Threads

By Dr. Yingwu Zhu

Review Multithreading Models

Many-to-one One-to-one Many-to-many

Many-to-one Model

Kernels do not support multiple threads of control

Multithreading can be implemented entirely as a user-level library

Schedule multiple threads onto the process’s single kernel thread; multiplexing multiple user threads on a single kernel thread

Many-to-one (cont.): Benefits

Cheap synchronization When a user thread wishes to perform

synchronization, the user-level thread lib. checks to see if the thread needs to block.

If a user thread does, the user-level thread lib. enqueues it, and dequeues another user thread from the lib.’s run queue, and swithes the active thread.

No system calls are required Cheap thread creation

The thread lib. need only create a context (i.e., a stack and registers) and enqueues it in the user-level run queue

Many-to-one (cont.): Benefits

Resource efficiency Kernel memory is not wasted on a stack for each

user thread Allows as many thread as VM permits

Portability User-level threads packages are implemented

entirely with standard UNIX and POSIX lib. calls

Many-to-one (cont.): Drawbacks

Single-threaded OS interface If a user thread blocks (e.g, blocking system calls),

the entire process blocks and so no other user thread can execute until the kernel thread (which is blocked in the system call) becomes available

Solution: using nonblocking system calls Can not utilize MP achitectures Examples: Java, Netscape

One-to-one Model

Each user thread has a kernel thread

One-to-one (cont.): Benefits

Scalable parallelism Each kernel thread is a different kernel-schedulable

entity; multiple threads can run concurrently on multiprocessors

Multithreaded OS interface When one user thread and its kernel thread block,

the other user threads can continue to execute since their kernel threads are unaffected

One-to-one (cont.): Drawbacks

Expensive synchronization Kernel threads require kernel involvement to be

scheduled; kernel thread synchronization will require a system call if the lock is not immediately acquired

If a trap is required, synchronization will be from 3-10 times more costly than many-to-one model

Expensive creation Every thread creation requires explicit kernel

involvement and consumes kernel resources 3-10 times more expensive than creating a user

thread

One-to-one (cont.): Drawbacks

Resource inefficiency Every thread created by the user requires kernel

memory for a stack, as well as some sort of kernel data structure to keep track of it

Many parts of many kernels cannot be paged out The presence of kernel threads is likely to displace

physical memory for applications

Many-to-Many Model

Combing the previous two models User threads are multiplexed on top of kernel

threads which in turn are scheduled on top of processors

Taking advantage of the previous two models while minimizing both’s disadvantages

Creating a user thread does not necessarily require the creation of a kernel threads; synchronization can be purely user-level

Pthread Tutorial

Creating and destroying threads How to use POSIX threads

How to compile?

$ gcc –o proj2 proj2.c –pthread The option specifies that pthreads library should be

linked causes the complier to properly handle multiple

threads in the code that it generates

Creating and Destroying Threads

Creating threads Step 1: create a thread Step 2: send the thread one or more parameters

Destroy threads Step 1: destroy a thread Step 2: retrieve one or more values that are returned

from the thread

Creating Threads

#include <pthread.h> int pthread_create (pthread_t *thread_id, pthread_attr_t *attr, void *(*thread_fun)(void

*), void *args);- The #1 para returns thread ID- The #2 para pointing to thread attr. NULL represents

using the default attr. settings- The #3 para as pointer to a function the thread is to

execute- The #4 para is the arguments to the function

Thread Terminates

Pthreads terminate when the function returns, or the thread calls pthread_exit()

int pthread_exit(void *status); status is the return value of the thread A thread_fun returns a void*, so calling “return (void

*) is the equivalent of this function

Thread termination

One thread can wait (or block) on the termination of another by using pthread_join()

You can collect the exit status of all threads you created by pthread_join()

int pthread_join(pthread_t thread_id, void **status)

The exit status is returned in status pthread_t pthread_self();

Get its own thread id int pthread_equal(pthread_t t1, pthread_t t2);

Compare two thread ids

Example

#include <pthread.h>void *thread_fun(void *arg) { int *inarg = (int *)arg; … return NULL;}

Int main() { pthread_t tid; void *exit_state; int val = 42; pthread_create(&tid, NULL, thread_fun, &value); pthread_join(tid, &exit_state); return 0;}

Kill Threads

Kill a thread before it returns normally using pthread_cancel()

But Make sure the thread has released any local

resources; unlike processes, the OS will not clean up the resources

Why? Threads in a process share resources

Exercise

Write a multithreaded program that calculates the summation of a non-negative integer in a separate thread

The non-negative integer is from command-line parameter

The summation result is kept in a global variable:int sum; // shared by threads

Step 1: write a thread function

void *thread_sum(void *arg) { int i; int m = (int)(*arg); sum = 0; //initialization for (i = 0; i <= sum; i++) sum += I; pthread_exit(0);}

Step 2: write the main()

int sum;int main(int argc, char *argv[]) { pthread_t tid; if (argc != 2) { printf(“Usage: %s <integer-para>\n”, argv[0]); return -1; } int i = atoi(argv[1]); if (i < 0) { printf(“integer para must be non-negative\n”); return -2; } pthread_create(&tid, NULL, thread_sum, &i); pthread_join(tid, NULL); printf(“sum = %d\n”, sum);}

Exercise

Write a program that creates 10 threads. Have each thread execute thesame function and pass each thread a unique number. Each thread should print “Hello, World (thread n)” five times where ‘n’ is replaced by the thread’s number. Use an array of pthread t objects to hold the various thread IDs. Be sure the program doesn’t terminate until all the threadsare complete. Try running your program on more than one machine. Are there any differences in how it behaves?

Returning Results from Threads

Thread function return a pointer to void: void * Pitfalls in return value

Pitfall #1

void *thread_function ( void *){ int code = DEFAULT_VALUE; return ( void *) code ;}

Only work in machines where integers can convert to a point and then back to an integer without loss of information

Pitfall #2

void *thread_function ( void *){ char buffer[64]; // fill up the buffer with sth good return ( void *) buffer;}

This buffer will disappear as the thread function returns

Pitfall #3

void *thread_function ( void *){ static char buffer[64]; // fill up the buffer with sth good return ( void *) buffer;}

It does not work in the common case of multiple threads running the same thread funciton

Right Way

void *thread_function ( void *){ char* buffer = (char *)malloc(64); // fill up the buffer with sth good return ( void *) buffer;}

Right Way

int main() { void *exit_state; char *buffer; …. pthread_join(tid, &exit_state); buffer = (char *) exit_state; printf(“from thread %d: %s\n”, tid, buffer); free(exit_state);

}

Exercise

Write a program that computes the square roots of the integers from 0 to 99 in a separate thread and returns an array of doubles containing the results. In the meantime the main thread should display a short message to the user and then display the results of the computation when they are ready.

Exercise

In textbook 4.7 and 4.9