Processes and Threads

• Processes and their scheduling• Multiprocessor scheduling• Threads• Distributed Scheduling/migration

Processes: Review

• Multiprogramming versus multiprocessing• Kernel data structure: process control block (PCB)• Each process has an address space

– Contains code, global and local variables..

• Process state transitions • Uniprocessor scheduling algorithms

– Round-robin, shortest job first, FIFO, lottery scheduling, EDF

• Performance metrics: throughput, CPU utilization, turnaround time, response time, fairness

Process Behavior

• Processes: alternate between CPU and I/O• CPU bursts

– Most bursts are short, a few are very long (high variance)

– Modeled using hyperexponential behavior

– If X is an exponential r.v.

• Pr [ X <= x] = 1 – e-x

• E[X] = 1/– If X is a hyperexponential r.v.

• Pr [X <= x] = 1 – p e-x -(1-p) e-x

• E[X] = p/ p)/

Process Scheduling

• Priority queues: multiples queues, each with a different priority– Use strict priority scheduling– Example: page swapper, kernel tasks, real-time tasks, user tasks

• Multi-level feedback queue– Multiple queues with priority– Processes dynamically move from one queue to another

• Depending on priority/CPU characteristics– Gives higher priority to I/O bound or interactive tasks– Lower priority to CPU bound tasks– Round robin at each level

Introduction

A thread is a path of execution through a program’s code, plus a set of resources (stack, register state, etc) assigned by the operating system.

Thread vs. Process• A Process is inert. A process never executes anything; it is

simply a container for threads. – One thread of control through a large, potentially sparse address space– Address space may be shared with other processes (shared mem)– Collection of systems resources (files, semaphores)

• Threads run in the context of a process. Each process has at least one thread.

• A thread represents a path of execution that has its own call stack and CPU state.

• Threads are confined to context of the process that created them.– A thread executes code and manipulates data within its process’s address

space.– If two or more threads run in the context of a single process they share a

common address space. They can execute the same code and manipulate the same data.

– Threads sharing a common process can share kernel object handles because the handles belong to the process, not individual threads.

Processes and Threads

• Thread (light weight process)– A flow of control through an address space

– Each address space can have multiple concurrent control flows

– Each thread has access to entire address space

– Potentially parallel execution, minimal state (low overheads)

– May need synchronization to control access to shared variables

Threads

• Each thread has its own stack, PC, registers– Share address space, files,…

Single and Multithreaded Processes

Why use Threads?

• Large multiprocessors need many computing entities (one per CPU)

• Switching between processes incurs high overhead• With threads, an application can avoid per-process

overheads– Thread creation, deletion, switching cheaper than processes

• Threads have full access to address space (easy sharing)• Threads can execute in parallel on multiprocessors

Starting a Process

• Every time a process starts, the system creates a primary thread.– The thread begins execution with the C/C++ run-time library’s

startup code.

– The startup code calls your main or WinMain and execution continues until the main function returns and the C/C++ library code calls ExitProcess.

Scheduling Threads• Windows 2000, NT and Win98 are preemptive multi-

tasking systems. Each task is scheduled to run for some brief time period before another task is given control of CPU.

• Threads are the basic unit of scheduling on current Win32 platforms. A thread may be in one of three possible states:– running

– blocked or suspended, using virtually no CPU cycles

– ready to run, using virtually no CPU cycles

Scheduling Threads (continued)• A running task is stopped by the scheduler if:

– it is blocked waiting for some system event or resource

– its time time slice expires and is placed back on the queue of ready to run threads

– it is suspended by putting itself to sleep for some time

– it is suspended by some other thread

– it is suspended by the operating system while the OS takes care of some other critical activity.

• Blocked threads become ready to run when an event or resource they wait on becomes available.

• Suspended threads become ready to run when their sleep interval has expired or suspend count is zero.

Threads (Benefits)• Responsiveness

– Program can continue even if part of it is blocked or is performing lengthy operation.

– Web browser can still allow user interaction in one thread during an image load thread

• Resource sharing– Several different threads of activity within the same address space --

share the code and data section• Economy

– Costly Memory and resource allocation for process– Comparatively time consuming to create and manage processes than

threads– Solaris: creating a process 30 times slower– Context switching 5 times slower

• Utilization of multiprocessor architecture

Benefits of using Threads• Keeping user interfaces responsive even if required processing takes a

long time to complete.– handle background tasks with one or more threads– service the user interface with a dedicated thread

• Your program may need to respond to high priority events. In this case, the design is easier to implement if you assign that event handler to a high priority thread.

