CS 134: Operating Systems - Threads · 06/12/2012 · CS 134: Operating Systems Threads 1/23 CS...

CS 134:Operating Systems

Threads

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CS 134:Operating Systems

Threads

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Overview

Wiki AnswersThread QuestionsScheduler QuestionsSynchronization Questions

ThreadsConceptsUsesModelsDesign

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Overview

Wiki AnswersThread QuestionsScheduler QuestionsSynchronization Questions

ThreadsConceptsUsesModelsDesign20

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Overview

Wiki Answers Thread Questions

Thread Questions (1)

What happens to a thread when it exits (i.e., calls thread_exit())?What about when it sleeps?

When a thread exits, it ensures the stack isn’t mangled, removesits virtual memory space and destroys it, decrements the counterof whatever vnode it may be poitning at, puts itself into a zombiestate, S_ZOMB, and preps itself to panic if it ever runs againbefore it dies. When it sleeps, it makes sure it’s not in an interrupthandler, yields control to the next thread, enters the S_SLEEPstate, and only starts taking control once more when wakeup() iscalled on its address.

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What happens to a thread when it exits (i.e., calls thread_exit())?What about when it sleeps?

When a thread exits, it ensures the stack isn’t mangled, removesits virtual memory space and destroys it, decrements the counterof whatever vnode it may be poitning at, puts itself into a zombiestate, S_ZOMB, and preps itself to panic if it ever runs againbefore it dies. When it sleeps, it makes sure it’s not in an interrupthandler, yields control to the next thread, enters the S_SLEEPstate, and only starts taking control once more when wakeup() iscalled on its address.20

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CS34Wiki Answers

Thread QuestionsThread Questions (1)



What function(s) handle(s) a context switch?

There are two functions that handle a context switch: mi_switch,which is the high level, machine-independent context switchfunction, and md_switch, which is the machine-independent codethat actually does the context switch. mi_switch is in thread.c, andmd_switch is in pcb.c

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What function(s) handle(s) a context switch?

There are two functions that handle a context switch: mi_switch,which is the high level, machine-independent context switchfunction, and md_switch, which is the machine-independent codethat actually does the context switch. mi_switch is in thread.c, andmd_switch is in pcb.c

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How many thread states are there? What are they?

There are four thread states - S_RUN, S_READY, S_SLEEP, andS_ZOMB. These states are defined in kern/thread/thread.c. Theyexpress whether the thread is running, ready to run, sleeping, or azombie.

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How many thread states are there? What are they?

There are four thread states - S_RUN, S_READY, S_SLEEP, andS_ZOMB. These states are defined in kern/thread/thread.c. Theyexpress whether the thread is running, ready to run, sleeping, or azombie.

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What does it mean to turn interrupts off? How is thisaccomplished? Why is it important to turn off interrupts in thethread subsystem code?

If interrupts are turned off, then even if an interrupt is signaled thehandler is not called until interrupts are turned back on. Interruptsare turned off using the function splhigh (set priority level high)and back on again using spl0 (set priority level zero). The prioritylevel can also be set to intermediate levels (or at least, it could ifOS/161 supported them) using the splx function. Turning offinterrupts for thread operations is necessary to ensure that theseoperations complete successfully and aren’t brokenmid-execution. For example, things could go pretty badly if thescheduler interrupted us in the middle of a context switch and triedto start executing a thread that wasn’t finished setting up its stack.And it would be really awful if someone interrupted us in themiddle of forking! 6 / 23


What does it mean to turn interrupts off? How is thisaccomplished? Why is it important to turn off interrupts in thethread subsystem code?

If interrupts are turned off, then even if an interrupt is signaled thehandler is not called until interrupts are turned back on. Interruptsare turned off using the function splhigh (set priority level high)and back on again using spl0 (set priority level zero). The prioritylevel can also be set to intermediate levels (or at least, it could ifOS/161 supported them) using the splx function. Turning offinterrupts for thread operations is necessary to ensure that theseoperations complete successfully and aren’t brokenmid-execution. For example, things could go pretty badly if thescheduler interrupted us in the middle of a context switch and triedto start executing a thread that wasn’t finished setting up its stack.And it would be really awful if someone interrupted us in themiddle of forking!

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What happens when a thread wakes up another thread? Howdoes a sleeping thread get to run again?

It removes the sleeping thread from the queue, and callsmake_runnable on the thread, which currently adds it to the end ofthe runqueue. The thread gets to run again when an mi_switch iscalled, and that thread is returned by the scheduler.

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What happens when a thread wakes up another thread? Howdoes a sleeping thread get to run again?

It removes the sleeping thread from the queue, and callsmake_runnable on the thread, which currently adds it to the end ofthe runqueue. The thread gets to run again when an mi_switch iscalled, and that thread is returned by the scheduler.

