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Questions and Answers
What is the main goal of the parallel version of the program?
What is the main goal of the parallel version of the program?
What is the default visibility of variables that already exist before the parallel section in OpenMP?
What is the default visibility of variables that already exist before the parallel section in OpenMP?
What is a critical section in the context of parallel programming?
What is a critical section in the context of parallel programming?
What is the purpose of a mutual exclusion mechanism?
What is the purpose of a mutual exclusion mechanism?
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What happens when a thread changes the value of a shared variable in OpenMP?
What happens when a thread changes the value of a shared variable in OpenMP?
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What is the purpose of the critical directive in OpenMP?
What is the purpose of the critical directive in OpenMP?
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What happens when several threads try to access the same shared variable at the same time?
What happens when several threads try to access the same shared variable at the same time?
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What is the main drawback of using the critical clause in OpenMP?
What is the main drawback of using the critical clause in OpenMP?
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Why is it necessary to use a mutual exclusion mechanism in the parallel version?
Why is it necessary to use a mutual exclusion mechanism in the parallel version?
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What is the purpose of the reduction clause in OpenMP?
What is the purpose of the reduction clause in OpenMP?
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What is the main challenge of parallel programming in this exercise?
What is the main challenge of parallel programming in this exercise?
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What is the purpose of time measurement in the sequential version?
What is the purpose of time measurement in the sequential version?
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What is the result of using the reduction clause in OpenMP?
What is the result of using the reduction clause in OpenMP?
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What is the expected result of both the sequential and parallel versions?
What is the expected result of both the sequential and parallel versions?
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What happens to the private values of each thread when using the reduction clause in OpenMP?
What happens to the private values of each thread when using the reduction clause in OpenMP?
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What is the main advantage of using the reduction clause over the critical clause in OpenMP?
What is the main advantage of using the reduction clause over the critical clause in OpenMP?
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What is the primary requirement for the Pthreads library to function?
What is the primary requirement for the Pthreads library to function?
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What is the purpose of the pthread_create function?
What is the purpose of the pthread_create function?
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What is the purpose of the linker option -lpthread?
What is the purpose of the linker option -lpthread?
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What is the data type of the thread_p parameter in the pthread_create function?
What is the data type of the thread_p parameter in the pthread_create function?
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What is the purpose of the pthread_join function?
What is the purpose of the pthread_join function?
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What is the purpose of the start_routine function in the pthread_create function?
What is the purpose of the start_routine function in the pthread_create function?
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What is the purpose of the arg_p parameter in the pthread_create function?
What is the purpose of the arg_p parameter in the pthread_create function?
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What is the purpose of the global variable sum in the example incremental application?
What is the purpose of the global variable sum in the example incremental application?
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What is the primary issue with using active waiting to synchronize access to a global state?
What is the primary issue with using active waiting to synchronize access to a global state?
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What is the main purpose of a condition variable?
What is the main purpose of a condition variable?
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Why is it necessary to associate a condition variable with a mutual exclusion mechanism?
Why is it necessary to associate a condition variable with a mutual exclusion mechanism?
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What is the main advantage of using condition variables over active waiting?
What is the main advantage of using condition variables over active waiting?
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What is the key restriction in the Reader-Writer problem?
What is the key restriction in the Reader-Writer problem?
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What is the primary benefit of using barrier synchronization?
What is the primary benefit of using barrier synchronization?
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What is the main challenge addressed by condition variables in the context of threads and global state?
What is the main challenge addressed by condition variables in the context of threads and global state?
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In the Producer-Consumer problem, what happens to the produced resources?
In the Producer-Consumer problem, what happens to the produced resources?
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What is the function of the barrier() function in Barrier Synchronization?
What is the function of the barrier() function in Barrier Synchronization?
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What is the condition for a Consumer thread to consume resources in the Producer-Consumer problem?
What is the condition for a Consumer thread to consume resources in the Producer-Consumer problem?
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What is the purpose of a mutual exclusion mechanism?
What is the purpose of a mutual exclusion mechanism?
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What is a typical case of synchronization?
What is a typical case of synchronization?
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What happens when all threads execute the barrier() function?
What happens when all threads execute the barrier() function?
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What is the purpose of the Reader-Writer problem?
What is the purpose of the Reader-Writer problem?
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What is the purpose of the cond_broadcast() function in barrier synchronization?
