 of the textbook Chapters 4 textbook slides

Lecture 18 – Monday, February 18, 2013

2Lecture 18

Threads

Thread the smallest sequence of programmed instructions that can be

managed independently by an operating system scheduler a light-weight process multiple threads can share the same address space

On a single processor, multithreading generally occurs by time-division multiplexing, the processor switches between different threads. This context switching generally happens frequently enough that the user perceives the threads or tasks as running at the same time.

On a multiprocessor or a multi-core system the threads actually run at the same time, with each processor or core running a particular thread or task

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Single- and multi-threaded processes

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Reasons for multithreading Reduce the execution time of a compute-intensive task. Better resource utilization – recall the imbalance between the CPU,

memory, and I/O device bandwidth/speed. Improve scalability Example: multithreaded execution on a two core system

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Challenges

Identify activities that can run in parallel. Divide the workload among the threads and balance the

load. Minimize the communication among threads. Avoid barrier-synchronization instances when all

threads have to wait until all of them reach a certain execution stage.

Debugging.

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Multithreading and client-server systems

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Common models for threaded programs

Manager/worker:  a single thread, the manager assigns work to other threads, the workers. Typically, the manager handles all input and parcels out work to the other tasks. At least two forms of the manager/worker model are common: static worker pool and dynamic worker pool.

Pipeline: a task is broken into a series of suboperations, each of which is handled in series, but concurrently, by a different thread. An automobile assembly line best describes this model.

Peer: similar to the manager/worker model, but after the main thread creates other threads, it participates in the work.

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Shared memory model for threads

All threads have access to the same global, shared memory Threads also have their own private data Programmers are responsible for synchronizing access (protecting)

globally shared data.

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Thread-safeness

The ability of an application to execute multiple threads simultaneously without "clobbering" shared data or creating "race" conditions.

For example, suppose that your application creates several threads, each of which makes a call to the same library routine: This library routine accesses/modifies a global structure or

location in memory. As each thread calls this routine it is possible that they may try to

modify this global structure/memory location at the same time. If the routine does not employ some sort of synchronization

constructs to prevent data corruption, then it is not thread-safe.

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Multiple threads can share a module

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User and kernel threads User threads- thread management done by user-level threads

library POSIX Pthreads Win32 threads Java threads

Kernel threads Windows XP/2000 Solaris Linux Tru64 UNIX Mac OS X

Recall that the kernel has to keep track of all processes on the system and uses the PCB to do that.

The kernel has to keep track of al threads as well.

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Relations between user and kernel threads Many-to-One several user-level threads mapped to single kernel

thread: Solaris Green Threads GNU Portable Threads

One-to-One each user-level thread maps to kernel thread: Windows NT/XP/2000 Linux Solaris 9 and later

Many-to many many user level threads mapped to many kernel threads Allows the operating system to create a sufficient number of

kernel threads Solaris prior to version 9 Windows NT/2000 with the ThreadFiber package

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