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Wiki Answers Scheduler Questions

Scheduler Questions (6)

What function is responsible for choosing the next thread to run?How does that function pick the next thread?

struct thread * scheduler(void); it uses a round-robin run queuethat schedules each thread in the queue in equal time-slicewithout priorities.

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What function is responsible for choosing the next thread to run?How does that function pick the next thread?

struct thread * scheduler(void); it uses a round-robin run queuethat schedules each thread in the queue in equal time-slicewithout priorities.

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Scheduler QuestionsScheduler Questions (6)

Wiki Answers Scheduler Questions


What role does the hardware timer play in scheduling? Whathardware independent function is called on a timer interrupt?

The interrupt handler for the hardware timer calls hardclock,defined in src/kern/thread/hardclock.c. The method hardclockfinishes by calling thread_yield every time it is run, forcing acontext switch.

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What role does the hardware timer play in scheduling? Whathardware independent function is called on a timer interrupt?

The interrupt handler for the hardware timer calls hardclock,defined in src/kern/thread/hardclock.c. The method hardclockfinishes by calling thread_yield every time it is run, forcing acontext switch.

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CS34Wiki Answers

Scheduler QuestionsScheduler Questions (7)

Wiki Answers Synchronization Questions

Synchronization Questions (8)

Describe how thread_sleep() and thread_wakeup() are used toimplement semaphores. What is the purpose of the argumentpassed to thread_sleep()?

thread_sleep is used in the P function of the semaphore. Thisfunction suspends the current thread until the semaphore count isgreater than zero.thread_wakeup() is used in the V function of the semaphore. Thisfunction wakes up all the suspended threads waiting on thecurrent semaphore.The addr argument that is passed in is the address of the object(in this case, semaphore) the sleeping thread is associated with.This is required so that when thread_wakeup is called on thesame semaphore, it can selectively wake up only the threadsassociated with that particular semaphore.

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Describe how thread_sleep() and thread_wakeup() are used toimplement semaphores. What is the purpose of the argumentpassed to thread_sleep()?

thread_sleep is used in the P function of the semaphore. Thisfunction suspends the current thread until the semaphore count isgreater than zero.thread_wakeup() is used in the V function of the semaphore. Thisfunction wakes up all the suspended threads waiting on thecurrent semaphore.The addr argument that is passed in is the address of the object(in this case, semaphore) the sleeping thread is associated with.This is required so that when thread_wakeup is called on thesame semaphore, it can selectively wake up only the threadsassociated with that particular semaphore.

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Synchronization Questions



Why does the lock API in OS/161 provide lock_do_i_hold(), butnot lock_get_holder()?

???

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Why does the lock API in OS/161 provide lock_do_i_hold(), butnot lock_get_holder()?

???

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The thread subsystem in OS/161 uses a queue structure tomanage some of its state. This queue structure does not containany synchronization primitives. Why not? Under whatcircumstances should you use a synchronized queue structure?

The runqueue queue used by the scheduler in the threadsubsystem is only accessed by a single scheduler thread, so doesnot need any synchronization primitives to prevent other(non-existent) threads from messing up the queue. You shoulduse a synchronized queue structure for any queue that multiplethreads could access simultaneously.

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The thread subsystem in OS/161 uses a queue structure tomanage some of its state. This queue structure does not containany synchronization primitives. Why not? Under whatcircumstances should you use a synchronized queue structure?

The runqueue queue used by the scheduler in the threadsubsystem is only accessed by a single scheduler thread, so doesnot need any synchronization primitives to prevent other(non-existent) threads from messing up the queue. You shoulduse a synchronized queue structure for any queue that multiplethreads could access simultaneously.

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Threads Concepts

Generalizing Processes

Simple view of process isAddress space

+ Thread of execution

Does the mapping need to be one-to-one?

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Simple view of process isAddress space

+ Thread of execution

Does the mapping need to be one-to-one?

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CS34Threads

Concepts


Threads Concepts

Possible Mappings

one process�one thread�

�

one process�multiple threads�

�

multiple processes�one thread per process�

�

multiple processes�multiple threads per process�

�

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Possible Mappings

one process�one thread�

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one process�multiple threads�

�

multiple processes�one thread per process�

�

multiple processes�multiple threads per process�

�

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CS34Threads

Concepts

Possible Mappings

Threads Concepts

Threads

Motivation:I Traditional processes: Virtual uniprocessor machineI Multithreaded processes: Virtual multiprocessor machine

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Threads

Motivation:I Traditional processes: Virtual uniprocessor machineI Multithreaded processes: Virtual multiprocessor machine

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CS34Threads

Concepts

Threads

Threads Uses

Uses of Threads

Various reasons why people use threadsI Performing foreground and background workI Supporting asynchronous processingI Speeding executionI Organizing programs

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Uses of Threads

Various reasons why people use threadsI Performing foreground and background workI Supporting asynchronous processingI Speeding executionI Organizing programs