What is the purpose of the cond_broadcast() function in barrier synchronization?
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In the producer-consumer problem, what is the condition for the consumer thread to execute?
In the producer-consumer problem, what is the condition for the consumer thread to execute?
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What is the purpose of the while loop in the producer thread?
What is the purpose of the while loop in the producer thread?
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What is the purpose of the mutex_lock() function in the producer-consumer problem?
What is the purpose of the mutex_lock() function in the producer-consumer problem?
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What is the purpose of the cond_wait() function in the producer-consumer problem?
What is the purpose of the cond_wait() function in the producer-consumer problem?
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What is the benefit of using condition variables in the producer-consumer problem?
What is the benefit of using condition variables in the producer-consumer problem?
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What is the purpose of the barrier synchronization?
What is the purpose of the barrier synchronization?
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What is the problem with active waiting in the producer-consumer problem?
What is the problem with active waiting in the producer-consumer problem?
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What is the primary issue with using active waiting to synchronize access to a global state?
What is the primary issue with using active waiting to synchronize access to a global state?
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When is a thread woken up from a condition variable?
When is a thread woken up from a condition variable?
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What is the main advantage of using condition variables over active waiting?
What is the main advantage of using condition variables over active waiting?
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What is necessary to associate with a condition variable?
What is necessary to associate with a condition variable?
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Why is active waiting not suitable for synchronizing access to a global state?
Why is active waiting not suitable for synchronizing access to a global state?
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What motivates the use of condition variables in parallel programming?
What motivates the use of condition variables in parallel programming?
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What is the primary issue with using active waiting to synchronize access to a global state?
What is the primary issue with using active waiting to synchronize access to a global state?
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What is the main purpose of using condition variables in the context of threads and global state?
What is the main purpose of using condition variables in the context of threads and global state?
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What is the key difference between pthread_cond_signal and pthread_cond_broadcast?
What is the key difference between pthread_cond_signal and pthread_cond_broadcast?
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What is the main advantage of using condition variables over active waiting?
What is the main advantage of using condition variables over active waiting?
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What is the purpose of the pthread_cond_wait function?
What is the purpose of the pthread_cond_wait function?
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Why is it necessary to associate a condition variable with a mutual exclusion mechanism?
Why is it necessary to associate a condition variable with a mutual exclusion mechanism?
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What is the primary purpose of a barrier in parallel programming?
What is the primary purpose of a barrier in parallel programming?
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What is active waiting in the context of parallel programming?
What is active waiting in the context of parallel programming?
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What is the purpose of a condition variable in the context of threads and global state?
What is the purpose of a condition variable in the context of threads and global state?
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What is the role of the cond_wait function in the producer-consumer problem?
What is the role of the cond_wait function in the producer-consumer problem?
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What is the difference between active waiting and passive waiting?
What is the difference between active waiting and passive waiting?
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What is the benefit of using condition variables in the producer-consumer problem?
What is the benefit of using condition variables in the producer-consumer problem?
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What is the purpose of the mutex_lock function in the producer-consumer problem?
What is the purpose of the mutex_lock function in the producer-consumer problem?
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What is the consequence of using active waiting in the producer-consumer problem?
What is the consequence of using active waiting in the producer-consumer problem?
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What is the primary function of MPI_Barrier?
What is the primary function of MPI_Barrier?
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What is the purpose of the root parameter in MPI_Bcast?
What is the purpose of the root parameter in MPI_Bcast?
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What happens when all processes invoke MPI_Barrier?
What happens when all processes invoke MPI_Barrier?
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What is the purpose of the sendbuf parameter in MPI_Reduce?
What is the purpose of the sendbuf parameter in MPI_Reduce?
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What is the role of the recvbuf parameter in MPI_Reduce?
What is the role of the recvbuf parameter in MPI_Reduce?
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What is the purpose of the op parameter in MPI_Reduce?
What is the purpose of the op parameter in MPI_Reduce?
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What happens when a process calls MPI_Bcast?
What happens when a process calls MPI_Bcast?
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What is the primary difference between MPI_Bcast and MPI_Reduce?
What is the primary difference between MPI_Bcast and MPI_Reduce?
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What is the primary difference between the MPI_Send function and the MPI_Ssend function?
What is the primary difference between the MPI_Send function and the MPI_Ssend function?
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What is the purpose of the MPI_Get_count function?
What is the purpose of the MPI_Get_count function?
