threadscs-3013 c-term 20081 threads (continued) cs-3013, operating systems c-term 2008 (slides...

ThreadsCS-3013 C-term 2008 1

Threads (continued)

CS-3013, Operating SystemsC-Term 2008

(Slides include materials from Operating System Concepts, 7th ed., by Silbershatz, Galvin, & Gagne and from Modern Operating Systems, 2nd ed., by Tanenbaum)


Review

• Threads introduced because• Processes are heavyweight in Windows and Linux

• Difficult to develop concurrent applications when address space is unique per process

• Thread — a particular execution of a program within the context of a Windows-Unix-Linux process

• Multiple threads in same address space at same time


This problem …

• … is partly an artifact of• Unix, Linux, and Windows

and of

• Big, powerful processors (e.g., Pentium, Athlon)

• … tends to occur in most large systems

• … is infrequent in small-scale systems• PDAs, cell phones

• Closed systems (i.e., controlled applications)


Characteristics

• A thread has its own• Program counter, registers, PSW

• Stack

• A thread shares• Address space, heap, static data, program code

• Files, privileges, all other resources

with all other threads of the same process


Address Space for Multiple Threads

0x00000000

0xFFFFFFFF

Virtual

address space

code(text)

static data

heap

thread 1 stack

PC (T2)

SP (T2)thread 2 stack

thread 3 stack

SP (T1)

SP (T3)

PC (T1)

PC (T3)

SP

PC


Who creates and manages threads?

• User-level implementation– done with function library (e.g., POSIX)– Runtime system – similar to process

management except in user space– Windows NT – fibers: a user-level thread

mechanism ???

• Kernel implementation – new system calls and new entity to manage– Linux: lightweight process (LWP)– Windows NT & XP: threads


Mutual Exclusion within Threads

extern void thread_yield(); extern int TestAndSet(int &i);

/* sets the value of i to 1 and returns the previous value of i. */

void enter_critical_region(int &lock) {while (TestAndSet(lock) == 1)

thread_yield(); /* give up processor */};

void leave_critical_region(int &lock) {lock = 0;

};


Reading Assignment

• Silbertshatz– Chapter 4 – “Threads”

• Robert Love, Linux Kernel Development– Chapter 3 – “Process Management”


Kernel Threads

• Supported by the Kernel• OS maintains data structures for thread state and

does all of the work of thread implementation.

• Examples• Solaris

• Tru64 UNIX

• Mac OS X

• Windows 2000/XP/Vista

• Linux version 2.6


Kernel Threads (continued)

• OS schedules threads instead of processes

• Benefits– Overlap I/O and computing in a process– Creation is cheaper than processes– Context switch can be faster than processes

• Negatives– System calls (high overhead) for operations– Additional OS data space for each thread


Threads – supported by processor

• E.g., Pentium 4 with Hyperthreading™• www.intel.com/products/ht/hyperthreading_more.htm

• Multiple processor cores on a single chip• True concurrent execution within a single process

• Requires kernel thread support• Re-opens old issues

• Deadlock detection

• Critical section management of synchronization primitives (especially in OS kernel)

http://www.intel.com/products/ht/hyperthreading_more.htm


Unix Processes vs. Threads

• On a 700 Mhz Pentium running Linux– Processes:

• fork()/exit(): 250 microsec

– Kernel threads:• pthread_create()/pthread_join(): 90 microsec

– User-level threads:• pthread_create()/pthread_join(): 5 microsec


POSIX pthread Interface

• Data type:– pthread_t

• int pthread_create(pthread_t *thread, const pthread_attr_t *attr, void*(*start_routine) (void), void *arg) ; – creates a new thread of control– new thread begins executing at start_routine

• pthread_exit(void *value_ptr)– terminates the calling thread

• pthread_join(pthread_t thread, void **value_ptr); – blocks the calling thread until the thread specified terminates

• pthread_t pthread_self() – Returns the calling thread's identifier


Threads and “small” operating systems

• Many “small” operating systems provide a common address space for all concurrent activities

• Each concurrent execution is like a Linux-Unix thread

• But its often called a process!

• pthread interface and tools frequently used for managing these processes

• …


Java Threads

• Thread class• Thread worker = new Thread();

• Configure it

• Run it• public class application extends Thread {

… }

• Methods to• Run, wait, exit

• Cancel, join

• Synchronize

• …


Threading Issues

• Semantics of fork() and exec() system calls for processes

• Thread cancellation

• Signal handling

• Kernel thread implementations– Thread pools– Thread specific data– Scheduler activations


Semantics of fork() and exec()

• Does fork() duplicate only the calling thread or all threads?– Easy if user-level threads

– Not so easy with kernel-level threads• Linux has special clone() operation – only forking

thread is created in child process

• Windows XP has something similar


Thread Cancellation

• Terminating a thread before it has finished

• Reason:–• Some other thread may have completed the joint

task

• E.g., searching a database

• Issue:–• Other threads may be depending cancelled thread

for resources, synchronization, etc.