• Take advantage of multiple processors available for a computation.

• Avoid low CPU activity when a thread is blocked waiting for response from a slow device or human by allowing other threads to continue.

More Benefits

• Improve robustness by isolating critical subsystems on their own threads of control.

• For simulations dealing with several interacting objects the program may be easier to design by assigning one thread to each object.

Potential Problems with Threads• Conflicting access to shared memory

– one thread begins an operation on shared memory, is suspended, and leaves that memory region incompletely transformed

– a second thread is activated and accesses the shared memory in the corrupted state, causing errors in its operation and potentially errors in the operation of the suspended thread when it resumes

• Race Conditions occur when:– correct operation depends on the order of completion of two or more

independent activities– the order of completion is not deterministic

• Starvation– a high priority thread dominates CPU resources, preventing lower priority

threads from running often enough or at all.

Problems with Threads (continued)

• Priority inversion– a low priority task holds a resource needed by a higher priority

task, blocking it from running

• Deadlock– two or more tasks each own resources needed by the other

preventing either one from running so neither ever completes and never releases its resource

Why Threads?

• Single threaded process: blocking system calls, no parallelism

• Finite-state machine [event-based]: non-blocking with parallelism

• Multi-threaded process: blocking system calls with parallelism

• Threads retain the idea of sequential processes with blocking system calls, and yet achieve parallelism

• Software engineering perspective– Applications are easier to structure as a collection of threads

• Each thread performs several [mostly independent] tasks

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

Multi-threaded Clients Example : Web Browsers

• Browsers such as IE are multi-threaded• Such browsers can display data before entire document

is downloaded: performs multiple simultaneous tasks– Fetch main HTML page, activate separate threads for other

parts

– Each thread sets up a separate connection with the server

• Uses blocking calls

– Each part (gif image) fetched separately and in parallel

– Advantage: connections can be setup to different sources

• Ad server, image server, web server…

Multi-threaded Server Example

• Apache web server: pool of pre-spawned worker threads– Dispatcher thread waits for requests

– For each request, choose an idle worker thread

– Worker thread uses blocking system calls to service web request

Solaris 2 Threads

Solaris Process

Thread Management

• Creation and deletion of threads– Static versus dynamic

• Critical sections– Synchronization primitives: blocking, spin-lock (busy-wait)

– Condition variables

• Global thread variables• Kernel versus user-level threads

Multithreading Models (Kernel Threads)

• Many-to-Many

• Many-to-One

• One-to-One

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

• 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

• Each user-level thread maps to kernel thread• Examples

• Windows NT/XP/2000

• Linux

• Solaris 9 and later

One-to-one Model

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

User Threads

• Thread management done by user-level threads library

• Examples

- POSIX Pthreads

- Mach C-threads

- Solaris threads

Kernel Threads

• Supported by the Kernel

• Examples

- Windows 95/98/NT/2000

- Solaris

- Tru64 UNIX

- BeOS

- Linux

User-level versus kernel threads

• Key issues:

• Cost of thread management– More efficient in user space

• Ease of scheduling• Flexibility: many parallel programming models and

schedulers• Process blocking – a potential problem

User-level Threads

• Threads managed by a threads library– Kernel is unaware of presence of threads

• Advantages: – No kernel modifications needed to support threads– Efficient: creation/deletion/switches don’t need system calls– Flexibility in scheduling: library can use different scheduling

algorithms, can be application dependent

• Disadvantages– Need to avoid blocking system calls [all threads block]– Threads compete for one another– Does not take advantage of multiprocessors [no real parallelism]

User-level threads

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)

Kernel-level threads

• Kernel aware of the presence of threads– Better scheduling decisions, more expensive

– Better for multiprocessors, more overheads for uniprocessors

Light-weight Processes

• Several LWPs per heavy-weight process• User-level threads package

– Create/destroy threads and synchronization primitives

• Multithreaded applications – create multiple threads, assign threads to LWPs (one-one, many-one, many-many)

• Each LWP, when scheduled, searches for a runnable thread [two-level scheduling]– Shared thread table: no kernel support needed

• When a LWP thread block on system call, switch to kernel mode and OS context switches to another LWP

LWP Example

Thread Packages

• Posix Threads (pthreads)– Widely used threads package

– Conforms to the Posix standard

– Sample calls: pthread_create,…

– Typical used in C/C++ applications

– Can be implemented as user-level or kernel-level or via LWPs

• Java Threads– Native thread support built into the language

– Threads are scheduled by the JVM

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

Pthreads

• 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.