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What is a requirement for successful message communication in MPI?
What is a requirement for successful message communication in MPI?
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What is the purpose of collective operations in MPI?
What is the purpose of collective operations in MPI?
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What is the behavior of the MPI_Recv function?
What is the behavior of the MPI_Recv function?
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What is the purpose of sender/receiver symmetry in MPI?
What is the purpose of sender/receiver symmetry in MPI?
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What is the difference between MPI_Send and MPI_Ssend in terms of error handling?
What is the difference between MPI_Send and MPI_Ssend in terms of error handling?
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What is the purpose of the MPI_Barrier function?
What is the purpose of the MPI_Barrier function?
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What is the purpose of the 'tag' parameter in the MPI_Send function?
What is the purpose of the 'tag' parameter in the MPI_Send function?
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What is the purpose of the 'status' parameter in the MPI_Recv function?
What is the purpose of the 'status' parameter in the MPI_Recv function?
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What is the purpose of the MPI_Barrier function?
What is the purpose of the MPI_Barrier function?
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What is the purpose of the MPI_Bcast function?
What is the purpose of the MPI_Bcast function?
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What is the purpose of the MPI_Reduce function?
What is the purpose of the MPI_Reduce function?
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What is the difference between MPI_Send and MPI_Ssend?
What is the difference between MPI_Send and MPI_Ssend?
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What happens when a process calls MPI_Barrier?
What happens when a process calls MPI_Barrier?
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What is the purpose of the 'count' parameter in the MPI_Send function?
What is the purpose of the 'count' parameter in the MPI_Send function?
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Study Notes
Parallel Programming Challenge
- Develop a program that increments a global variable N times, with a sequential version relying on a single thread and a parallel version using more than 1 thread.
- Correctness: both versions must always produce the same result.
- Performance: the parallel version is expected to be n times faster for n threads.
Implementation Exercise
- Implement a sequential version that increments a global variable, considering a parameter #iterations.
- Implement a parallel version that:
- Makes use of threads.
- Allows configuring the number of threads.
- Distributes the cycle size among the threads.
Race Condition and Critical Section
- A race condition occurs when multiple threads try to access the same shared variable simultaneously.
- Hardware characteristics can cause inconsistencies in shared variable values when subject to read/write operations.
- A critical section is code executed by multiple threads that accesses a shared variable and must be protected from simultaneous access.
Mutual Exclusion Mechanism
- Mutual exclusion mechanisms prevent multiple threads from executing critical sections simultaneously.
- These mechanisms ensure that only one thread can access shared resources at a time.
Solution to Get it Correct in the Parallel Version
- Use a mutual exclusion mechanism to prevent simultaneous access to shared variables.
- Identify:
- Shared variables.
- Thread code that accesses the variable.
- Rotate this code by a mutual exclusion mechanism.
Mutual Exclusion Mechanisms in OpenMP
- Variable visibility: shared and private.
- Shared variables are accessed by all threads, while private variables are only visible to the thread itself.
- Changing the value of a shared variable makes it visible to all other threads.
Critical Directive
-
#pragma omp critical
directive indicates a critical section protected by a mutual exclusion mechanism. - Only one thread can execute this code at a time, not simultaneously.
Accumulation of Results in Shared Global Variable
- The critical clause guarantees correctness but creates serialization, losing the advantage of parallelism.
- The reduction clause allows using a shared global variable as if it were private, accumulating results at the end of execution.
Reduction Clause
-
reduction(:)
clause allows using a shared global variable as if it were private. - At the end of execution, private values are accumulated into the shared global variable using the operator.
- Equivalent to using an auxiliary private variable to accumulate internal values and then integrating them into the shared global value.
Pthreads in High Performance Computing
- Pthreads is a library for developing parallel applications using shared memory.
- It assumes a POSIX-compliant operating system as its base.
- The library can be embedded in any programming language, usually C.
- The use of threads is achieved by invoking functions from the library.
