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Shared-Memory
Programming
with Threads
Adapted by Aleksey Zimin from
http://navet.ics.hawaii.edu/~casanova/courses
/ics632_fall07/slides/ics632_threads.ppt
The concept of a “process”
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Processes are the very basic elements in O.S.
Unit of resources ownership
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Allocated with virtual address space + control of other
resources such as I/O, files….
Unit of dispatching (allocating computer
resources)
Execution path and state, dispatching priority.
 Controlled by OS
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What is a thread?
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A thread is an execution path in the code
segment
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O.S. provides an individual Program Counter
(PC) for each execution path
Comments
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Traditional program is one thread per process.
The main thread starts with main()
Only one thread or program counter (PC) is
allowed to execute the code segment
To add a new PC, you need to fork() to have
another PC to execute in another process
address space.
Unix’s fork() revisited
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Process management:
pid = fork() – create a child process identical to the
parent
 pid = waitpid(pid,&statloc,options) – wait for child
to terminate
 exit(status) – terminate process execution and return
status
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fork() example
void main() {
if (fork() == 0)
printf(“ in the child process”);
else
printf(“ in the parent process”);
}
Key benefits of multithreading
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Less time to create a thread than a process
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Less time to terminate a thread than a process
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Less time to switch a thread
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Enhance efficiency in communication: no need
for kernel to intervene
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Smaller chance of driving you crazy while
writing code / debugging
Shared memory programming
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The “easiest” form of parallel programming
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Can be used to parallelize a sequential code in an
incremental way:
 take a sequential code
 parallelize a small section
 check that it works
 check that it speeds things up a bit
 move on to another section
Thread
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A thread is a stream of instructions that can be
scheduled as an independent unit.
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A process is created by an operating system
 contains information about resources
 process id, file descriptors, ...
 contains information on the execution state
 program counter, stack, ...
Thread
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The concept of a thread requires that we make a
separation between these two kinds of information in a
process
 resources available to the entire process
 program instructions, global data, working
directory
 schedulable entities
 program counters and stacks.
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A thread is an entity within a process which consists of
the schedulable part of the process.
Process is still there, what’s new for
thread?
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With process
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Virtual address space (holding process image)
Protected access to CPU, files, and I/O resources
With thread (each thread has its own..)
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Thread execution state
Saved thread context (an independent PC within a process)
An execution stack
Per-thread static storage for local variables
Access to memory and resources of its process, shared with
all other threads in that process
Possible combination of thread and processes
One process one thread
Multiple processes
One thread per process
One process multiple thread
Multiple processes multiple
Threads per process
Parallelism with Threads
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Create threads within a process
Each thread does something (hopefully) useful
Threads may be working truly concurrently
Multi-processor
 Multi-core
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Or just pseudo-concurrently
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Single-proc, single-core
Example
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Say I want to compute the sum of two arrays
I can just create N threads, each of which sums 1/Nth
of both arrays and then combine their results
I can also create N threads that each increment some
sum variable element-by-element, but then I’ve got to
make sure they don’t step on each other’s toes
The first version is a bit less “shared-memory”, but is
probably more efficient
Multi-threading issues
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There are really two main issues when writing multi-threaded
code:
Issue #1: Load Balancing
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Make sure that no processors/cores is left idle when it could be doing
useful work
Issue #2: Correct access to shared variables
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Implemented via mutual exclusion: create sections of code that only a
single thread can be in at a time
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Called “critical sections”
Classical variable update example
Done via “locks” and “unlocks”
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Warning: locks are NOT on variables, but on sections of code
Threads in Practice
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Pthreads
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Popular C library
Flexible
Will discuss these
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OpenMP
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Java Threads
Pthreads
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A POSIX standard (IEEE 1003.1c) API for thread
creation and synchronization
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The API specifies the standard behavior
Implementation choices are up to developers
And implementations vary from systems to systems, with
some better than some others
Common in all UNIX operating systems
Some people have written it for Win32
The most portable threading library out there
What do threads look like in UNIX?