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

• …

Windows 2000 Threads

• Implements the one-to-one mapping.• Each thread contains

- a thread id

- register set

- separate user and kernel stacks

- private data storage area

Linux Threads

• Linux refers to them as tasks rather than threads.• Thread creation is done through clone() system call.• Clone() allows a child task to share the address space of

the parent task (process)

Java Threads

• Java threads may be created by:

– Extending Thread class

– Implementing the Runnable interface

• Java threads are managed by the JVM.

Java Thread States

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• …

Modern Linux Threads (continued)

• 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

Modern Linux Threads (continued)

• 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

Synchronization• A program may need multiple threads to share some data.• If access is not controlled to be sequential, then shared

data may become corrupted.– One thread accesses the data, begins to modify the data, and then

is put to sleep because its time slice has expired. The problem arises when the data is in an incomplete state of modification.

– Another thread awakes and accesses the data, that is only partially modified. The result is very likely to be corrupt data.

• The process of making access serial is called serialization or synchronization.

Thread Safety

• Note that MFC is not inherently thread-safe. The developer must serialize access to all shared data.

• MFC message queues have been designed to be thread safe. Many threads deposit messages in the queue, the thread that created the (window with that) queue retrieves the messages.

• For this reason, a developer can safely use PostMessage and SendMessage from any thread.

• All dispatching of messages from the queue is done by the thread that created the window.

• Also note that Visual C++ implementation of the STL library is not thread-safe, and should not be used in a multi-threaded environment. I hope that will be fixed with the next release of Visual Studio, e.g., Visual Studio.Net.

MFC Support for Threads• CWinThread is MFC’s encapsulation of threads

and the Windows 2000 synchronization mechanisms, e.g.:– Events– Critical Sections– Mutexes– Semaphores

MFC Threads• User Interface (UI) threads create windows and process

messages sent to those windows

• Worker threads receive no direct input from the user.– Worker threads must not access a window’s member functions using a

pointer or reference. This will often cause a program crash.

– Worker threads communicate with a program’s windows by calling the PostMessage and SendMessage functions.

• Often a program using worker threads will create user defined messages that the worker thread passes to a window to indirectly call some (event-handler) function. Inputs to the function are passed via the message’s WPARAM and LPARAM arguments.

Creating Worker Threads in MFC

• AfxBeginThread – function that creates a thread:

CWinThread *pThread = AfxBeginThread(ThreadFunc, &ThreadInfo);

• ThreadFunc – the function executed by the new thread.

AFX_THREADPROC ThreadFunc(LPVOID pThreadInfo)

• LPVOID pThreadInfo – a pointer to an arbitrary set of input parameters, often created as a structure.– We create a pointer to the structure, then cast to a pointer to void and pass to

the thread function.– Inside the thread function we cast the pointer back to the structure type to

extract its data.

Creating UI Threads in MFC

• Usually windows are created on the application’s main thread.

• You can, however, create windows on a secondary UI thread. Here’s how you do that:– Create a class, say CUIThread, derived from CWinThread.

– Use DECLARE_DYNCREATE(CUIThread) macro in the class declaration.

– Use IMPLEMENT_DYNCREATE(CUIThread, CWinThread) in implementation.

– Create windows

– Launch UI thread by calling: CWinThread *pThread = AfxBeginThread(RUNTIME_CLASS(CUIThread));

Creating Win32 Threads

• HANDLE hThrd = (HANDLE)_beginthread(ThreadFunc, 0, &ThreadInfo);

• ThreadFunc – the function executed by the new thread

void _cdecl ThreadFunc(void *pThreadInfo);

• pThreadInfo – pointer to input parameters for the thread– Works just like the pThreadInfo on the previous slide.

• For both threads created with AfxBeginThread and _beginthread the thread function, ThreadFunc, must be a global function or static member function of a class. It can not be a non-static member function.

Suspending and Running Threads• Suspend a thread’s execution by calling SuspendThread. This

increments a suspend count. If the thread is running, it becomes suspended.

pThread -> CWinThread::SuspendThread();

• Calling ResumeThread decrements the suspend count. When the count goes to zero the thread is put on the ready to run list and will be resumed by the scheduler.

pThread -> CWinThread::ResumeThread();

• A thread can suspend itself by calling SuspendThread. It can also relinquish its running status by calling Sleep(nMS), where nMS is the number of milliseconds that the thread wants to sleep.