Compilation and Execution of Pthreads Programs
- Include library headers:
#include
- Linker option:
-lpthread
- Example compilation command:
gcc -lpthread hello.c -o hello
Pthread API to Create and Join Threads
-
pthread_create
function:-
pthread_t* thread_p
: thread object reference -
const pthread_attr_t* attr_p
: creation attributes, can be NULL -
void* (*start_routine)(void*)
: function to execute -
void* arg_p
: function argument
-
- Generic function header for
start_routine
:void* start_routine(void* args_p);
Pthread API to Join Threads
-
pthread_join
function:-
pthread_t thread
: thread to join -
void** ret_val_p
: return value of the joined thread
-
Example Incremental Application
- Global variables:
-
long long n
: number of iterations -
long long thread_count
: number of threads -
long long sum
: global sum value
-
- Thread operation:
-
void* Increment(void* rank)
:- Calculate
my_n
andmy_first_i
based onn
andthread_count
- Perform
printf
statement to display thread range - Perform a
for
loop to increment the sum value
- Calculate
-
Thread Synchronization Problems
- High-performance computing and Master in Applied Artificial Intelligence involve thread execution ordering and synchronization problems.
Thread Execution Ordering
- The Mutual Exclusion mechanism ensures that only one thread executes code that manipulates the same resource at a time.
- Threads have an implicit execution order, even though they are concurrent.
- Typical synchronization problems include:
- Producer-Consumer
- Reader-Writer
- Barrier
Producer-Consumer Problem
- Involves two types of threads: Producer and Consumer.
- Producer produces resources and stores them in a shared buffer.
- Consumer consumes produced resources and removes them from the shared buffer.
- Conditions:
- Producer can produce only if the buffer has available space.
- Consumer can consume only if the buffer has available resources.
Reader-Writer Problem
- Involves two types of threads: Readers and Writers.
- Restrictions:
- Multiple readers can access data concurrently without risk of inconsistencies.
- When a writer wants to alter data, there can be no simultaneous access, reading or writing.
Barrier Problem
- Threads invoke the
barrier()
function, which blocks execution until all threads reach this point. - When all threads execute this function, it terminates, and all threads are unblocked.
- Acts as a synchronization point in the execution of each thread.
Barrier Example using Condition Variables
- Pseudocode:
-
Mutex_lock(mutex);
-
counter++;
-
if (counter == thread_count):
-
counter = 0;
-
Cond_broadcast(cond_var);
-
-
else:
-
while (...);
-
cond_wait(cond_var, mutex);
-
-
Mutex_unlock(mutex);
-
Producer-Consumer Example
- Unbounded buffer example:
- Thread Producer generates items for a buffer (list) with limited size.
- Thread Consumer extracts items from the same list.
- Cases:
- Consumer can execute only if the list has at least one item.
- Producer can execute only if the list has available space for one more item.
Producer-Consumer Unbounded Buffer Example (Pseudo-code)
-
mutex_t mutex;
-
cond_t notEmptyforConsumer;
-
cond_t notFullforProducer;
-
void* buffer[];
-
int firstptr, lastptr, count;
-
int buf_size;
Producer-Consumer Unbounded Buffer Pseudo-code
-
void* produce(void* item) :
-
mutex_lock(&mutex);
-
while (count == buffer_size) pthread_cond_wait(&mutex, &notFullforProducer);
-
buffer[lastptr] = item;
-
lastptr = lastptr + 1 % buffer_size;
-
count += 1;
-
pthread_cond_signal(&notEmptyforConsumer);
-
mutex_unlock(&mutex);
-
-
void* consume() :
-
mutex_lock(&mutex);
-
while (count == 0) pthread_cond_wait(&mutex, &notEmptyforConsumer);
-
item = buffer[firstptr];
-
firstptr = firstptr + 1 % buffer_size;
-
count -= 1;
-
pthread_cond_signal(&notFullforProducer);
-
mutex_unlock(&mutex);
-
return item;
-
Condition Variables
-
Motivation:
- Threads need to check a global condition.
- Need to access a common global state.
- Need to wait for the state to change without blocking other threads.
-
Solution:
- Use condition variables to suspend execution until a certain event occurs.
- Associated with a mutual exclusion (mutex) mechanism.
- Signalling mechanism "wakes up" locked threads to continue execution.
Condition Variables
- Condition variables are used to synchronize threads that need to access a common global state.
- A condition variable allows threads to suspend execution until a certain event or condition occurs.
- When the event occurs, a signaling mechanism "wakes up" the locked threads to continue execution.
- A condition variable must be associated with a mutual exclusion (mutex) mechanism.
