operating systems - university of limerick

AnnouncementsThreads – §4, SGG

Operating Systems

P. Healy

CS1-08Computer Science Bldg.

tel: [email protected]

Autumn 2019–2020

P. Healy (University of Limerick) CS4023 Autumn 2019–2020 1 / 24


Outline

1 Announcements

2 Threads – §4, SGGMultithreading ModelsThreading IssuesThread Libraries



Announcements

Tute / problem solving session now moved to Monday,13.00, KBG12 today onwards!SETMid-term: bubbling up to the surface of my head



Password Problems

If you’ve forgotten your password ...... but then, one thing leads to another


https://www.youtube.com/watch?v=p0Gwe5gKgjo

https://www.youtube.com/watch?v=VKTT-sy0aLg


Multithreading ModelsThreading IssuesThread Libraries

Outline

1 Announcements





Single and Multithreaded Processes

Process: unit of resourcesThread: unit of scheduling and execution




Benefits of Threads

ResponsivenessResource SharingEconomy

in Solaris, creating a process is aboutthan is creating a thread, and context switching is about

Utilization of multiprocessor architectures




Kernel vs. User Threads

Kernel threads are supported by almost every operatingsystem currently; relatively expensive to createUser threads Thread management done by user-levelthreads library

Three primary thread libraries:POSIX PthreadsWin32 threadsJava threads

Ultimately, some relationship that maps from user- tokernel-thread must exist between the two: “shadowing”

StackOverflow Explanation


https://stackoverflow.com/questions/15983872/difference-between-user-level-and-kernel-supported-threads



Three Thread Models

Many-to-OneOne-to-OneMany-to-Many

Wikipedia’s Processes, kernel threads and Kernel models


http://en.wikipedia.org/wiki/Thread_(computing)#Processes.2C_kernel_threads.2C_user_threads.2C_and_fibers

http://en.wikipedia.org/wiki/Thread_(computing)#Models



Many-to-One Thread Model

Many user-level threadsmapped to single

All threads blocked ifkernel thread is blockedExamples:

Solaris Green ThreadsGNU Portable Threads

One kernel thread ⇒ noparallelism (multi-coresuseless)




One-to-One Thread Model

Each user-level thread maps to kernel threadBlocking on a single kernel thread now not an issue butrelatively high cost of kernel thread setup works againstthis approachExamples

Windows NT/XP/2000LinuxSolaris 9 and later




Many-to-Many Thread Model

Allows many user levelthreads to be mappedto many kernel threadsAllows the operatingsystem to create asufficient number ofkernel threads

Solaris prior to v. 9Windows NT/2000with the ThreadFiberpackage




Two-level Model

Similar toprevious M:M,except that itallows a userthread to bebound to a kernelthreadExamples

IRIXHP-UXTru64 UNIXSolaris 8 andearlier




Outline

1 Announcements





Some concerns

Semantics of fork() and exec() system callsDoes fork() duplicate only the calling thread or allthreads?

Thread cancellationTerminating a thread before it has finishedTwo general approaches:

Asynchronous cancellation terminates the target threadimmediatelyDeferred cancellation allows the target thread toperiodically check if it should be cancelled




Some concerns (contd.)

Signal handlingSignals are used in UNIX systems to notify a process that aparticular event has occurredA signal handler is used to process signals:

Signal is generated by particular eventSignal is delivered to a processSignal is handled

Possibilities:Deliver the signal to the thread to which the signal appliesDeliver the signal to every thread in the processDeliver the signal to certain threads in the processAssign a specific thread to receive all signals for the process




Some concerns (contd.)

Thread poolsCreate a number of threads in a pool where they await workAdvantages

Usually slightly faster to service a request with an existingthread than create a new threadAllows the number of threads in the application(s) to bebound to the size of the pool

Thread-specific dataAllows each thread to have its own copy of dataUseful when you do not have control over the threadcreation process (i.e., when using a thread pool)




Scheduler activations

Both M:M and Two-level models require communication tomaintain the appropriate number of kernel threadsallocated to the applicationScheduler activations provide upcalls - a communicationmechanism from the kernel to the thread libraryThis communication allows an application to maintain thecorrect number of kernel threads




Outline

1 Announcements





Pthreads

A POSIX standard (IEEE 1003.1c) API for thread creationand synchronizationAPI specifies behavior of the thread library, implementationis up to development of the libraryCommon in UNIX operating systems (Solaris, Linux, MacOS X)




Java Threads

Java threads are managed by the JVM which runs onsome OSJava threads may be created by:

Extending Thread classImplementing the Runnable interface

Java thread states:




Linux Threads

Linux refers to them as tasks rather than threadsThread creation is done through clone() system callclone() allows a child task to share the address space ofthe parent task (process)amount of “data sharing” with parent controlled by followingflags




Windows XP Threads

Implements the one-to-one mappingEach thread contains

A thread idRegister setSeparate user and kernel stacksPrivate data storage area

The register set, stacks, and private storage area areknown as the context of the threadsThe primary data structures of a thread include:

ETHREAD (executive thread block)KTHREAD (kernel thread block)TEB (thread environment block)




Windows Vista Enhancements

Thread pool significantly enhanced to improve reliability,performance, and API simplicitySome specific examples of Windows Vista thread poolenhancements include:

Cleanup groups - introduce the ability to clean up pendingthread pool requests on process shutdownMultiple pools per process, each of which is scheduledindependently. This significantly improves responsivenessas well as code isolationPerformance. The thread pool is optimized to provide thebest possible processor usage and to reduce pressure onsystem memory. Thread recycling, maintaining a number ofrunning threads that is proportional to the number ofprocessors, smart load balancing, and work assignment areused to gain maximum performance from the thread pool.


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