Thread Termination• ThreadFunc returns

– Worker thread only

– Return value of 0 a normal return condition code

• WM_QUIT– UI thread only

• AfxEndThread( UINT nExitCode )– Must be called by the thread itself

• ::GetExitCode(hThread, &dwExitCode)– Returns the exit code of the last work item (thread, process)

that has been terminated.

Wait For Objects• WaitForSingleObject makes one thread wait for:

– Termination of another thread

– An event

– Release of a mutex

– Syntax: WaitForSingleObject(objHandle, dwMillisec)

• WaitForMultipleObjects makes one thread wait for the elements of an array of kernel objects, e.g., threads, events, mutexes.– Syntax:

WaitForMultipleObjects(nCount, lpHandles, fwait, dwMillisec)

– nCount: number of objects in array of handles

– lpHandles: array of handles to kernel objects

– fwait: TRUE => wait for all objects, FALSE => wait for first object

– dwMillisec: time to wait, can be INFINITE

Process Priority

• IDLE_PRIORITY_CLASS– Run when system is idle

• NORMAL_PRIORITY_CLASS– Normal operation

• HIGH_PRIORITY_CLASS– Receives priority over the preceding two classes

• REAL_TIME_PRIORITY_CLASS– Highest Priority

– Needed to simulate determinism

Thread Priority• You use thread priority to balance processing performance

between the interfaces and computations. – If UI threads have insufficient priority the display freezes while

computation proceeds.– If UI threads have very high priority the computation may suffer.– We will look at an example that shows this clearly.

• Thread priorities take the values:– THREAD_PRIORITY_IDLE– THREAD_PRIORITY_LOWEST– THREAD_PRIORITY_BELOW_NORMAL– THREAD_PRIORITY_NORMAL– THREAD_PRIORITY_ABOVE_NORMAL– THREAD_PRIORITY_HIGHEST– THREAD_PRIORITY_TIME_CRITICAL

Thread Synchronization• Synchronizing threads means that every access to data shared

between threads is protected so that when any thread starts an operation on the shared data no other thread is allowed access until the first thread is done.

• The principle means of synchronizing access to shared data are:– Interlocked increments

• only for incrementing or decrementing integers– Critical Sections

• Good only inside one process– Mutexes

• Named mutexes can be shared by threads in different processes.– Events

• Useful for synchronization as well as other event notifications.

Interlocked Operations

• InterlockedIncrement increments a 32 bit integer as an atomic operation. It is guaranteed to complete before the incrementing thread is suspended.

long value = 5; InterlockedIncrement(&value);

• InterlockedDecrement decrements a 32 bit integer as an atomic operation:

InterlockedDecrement(&value);

Win32 Critical Sections• Threads within a single process can use critical sections to ensure mutually

exclusive access to critical regions of code. To use a critical section you:– allocate a critical section structure– initialize the critical section structure by calling a win32 API function– enter the critical section by invoking a win32 API function– leave the critical section by invoking another win32 function.– When one thread has entered a critical section, other threads requesting entry are

suspended and queued waiting for release by the first thread.

• The win32 API critical section functions are:– InitializeCriticalSection(&GlobalCriticalSection)– EnterCriticalSection(&GlobalCriticalSection)– TryEnterCriticalSection(&GlobalCriticalSection)– LeaveCriticalSection(&GlobalCriticalSection)– DeleteCriticalSection(&GlobalCriticalSection)

MFC Critical Sections• A critical section synchronizes access to a resource shared

between threads, all in the same process.– CCriticalSection constructs a critical section object

– CCriticalSection::Lock() locks access to a shared resource for a single thread.

– CCriticalSection::Unlock() unlocks access so another thread may access the shared resource

CCriticalSection cs;cs.Lock(); // operations on a shared resource, e.g., data, an iostream, filecs.Unlock();

Win32 Mutexes• Mutually exclusive access to a resource can be guaranteed

through the use of mutexes. To use a mutex object you:– identify the resource (section of code, shared data, a device) being shared

by two or more threads

– declare a global mutex object

– program each thread to call the mutex’s acquire operation before using the shared resource

– call the mutex’s release operation after finishing with the shared resource

• The mutex functions are:– CreateMutex

– WaitForSingleObject

– WaitForMultipleObjects

– ReleaseMutex

MFC Mutexes• A mutex synchronizes access to a resource shared between two or

more threads. Named mutexes are used to synchronize access for threads that reside in more than one process.– CMutex constructs a mutex object– Lock locks access for a single thread– Unlock releases the resource for acquisition by another thread

CMutex cm;cm.Lock(); // access a shared resourcecm.Unlock();

– CMutex objects are automatically released if the holding thread terminates.