Condition Variables API
-
pthread_cond_wait(cv, mt)
: waits for the event to occur (passive waiting). -
pthread_cond_signal(cv)
: signals the occurrence of the event for one locked thread. -
pthread_cond_broadcast(cv)
: signals the occurrence of the event for all locked threads. -
pthread_cond_init()
: initializes a condition variable. -
pthread_cond_destroy()
: destroys a condition variable. -
pthread_cond_t
: type of auxiliary structure.
Condition Variables Usage Pattern
- Problem of synchronization between threads without using active wait.
- Threads invoke
wait()
function, which only returns when the event is signaled. - All threads update global state (with mutex), and block.
- The last thread, which detects the event or condition, signals to all:
broadcast()
.
Barrier Example using Condition Variables
-
Barrier Pseudocode
:-
Mutex_lock(mutex)
. -
counter++
. - If
counter == thread_count
:counter = 0
,Cond_broadcast( cond_var)
. - Else:
while (...)
,cond_wait( cond_var, mutex)
. -
Mutex_unlock(mutex)
.
-
Producer-Consumer Example
- An example of producer-consumer synchronization problem is the unbounded buffer.
- Thread Producer generates items for a buffer (list) with limited size.
- Thread Consumer extracts items from that same list.
- Cases:
- Consumer can only execute when there is at least 1 item in the list.
- Producer can only execute if the list has space available for 1 more item (assuming limited size).
Producer-Consumer UnBounded Buffer Example (pseudo-code)
-
mutex_t mutex;
,cond_t notEmptyforConsumer;
,cond_t notFullforProducer;
. -
void* produce(void* item)
:-
mutex_lock( &mutex)
. -
while (count == buffer_size)
,pthread_cond_wait( &mutex, &notFullforProducer)
. -
buffer[lastptr] = item
. -
lastptr = lastptr + 1 % buffer_size
. -
count += 1
. -
pthread_cond_signal( &notEmptyforConsumer)
. -
mutex_unlock( &mutex)
.
-
-
void* consume()
:-
mutex_lock( &mutex)
. -
while (count == 0)
,pthread_cond_wait( &mutex, &notEmptyforConsumer)
. -
item = buffer[firstptr]
. -
firstptr = firstptr + 1 % buffer_size
. -
count -= 1
. -
pthread_cond_signal( &notFullforProducer)
. -
mutex_unlock( &mutex)
. -
return item
.
-
MPI Basics
- MPI (Message Passing Interface) is a library for parallel computing
- It provides a way to communicate between processes
Barrier Synchronization
- MPI_Barrier: blocks all processes until all processes in the comm group call the function
- Used for synchronization of processes
Broadcast
- MPI_Bcast: sends a message from one process to all processes in the comm group
- The root process sends the data, and all other processes receive it
- Parameters: buffer (memory address of data to send), count (number of data to send), datatype (type of data to send), root (rank of the process that sends the data), comm (communication group)
Scatter/Gather
- Sending from one process to all, and sending from all to one process
Reduce
- MPI_Reduce: collects a value from all processes, applies an aggregation function, and merges the result in the root process
- Parameters: sendbuf (memory pointer to data to be collected in all processes), recvbuf (memory pointer to the final aggregate value in the root process), count (number of items in the buffer), datatype (type of data to send), op (operation to apply to aggregate the results), root (rank of the process which will have the only global result), comm (communication group)
MPI Data Types
- MPI_Datatype: specifies the type of data to send or receive
MPI Count
- MPI_Get_count: returns the number of elements received by the last message
- Parameters: status (status of the receive operation), datatype (type of data of the message), count (number of elements received)
Message Exchange
- Sending a message: uses Send or Ssend function
- Receiving a message: uses Recv function
- Synchronization: Send can have synchronous or deferred sending behavior, Recv is always synchronous
- Ssend: always synchronous and blocking, ends when the message reaches the destination
Synchronization Model
- Sender/Receiver Symmetry: sending and receiving functions must be symmetrical in sender and receiver
- If not matched, the process may block
Collectives
- Motivation: need to exchange messages between all processes
- Optimized by the implementation of the library and the communication hardware
MPI Compilation and Execution
- Compile: use mpicc wrapper around the system compiler (typically gcc)
- Execute: use mpirun with -n option to specify the number of processes
Point-to-Point Communication
- Communication between processes happens by sending and receiving messages
- Each process executes a different part of the same code, through "if"s
- Each process is identified by its rank value
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Description
Develop a parallel program to increment a global variable N times, using more than one thread to increment the global counter variable, ensuring correctness and performance.