Parallel Programming Challenge: Mutual Exclusion

Questions and Answers

What is the main goal of the parallel version of the program?

To use only one thread for simplicity

To test the correctness of the sequential version

To make the program run n times faster for n threads (correct)

To make the program run slower

What is the default visibility of variables that already exist before the parallel section in OpenMP?

Private

Local

Shared (correct)

Internal

What is a critical section in the context of parallel programming?

A section of code that can be accessed by only one thread

A section of code that is only used in the sequential version

A section of code that can be accessed by multiple threads simultaneously

A section of code that accesses a shared variable and must be protected from simultaneous access (correct)

What is the purpose of a mutual exclusion mechanism?

To ensure that only one thread can access a shared resource at a time Signup and view all the answers

What happens when a thread changes the value of a shared variable in OpenMP?

The new value is visible to all other threads. Signup and view all the answers

What is the purpose of the critical directive in OpenMP?

To protect a code section from simultaneous access by multiple threads. Signup and view all the answers

What happens when several threads try to access the same shared variable at the same time?

A race condition arises Signup and view all the answers

What is the main drawback of using the critical clause in OpenMP?

It creates a serialization of execution, losing the advantage of parallelism. Signup and view all the answers

Why is it necessary to use a mutual exclusion mechanism in the parallel version?

To prevent race conditions and ensure correctness Signup and view all the answers

What is the purpose of the reduction clause in OpenMP?

To accumulate local results from each thread into a shared global variable. Signup and view all the answers

What is the main challenge of parallel programming in this exercise?

Ensuring the correctness of the result Signup and view all the answers

What is the purpose of time measurement in the sequential version?

To compare the performance of the sequential and parallel versions Signup and view all the answers

What is the result of using the reduction clause in OpenMP?

The private values of each thread are accumulated into the shared global variable. Signup and view all the answers

What is the expected result of both the sequential and parallel versions?

The results will always be the same Signup and view all the answers

What happens to the private values of each thread when using the reduction clause in OpenMP?

They are accumulated into the shared global variable. Signup and view all the answers

What is the main advantage of using the reduction clause over the critical clause in OpenMP?

It allows parallel execution of the program, preserving the advantage of parallelism. Signup and view all the answers

What is the primary requirement for the Pthreads library to function?

A POSIX-compliant operating system Signup and view all the answers

What is the purpose of the pthread_create function?

To create a new thread and specify its attributes Signup and view all the answers

What is the purpose of the linker option -lpthread?

To link the Pthreads library to the program Signup and view all the answers

What is the data type of the thread_p parameter in the pthread_create function?

pthread_t Signup and view all the answers

What is the purpose of the pthread_join function?

To join a thread with its parent thread Signup and view all the answers

What is the purpose of the start_routine function in the pthread_create function?

To execute the thread's code Signup and view all the answers

What is the purpose of the arg_p parameter in the pthread_create function?

To pass an argument to the thread's start routine Signup and view all the answers

What is the purpose of the global variable sum in the example incremental application?

To store the global sum value Signup and view all the answers

What is the primary issue with using active waiting to synchronize access to a global state?

It wastes CPU cycles and reduces overall system performance Signup and view all the answers

What is the main purpose of a condition variable?

To allow threads to wait for a certain event or condition to occur Signup and view all the answers

Why is it necessary to associate a condition variable with a mutual exclusion mechanism?

To ensure that only one thread can signal the condition variable at a time Signup and view all the answers

What is the main advantage of using condition variables over active waiting?

Improved system performance and reduced CPU waste Signup and view all the answers

What is the key restriction in the Reader-Writer problem?

No writer can access the data while readers are accessing it. Signup and view all the answers

What is the primary benefit of using barrier synchronization?

Ensuring that all threads reach a certain point before proceeding Signup and view all the answers

What is the main challenge addressed by condition variables in the context of threads and global state?

Synchronizing access to a global state without active waiting or blocking threads Signup and view all the answers

In the Producer-Consumer problem, what happens to the produced resources?

They are stored in a shared buffer. Signup and view all the answers

What is the function of the barrier() function in Barrier Synchronization?

It blocks the execution of a thread until all threads are in this position. Signup and view all the answers

What is the condition for a Consumer thread to consume resources in the Producer-Consumer problem?

Only resources already produced can be consumed. Signup and view all the answers

What is the purpose of a mutual exclusion mechanism?

To guarantee that at no time does more than one thread execute code that manipulates the same resource. Signup and view all the answers

What is a typical case of synchronization?

Barrier Signup and view all the answers

What happens when all threads execute the barrier() function?

The function terminates for all threads. Signup and view all the answers

What is the purpose of the Reader-Writer problem?

To synchronize access to shared resources. Signup and view all the answers

What is the purpose of the cond_broadcast() function in barrier synchronization?

To signal all threads waiting on a condition variable Signup and view all the answers

In the producer-consumer problem, what is the condition for the consumer thread to execute?

The buffer has at least one item Signup and view all the answers

What is the purpose of the while loop in the producer thread?

To wait for space to become available in the buffer Signup and view all the answers

What is the purpose of the mutex_lock() function in the producer-consumer problem?

To protect the critical section of code from concurrent access Signup and view all the answers

What is the purpose of the cond_wait() function in the producer-consumer problem?

To wait for space to become available in the buffer Signup and view all the answers

What is the benefit of using condition variables in the producer-consumer problem?

It allows threads to wait efficiently for a condition to occur Signup and view all the answers

What is the purpose of the barrier synchronization?