User-level / Kernel-level
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User-level threads: Many-to-one thread mapping
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Implemented by user-level runtime libraries
 Create, schedule, synchronize threads at user-level
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OS is not aware of user-level threads
 OS thinks each process contains only a single
thread of control
User-level / Kernel-level
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Advantages
 Does not require OS support; Portable
 Can tune scheduling policy to meet application
demands
 Lower overhead thread operations since no system
calls
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Disadvantages
 Cannot leverage multiprocessors
 Entire process blocks when one thread blocks
User-level / Kernel-level
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Kernel-level threads: One-to-one thread
mapping
OS provides each user-level thread with a kernel
thread
 Each kernel thread scheduled independently
 Thread operations (creation, scheduling,
synchronization) performed by OS
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User-level / Kernel-level
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Advantages
Each kernel-level thread can run in parallel on a
multiprocessor
 When one thread blocks, other threads from process
can be scheduled
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Disadvantages
Higher overhead for thread operations
 OS must scale well with increasing number of
threads
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Using the Pthread Library
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Pthread library typically uses kernel-threads
Programs must include the file pthread.h
Programs must be linked with the pthread library (lpthread)
The API contains functions to
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create threads
control threads
manage threads
synchronize threads
pthread_self()
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Returns the thread identifier for the calling
thread
At any point in its instruction stream a thread can
figure out which thread it is
 Convenient to be able to write code that says: “If
you’re thread 1 do this, otherwise to that”
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#include <pthread.h>
pthread_t pthread_self(void);
pthread_create()
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Creates a new thread of control
#include <pthread.h>
int pthread_create (
pthread_t *thread,
pthread_attr_t *attr,
void * (*start_routine) (void *),
void *arg);
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Returns 0 to indicate success, otherwise returns error code
thread: output argument that will contain the thread id of the new thread
attr: input argument that specifies the attributes of the thread to be created (NULL =
default attributes)
start_routine: function to use as the start of the new thread must have prototype:
void * foo(void*)
arg: argument to pass to the new thread routine
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If the thread routine requires multiple arguments, they must be passed bundled up in an array or a
structure
pthread_create() example
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Want to create a thread to compute the sum of the elements of
an array
void *do_work(void *arg);
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Needs three arguments
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the array, its size, where to store the sum
we need to bundle them in a structure
struct arguments {
long int *array;
long int size;
long int *sum;
}
pthread_create() example
int main(void) {
long int array[ARRAY_SIZE], sum, i;
pthread_t worker_thread;
struct arguments *arg;
for(i=0;i<ARRAY_SIZE;i++) array[i]=1;
arg = calloc(1,sizeof(struct arguments));
arg->array = array;
arg->size=ARRAY_SIZE;
arg->sum = &sum;
if (pthread_create(&worker_thread, NULL, do_work, (void *)arg))
{
fprintf(stderr,"Error while creating thread");
exit(1);
}
...
exit(0);
}
pthread_create() example
void *do_work(void *arg){
long int i, size;
long int *array;
long int *sum;
size = ((struct arguments *)arg)->size;
array = ((struct arguments *)arg)->array;
sum = ((struct arguments *)arg)->sum;
*sum = 0;
for (i=0;i<size;i++)
*sum += array[i];
return NULL;
}
Comments about the example
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The “parent thread” continues its normal execution
after creating the “child thread”
Memory is shared by the parent and the child (the array,
the location of the sum)
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nothing prevents from the parent doing something to it while
the child is still executing
which may lead to a wrong computation
The bundling and unbundling of arguments is a bit
tedious, but nothing compared to what’s needed with
shared memory segments and processes
pthread_exit()
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Terminates the calling thread
#include <pthread.h>
void pthread_exit(
void *retval);
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The return value is made available to another thread calling a
pthread_join() (see later)
The previous example had the thread just return from
function do_work()
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In this case the call to pthread_exit() is implicit
The return value of the function serves as the argument to the
(implicitly called) pthread_exit().
pthread_join()
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Causes the calling thread to wait for another thread to terminate
#include <pthread.h>
int pthread_join(
pthread_t thread,
void **value_ptr);
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thread: input parameter, id of the thread to wait on
value_ptr: output parameter, value given to pthread_exit() by
the terminating thread (which happens to always be a void *)
returns 0 to indicate success, error code otherwise
multiple simultaneous calls for the same thread are not allowed
pthread_kill()
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Causes the termination of a thread
#include <pthread.h>
int pthread_kill(
pthread_t thread,
int sig);
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thread: input parameter, id of the thread to terminate
sig: signal number
returns 0 to indicate success, error code otherwise
pthread_join() example
int main(void) {
long int array[100];
long int sum;
pthread_t worker_thread;
struct arguments *arg;
arg = (struct arguments *)calloc(1,sizeof(struct arguments));
arg->array = array;
arg->size=100;
arg->sum = &sum;
if (pthread_create(&worker_thread, NULL,
do_work, (void *)arg)) {
fprintf(stderr,”Error while creating thread\n”);
exit(1);
}
...