Win32 Events• Events are objects which threads can use to serialize access to

resources by setting an event when they have access to a resource and resetting the event when through. All threads use WaitForSingleObject or WaitForMultipleObjects before attempting access to the shared resource.

• Unlike mutexes and semaphores, events have no predefined semantics.– An event object stays in the nonsignaled stated until your program sets its

state to signaled, presumably because the program detected some corresponding important event.

– Auto-reset events will be automatically set back to the non-signaled state after a thread completes a wait on that event.

– After a thread completes a wait on a manual-reset event the event will return to the non-signaled state only when reset by your program.

Win32 Events (continued)

• Event functions are:– CreateEvent

– OpenEvent

– SetEvent

– PulseEvent

– WaitForSingleEvent

– WaitForMultipleEvents

MFC Events• An event can be used to release a thread waiting on some shared

resource (refer to the buffer writer/reader example in pages 1018-1021).

• A named event can be used across process boundaries.• CEvent constructs an event object.• SetEvent() sets the event.• Lock() waits for the event to be set, then automatically resets it.

CEvent ce; :

ce.Lock(); // called by reader thread to wait for writer :ce.SetEvent(); // called by writer thread to release reader

CSingleLock & CMultiLock• CSingleLock and CMultiLock classes can be used to wrap critical

sections, mutexes, events, and semaphores to give them somewhat different lock and unlock semantics.

CCriticalSection cs;CSingleLock slock(cs);slock.Lock(); // do some work on a shared resourceslock.Unlock();

This CSingleLock object will release its lock if an exception is thrown inside the synchronized area, because its destructor is called. That does not happen for the unadorned critical section.

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

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

Sample Pthreads Program in C, C++

• The program in C++ calls the pthread.h header file. Pthreads related statements are preceded by the pthread_ prefix (except for semaphores). Knowing how to manipulate pointers is important.

1 //****************************************************************2 // This is a sample threaded program in C++. The main thread creates3 // 4 daughter threads. Each daughter thread simply prints out a message4 // before exiting. Notice that I’ve set the thread attributes to joinable and5 // of system scope.6 //****************************************************************7 #include <iostream.h>8 #include <stdio.h>9 #include <pthread.h>10 11 #define NUM_THREADS 412 13 void *thread_function( void *arg );14 15 int main( void )16 {17 int i, tmp;18 int arg[NUM_THREADS] = {0,1,2,3};19 20 pthread_t thread[NUM_THREADS];21 pthread_attr_t attr;22 23 // initialize and set the thread attributes24 pthread_attr_init( &attr );25 pthread_attr_setdetachstate( &attr, PTHREAD_CREATE_JOINABLE );26 pthread_attr_setscope( &attr, PTHREAD_SCOPE_SYSTEM );27

28 // creating threads 29 for ( i=0; i<NUM_THREADS; i++ )30 {31 tmp = pthread_create( &thread[i], &attr, thread_function, (void *)&arg[i] );32 33 if ( tmp != 0 )34 {35 cout << "Creating thread " << i << " failed!" << endl;36 return 1;37 }38 }39 40 // joining threads41 for ( i=0; i<NUM_THREADS; i++ )42 {43 tmp = pthread_join( thread[i], NULL );44 if ( tmp != 0 )45 {46 cout << "Joing thread " << i << " failed!" << endl;47 return 1;48 }49 }50 51 return 0;52 }53

54 //***********************************************************55 // This is the function each thread is going to run. It simply asks56 // the thread to print out a message. Notice the pointer acrobat ics.57 //***********************************************************58 void *thread_funct ion( void *arg )59 {60 int id;61 62 id = *(( int *)arg);63 64 printf( "Hello from thread %d!\n", id ) ;65 pthread_exit( NULL ) ;66 }

• How to compile:• in Linux use:

> {C++ comp} –D_REENTRANT hello.cc –lpthread –o hello

• it might also be necessary for some systems to define the _POSIX_C_SOURCE (to 199506L)

• Creating a thread:int pthread_create( pthread_t *thread, pthread_attr_t *attr, void

*(*thread_function)(void *), void *arg );

• first argument – pointer to the identifier of the created thread

• second argument – thread attributes

• third argument – pointer to the function the thread will execute

• fourth argument – the argument of the executed function (usually a struct)

• returns 0 for success