• May not be able to cancel one until all can be cancelled


Thread Cancellation (continued)

• Two general approaches:– Asynchronous cancellation terminates the

target thread immediately– Deferred cancellation allows the target thread

to periodically check if it should cancel itself

• pthreads provides cancellation points


Signal Handling

• Signals are used in Unix-Linux to notify process that a particular event has occurred — e.g.– Divide-by-zero– Illegal memory access, stack overflow, etc.– CTL-C typed, or kill command issued at console– Timer expiration; external alarm– …

• A signal handler is used to process signals– Signal is generated by particular event– Signal is delivered to a process– Signal is handled by a signal handler

• All processes provided with default signal handler• Applications may install own handlers for specific signals


Signal Handling Options

• Deliver signal to specific thread to which it applies• E.g., illegal memory access, divide-by-zero, etc.

• Deliver signal to every thread in the process• CTL-C typed

• Deliver signal to certain threads in the process• I.e., threads that have agreed to receive such signals

(or not blocked them)

• Assign a specific thread to receive all signals for the process

• …


Kernel Thread Implementations

• Linux

• Windows

• Others


Modern Linux Thread Implementation

• Implemented directly in kernel

• Primary unit of scheduling and computation implemented by Linux 2.6 kernel

• “A thread is just a special kind of process.”• Robert Love, Linux Kernel Development, p.23

• Every thread has its own task_struct in kernel

• …


Definition

• Task (from point of view of Linux kernel):–– Process– Thread– Kernel thread (see later)


Modern Linux Threads (continued)

• Process task_struct has pointer to own memory & resources

• Thread task_struct has pointer to process’s memory & resources

• Kernel thread task_struct has null pointer to memory & resources

• fork() and pthread_create() are library functions that invoke clone() system call

• Arguments specify what kind of clone

• …



• Threads are scheduled independently of each other

• Threads can block independently of each other

• Even threads of same process

• Threads can make their own kernel calls• Kernel maintains a small kernel stack per thread

• During kernel call, kernel is in process context


Process Context

0x00000000

0xFFFFFFFF

Virtual

address space

code(text)

static data

heap(dynamically allocated)

Kernel Code and Data

PC

SP1

User Space

stack(dynamically allocated)

Kernel Space

32-bit Linux & Win XP – 3G/1G user space/kernel space

stack(dynamically allocated) SP2



• Multiple threads can be executing in kernel at same time

• When in process context, kernel can• sleep on behalf of its thread

• take pages faults on behalf of its thread

• move data between kernel and process address space on behalf of thread

• …


Linux Kernel Threads

• Kernel has its own threads• No associated process context

• Supports concurrent activity within kernel• Multiple devices operating at one time• Multiple application activities at one time• Multiple processors in kernel at one time

• A useful tool• Special kernel thread packages, synchronization

primitives, etc.• Useful for complex OS environments


Windows XP Threads

• Much like to Linux 2.6 threads• Primitive unit of scheduling defined by kernel

• Threads can block independently of each other

• Threads can make kernel calls

• …

• Process is a higher level (non-kernel) abstraction

• See Silbershatz, §22.3.2.2


Other Implementations of Kernel Threads

• Many-to-Many

• Many-to-One

• One-to-One


Thread Pools (Implementation technique)

• Create a number of threads in a pool where they await work

• Advantages:– Usually slightly faster to service a request with

an existing thread than create a new thread– Allows the number of threads in application(s)

to be bounded by size of pool


Many-to-Many Model

• 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 prior to version 9

• Windows NT/2000 with the Thread/Fiber package


Many-to-Many Model


Two-level Model

• Similar to M:M, except it allows a specific thread to be bound to one kernel thread

• Examples• IRIX

• HP-UX

• Tru64 UNIX

• Solaris 8 and earlier


Two-level Model


Many-to-One

• Many user-level threads mapped to single kernel thread

• Examples:• Solaris Green Threads

• GNU Portable Threads


Many-to-One Model


One-to-One

• Each user-level thread maps to kernel thread

• Examples• Windows NT/XP/2000

• Linux

• Solaris 9 and later


One-to-one Model


Threads – Summary

• Threads were invented to counteract the heavyweight nature of Processes in Unix, Windows, etc.

• Provide lightweight concurrency within a single address space

• Have evolved to become primitive abstraction defined by kernel

• Fundamental unit of scheduling in Linux, Windows, etc


Reading Assignment

• Silbertshatz– Chapter 4 – “Threads”

• Robert Love, Linux Kernel Development– Chapter 3 – “Process Management”


Questions?

threadscs-3013 c-term 20081 threads (continued) cs-3013, operating systems c-term 2008 (slides...

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