if (pthread_join(worker_thread, NULL)) {
fprintf(stderr,”Error while waiting for thread\n”);
exit(1);
}
}
Synchronizing pthreads
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As we’ve seen earlier, we need a system to implement locks
to create mutual exclusion for variable access, via critical
sections
Lock creation
int pthread_mutex_init(
pthread_mutex_t *mutex,
const pthread_mutexattr_t *attr);
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returns 0 on success, an error code otherwise
mutex: output parameter, lock
attr: input, lock attributes
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NULL: default
There are functions to set the attribute (look at the man pages if you’re
interested)
Synchronizing pthreads
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Locking a lock
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If the lock is already locked, then the calling thread is blocked
If the lock is not locked, the the calling thread acquires it
int pthread_mutex_lock(
pthread_mutex_t *mutex);
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returns 0 on success, an error code otherwise
mutex: input parameter, lock
Just checking
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Returns instead of locking
int pthread_mutex_trylock(
pthread_mutex_t *mutex);
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returns 0 on success, EBUSY is the lock is locked, an error code otherwise
mutex: input parameter, lock
Synchronizing pthreads
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Releasing a lock
int pthread_mutex_unlock(
pthread_mutex_t *mutex);
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returns 0 on success, an error code otherwise
mutex: input parameter, lock
With locking, trylocking, and unlocking, one can
avoid all race conditions and protect access to
shared variables
Mutex Example:
...
pthread_mutex_t mutex;
pthread_mutex_init(&mutex, NULL);
...
pthread_mutex_lock(&mutex);
Critical Section
count++;
pthread_mutex_unlock(&mutex);
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To “lock” variable count, just put a pthread_mutex_lock()
and pthread_mutex_unlock() around all sections of the code
that write to variable count
Again, you’re really locking code, not variables
Cleaning up memory
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Releasing memory for a mutex attribute
int pthread_mutex_destroy(
pthread_mutex_t *mutex);
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Releasing memory for a mutex
int pthread_mutexattr_destroy(
pthread_mutexattr_t *mutex);
Signaling
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Allows a thread to wait until some process signals that some
condition is met
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provides a more sophisticated way to synchronize threads than just mutex
locks
Done with “condition variables”
Example:
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You have to implement a server with a main thread and many threads that
can be assigned work (e.g., an incoming request)
You want to be able to “tell” a thread: “there is work for you to do”
Inconvenient to do with mutex locks
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the main thread must carefully manage a lock for each worker thread
everybody must constantly be polling locks
Condition Variables
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Condition variables are used in conjunction with mutexes
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Create a condition variable
Create an associated mutex
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Waiting on a condition
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We will see why it’s needed later
lock the mutex
wait on condition variable
unlock the mutex
Signaling
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Lock the mutex
Signal on the condition variable
Unlock mutex
pthread_cond_init()
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Creating a condition variable
int pthread_cond_init(
pthread_cond_t *cond,
const pthread_condattr_t *attr);
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returns 0 on success, an error code otherwise
cond: output parameter, condition
attr: input parameter, attributes (default = NULL)
pthread_cond_wait()
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Waiting on a condition variable
int pthread_cond_wait(
pthread_cond_t *cond,
pthread_mutex_t *mutex);
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returns 0 on success, an error code otherwise
cond: input parameter, condition
mutex: input parameter, associated mutex
pthread_cond_signal()
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Signaling a condition variable
int pthread_cond_signal(
pthread_cond_t *cond;
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returns 0 on success, an error code otherwise
cond: input parameter, condition
“Wakes up” one thread out of the possibly
many threads waiting for the condition
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The thread is chosen non-deterministically
pthread_cond_broadcast()
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Signaling a condition variable
int pthread_cond_broadcast(
pthread_cond_t *cond;
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returns 0 on success, an error code otherwise
cond: input parameter, condition
“Wakes up” ALL threads waiting for the
condition
Condition Variable: example
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Say I want to have multiple threads wait until a counter reaches a maximum
value and be awakened when it happens
pthread_mutex_lock(&lock);
while (count < MAX_COUNT) {
pthread_cond_wait(&cond,&lock);
}
pthread_mutex_unlock(&lock)
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Locking the lock so that we can read the value of count without the possibility of
a race condition
Calling pthread_cond_wait() in a loop to avoid “spurious wakes ups”
When going to sleep the pthread_cond_wait() function implicitly releases
the lock
When waking up the pthread_cond_wait() function implicitly acquires the
lock (and may thus sleep)
Unlocking the lock after exiting from the loop
pthread_cond_timed_wait()
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Waiting on a condition variable with a
timeout
int pthread_cond_timedwait(
pthread_cond_t *cond,
pthread_mutex_t *mutex,
const struct timespec *delay);
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returns 0 on success, an error code otherwise
cond: input parameter, condition
mutex: input parameter, associated mutex
delay: input parameter, timeout (same fields as the one
used for gettimeofday)
PThreads: Conclusion
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A popular way to write multi-threaded code
If you know pthreads, you’ll have no problem adapting
to other multi-threading techniques
Condition variables are a bit odd, but very useful
For you project you may want to use pthreads
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More information
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Man pages
PThread Tutorial:
http://www.llnl.gov/computing/tutorials/pthreads/