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Chapter 4: Threads
Threads
 A thread (or lightweight process) is a basic unit of CPU utilization; it
consists of:

program counter

register set

stack space
 A thread shares with its peer threads its:

code section

data section

operating-system resources
 A traditional or heavyweight process is equal to a task with one
thread
Operating System Concepts
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Threads (Cont.)
 In a multiple threaded process, while one server thread is blocked and
waiting, a second thread in the same process can run.

Cooperation of multiple threads in same job confers higher
throughput and improved performance.

Applications that require sharing a common buffer (i.e., producerconsumer) benefit from thread utilization.
 Threads provide a mechanism that allows sequential processes to
make blocking system calls while also achieving parallelism.
 Kernel-supported threads (Mach and OS/2).
 User-level threads; supported above the kernel, via a set of library
calls at the user level (Project Andrew from CMU).
 Hybrid approach implements both user-level and kernel-supported
threads (Solaris 2).
Operating System Concepts
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Silberschatz, Galvin and Gagne ©2005
Multiple Threads within a Process (task)
Operating System Concepts
4.4
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Single and Multithreaded Processes
Operating System Concepts
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Benefits
 Responsiveness
 Resource Sharing
 Economy
 Utilization of MP Architectures
Operating System Concepts
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User Threads
 Thread management is done by user-level threads library
 Three primary thread libraries:

POSIX Pthreads

Win32 threads

Java threads
Operating System Concepts
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Kernel Threads
 Supported by the Kernel
 Examples

Windows XP/2000

Solaris

Linux

Tru64 UNIX

Mac OS X
Operating System Concepts
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Silberschatz, Galvin and Gagne ©2005
Multithreading Models
 Many-to-One
 One-to-One
 Many-to-Many
Operating System Concepts
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Many-to-One
 Many user-level threads mapped to single kernel thread
 Examples:

Solaris Green Threads

GNU Portable Threads
Operating System Concepts
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Many-to-One Model
Operating System Concepts
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One-to-One
 Each user-level thread maps to kernel thread
 Examples

Windows NT/XP/2000

Linux

Solaris 9 and later
Operating System Concepts
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One-to-one Model
Operating System Concepts
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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 ThreadFiber package
Operating System Concepts
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Many-to-Many Model
Operating System Concepts
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Two-level Model
 Similar to M:M, except that it allows a user thread to be
bound to kernel thread
 Examples

IRIX

HP-UX

Tru64 UNIX

Solaris 8 and earlier
Operating System Concepts
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Silberschatz, Galvin and Gagne ©2005
Two-level Model
Operating System Concepts
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Threading Issues
 Semantics of fork() and exec() system calls
 Thread cancellation
 Signal handling
 Thread pools
 Thread specific data
 Scheduler activations
Operating System Concepts
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Semantics of fork() and exec()
 Does fork() duplicate only the calling thread or all threads?
Operating System Concepts
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Thread Cancellation
 Terminating a thread before it has finished
 Two general approaches:

Asynchronous cancellation terminates the target
thread immediately

Deferred cancellation allows the target thread to
periodically check if it should be cancelled
Operating System Concepts
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Signal Handling

Signals are used in UNIX systems to notify a process that a
particular event has occurred

A signal handler is used to process signals

1.
Signal is generated by particular event
2.
Signal is delivered to a process
3.
Signal is handled
Options:

Deliver the signal to the thread to which the signal applies

Deliver the signal to every thread in the process

Deliver the signal to certain threads in the process

Assign a specific threa to receive all signals for the process
Operating System Concepts
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Silberschatz, Galvin and Gagne ©2005
Thread Pools
 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 the application(s) to be
bound to the size of the pool
Operating System Concepts
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Thread Specific Data
 Allows each thread to have its own copy of data
 Useful when you do not have control over the thread
creation process (i.e., when using a thread pool)
Operating System Concepts
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Scheduler Activations
 Both M:M and Two-level models require communication to
maintain the appropriate number of kernel threads allocated
to the application
 Scheduler activations provide upcalls - a communication
mechanism from the kernel to the thread library
 This communication allows an application to maintain the
correct number kernel threads
Operating System Concepts
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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 (Solaris, Linux,
Mac OS X)
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Pthread Example
/**
* A pthread program illustrating how to
* create a simple thread and some of the pthread API
* This program implements the summation function where
* the summation operation is run as a separate thread.
*
* Compilation:
* gcc –lpthread thrd-posix.c
*
*/
#include <pthread.h>
#include <stdio.h>
int sum; /* this data is shared by the thread(s) */
void *runner(void *param); /* the thread */
Operating System Concepts
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Pthread Example (cont.)
int main(int argc, char *argv[]) {
pthread_t tid; /* the thread identifier */
pthread_attr_t attr; /* set of attributes for the thread */
if (argc != 2) {
fprintf(stderr,"usage: a.out <integer value>\n"); return -1;
}
if (atoi(argv[1]) < 0) {
fprintf(stderr,"Argument %d must be non-negative\n",atoi(argv[1])); return -1;
}
/* get the default attributes */
pthread_attr_init(&attr);
/* create the thread */
pthread_create(&tid,&attr,runner,argv[1]);
/* now wait for the thread to exit */
pthread_join(tid,NULL);
printf("sum = %d\n",sum);
return 0;
}
Operating System Concepts
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Pthread Example (cont.)
/**
* The thread will begin control in this function
*/
void *runner(void *param) {
int i, upper = atoi(param);
sum = 0;
if (upper > 0) {
for (i = 1; i <= upper; i++)
sum += i;
}
pthread_exit(0);
}
Operating System Concepts
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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)
Operating System Concepts
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Java Threads
 Java threads are managed by the JVM
 Java threads may be created by:

Extending Thread class

Implementing the Runnable interface
 studied in the course: Advanced Programming
Operating System Concepts
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Java Thread States
Operating System Concepts
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End of Chapter 4