To ensure that all threads reach a certain point before proceeding Signup and view all the answers

What is the problem with active waiting in the producer-consumer problem?

It is inefficient and wasteful of CPU resources Signup and view all the answers

What is the primary issue with using active waiting to synchronize access to a global state?

It wastes CPU cycles and is inefficient Signup and view all the answers

When is a thread woken up from a condition variable?

When another thread signals the condition variable Signup and view all the answers

What is the main advantage of using condition variables over active waiting?

It is more efficient and does not waste CPU cycles Signup and view all the answers

What is necessary to associate with a condition variable?

A mutex Signup and view all the answers

Why is active waiting not suitable for synchronizing access to a global state?

Because it wastes CPU cycles and is inefficient Signup and view all the answers

What motivates the use of condition variables in parallel programming?

To avoid busy waiting and improve responsiveness Signup and view all the answers

What is the primary issue with using active waiting to synchronize access to a global state?

It wastes CPU resources Signup and view all the answers

What is the main purpose of using condition variables in the context of threads and global state?

To synchronize access to shared variables Signup and view all the answers

What is the key difference between pthread_cond_signal and pthread_cond_broadcast?

pthread_cond_signal signals only one thread, while pthread_cond_broadcast signals all threads Signup and view all the answers

What is the main advantage of using condition variables over active waiting?

Condition variables reduce CPU usage Signup and view all the answers

What is the purpose of the pthread_cond_wait function?

To unlock a mutex and wait for a signal Signup and view all the answers

Why is it necessary to associate a condition variable with a mutual exclusion mechanism?

To ensure thread safety Signup and view all the answers

What is the primary purpose of a barrier in parallel programming?

To synchronize threads that reach a common point Signup and view all the answers

What is active waiting in the context of parallel programming?

A thread continuously checking a condition Signup and view all the answers

What is the purpose of a condition variable in the context of threads and global state?

To suspend thread execution until a certain event occurs Signup and view all the answers

What is the role of the cond_wait function in the producer-consumer problem?

To block the thread until a resource becomes available Signup and view all the answers

What is the difference between active waiting and passive waiting?

Active waiting uses a loop, while passive waiting uses a condition variable Signup and view all the answers

What is the benefit of using condition variables in the producer-consumer problem?

It avoids busy waiting and improves responsiveness Signup and view all the answers

What is the purpose of the mutex_lock function in the producer-consumer problem?

To acquire the lock on a shared resource Signup and view all the answers

What is the consequence of using active waiting in the producer-consumer problem?

It causes the thread to waste CPU cycles Signup and view all the answers

What is the primary function of MPI_Barrier?

To synchronize all processes in a communication group Signup and view all the answers

What is the purpose of the root parameter in MPI_Bcast?

To indicate the rank of the process that sends the data Signup and view all the answers

What happens when all processes invoke MPI_Barrier?

All processes are blocked until all processes in the comm group call the function Signup and view all the answers

What is the purpose of the sendbuf parameter in MPI_Reduce?

To specify the memory pointer to the data to be collected in all processes Signup and view all the answers

What is the role of the recvbuf parameter in MPI_Reduce?

To specify the memory pointer to the final aggregate value (in the root process) Signup and view all the answers

What is the purpose of the op parameter in MPI_Reduce?

To define the operation to apply to the aggregated results Signup and view all the answers

What happens when a process calls MPI_Bcast?

The process sends data to all processes, including itself Signup and view all the answers

What is the primary difference between MPI_Bcast and MPI_Reduce?

MPI_Bcast sends data from one process to all, while MPI_Reduce collects data from all processes to one Signup and view all the answers

What is the primary difference between the MPI_Send function and the MPI_Ssend function?

MPI_Send is always asynchronous and non-blocking, whereas MPI_Ssend is synchronous and blocking Signup and view all the answers

What is the purpose of the MPI_Get_count function?

To get the number of elements received in a message Signup and view all the answers

What is a requirement for successful message communication in MPI?

The sender and receiver must use the same datatype Signup and view all the answers

What is the purpose of collective operations in MPI?

To exchange messages between all processes in a communicator Signup and view all the answers

What is the behavior of the MPI_Recv function?

It blocks until the message is received Signup and view all the answers

What is the purpose of sender/receiver symmetry in MPI?

To ensure that the sender and receiver have matching parameters Signup and view all the answers

What is the difference between MPI_Send and MPI_Ssend in terms of error handling?

MPI_Send does not return an error code, whereas MPI_Ssend does Signup and view all the answers

What is the purpose of the MPI_Barrier function?

To synchronize all processes in a communicator Signup and view all the answers

What is the purpose of the 'tag' parameter in the MPI_Send function?

To distinguish message channels Signup and view all the answers

What is the purpose of the 'status' parameter in the MPI_Recv function?

To retrieve the status of the receive operation Signup and view all the answers

What is the purpose of the MPI_Barrier function?

To synchronize all processes in a communicator Signup and view all the answers

What is the purpose of the MPI_Bcast function?

To send data from one process to all others Signup and view all the answers

What is the purpose of the MPI_Reduce function?

To combine data from all processes Signup and view all the answers

What is the difference between MPI_Send and MPI_Ssend?

MPI_Ssend is used for synchronous communication Signup and view all the answers

What happens when a process calls MPI_Barrier?

The process waits until all processes in the communicator have reached the barrier Signup and view all the answers

What is the purpose of the 'count' parameter in the MPI_Send function?

To specify the number of elements in the message Signup and view all the answers

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 and my_first_i based on n and thread_count
  - Perform printf statement to display thread range
  - Perform a for loop to increment the sum value

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.