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UsesUses of Threads

Threads Uses

Uses of Threads—Example

Dispatcher thread

Worker thread

Web page cache

Kernel

Network�connection

Web server process

User�space

Kernel�space

/* Dispatcher Thread */for ( ; ; ) {url = get_next_request();handoff_work(url);

}

/* Worker Thread */ \\for ( ; ; ) {url = wait_for_work();page = look_in_cache(url);if (page == NULL)page = generate_page(url);

send_page(page);}

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Uses of Threads—Example

Dispatcher thread

Worker thread

Web page cache

Kernel

Network�connection

Web server process

User�space

Kernel�space

/* Dispatcher Thread */for ( ; ; ) {url = get_next_request();handoff_work(url);

}

/* Worker Thread */ \\for ( ; ; ) {

url = wait_for_work();page = look_in_cache(url);if (page == NULL)page = generate_page(url);

send_page(page);}

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CS34Threads

UsesUses of Threads—Example

Threads Uses

Class Exercise

Can an application implement threads without built-in threadsupport in the OS?

If so, what does it need from the from the OS to support threads?

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Class Exercise

Can an application implement threads without built-in threadsupport in the OS?

If so, what does it need from the from the OS to support threads?

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CS34Threads

UsesClass Exercise

Threads Models

Model for User Threads

P��

User�Space�

�

Threads�Library�

�KernelSpace�

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Pure user-level��

Key:

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User-level thread��Kernel-level thread�Process��

Class ExerciseWhat are the pros and cons of this approach?

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P��

User�Space�

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�KernelSpace�

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Key:

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CS34Threads

ModelsModel for User Threads

So, maybe we should put the threads in the kernel?

Threads Models


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User�Space�

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�KernelSpace�

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Key:

P��


+ No kernel overhead for thread library calls+ Own scheduler = Application-specific scheduling policy?− I/O issues− Can’t (easily) take advantage of multiprocessing

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P��

User�Space�

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�KernelSpace�

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Key:

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+ No kernel overhead for thread library calls+ Own scheduler = Application-specific scheduling policy?− I/O issues− Can’t (easily) take advantage of multiprocessing

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CS34Threads

ModelsModel for User Threads

So, maybe we should put the threads in the kernel?

Threads Models

Model for Kernel-Level Threads

P��

User�Space�

�KernelSpace�

�

Pure kernel-level��

Key:

P��



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P��

User�Space�

�KernelSpace�

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Key:

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CS34Threads

ModelsModel for Kernel-Level Threads

Threads Models


P��

User�Space�

�KernelSpace�

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Key:

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Now we have kernel overheads:I Kernel data structuresI Mode switch to kernel

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P��

User�Space�

�KernelSpace�

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Key:

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Now we have kernel overheads:I Kernel data structuresI Mode switch to kernel

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CS34Threads

ModelsModel for Kernel-Level Threads

Threads Models

Hybrid Thread Schemes

P��

P��

User�Space��


�KernelSpace��

Combined��

Key:

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Hybrid Thread Schemes

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Combined��

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ModelsHybrid Thread Schemes

Threads Models

Traditional vs. Multithreaded Processes

Single-Threaded�Process Model�

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Process�Control�Block�

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User�Address�Space�

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User�Stack�

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Kernel�Stack�

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Multithreaded�Process Model�

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Thread��

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Class QuestionBut what’s per-process and what’s per-thread?

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Traditional vs. Multithreaded Processes

Single-Threaded�Process Model�

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User�Stack�

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Kernel�Stack�

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Multithreaded�Process Model�

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Class QuestionBut what’s per-process and what’s per-thread?

2012

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ModelsTraditional vs. Multithreaded Processes

Threads Design

Per-Process vs. Per-Thread—You Decide. . .

I Execution stateI RegistersI Program counterI Program status wordI Stack pointer

I Scheduling informationI Process stateI PriorityI Class, etc.

I MemoryI Text areaI Data areaI Stack area

I Security/Authentication InfoI User IDI Group ID

I I/O StateI File descriptorsI Working directoryI Root directory

I Event NotificationsI Signals waitingI Signal maskI Time of next alarm

I OtherI Process IDI Parent processI Process groupI Controlling terminalI Start timeI CPU timeI Children’s CPU time

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I Execution stateI RegistersI Program counterI Program status wordI Stack pointer

I Scheduling informationI Process stateI PriorityI Class, etc.

I MemoryI Text areaI Data areaI Stack area

I Security/Authentication InfoI User IDI Group ID

I I/O StateI File descriptorsI Working directoryI Root directory

I Event NotificationsI Signals waitingI Signal maskI Time of next alarm

I OtherI Process IDI Parent processI Process groupI Controlling terminalI Start timeI CPU timeI Children’s CPU time

2012

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CS34Threads

Design


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