C Review

Array / pointer duality

name of array is equivalent to pointer to first element of that array
pointer treated like an array whose first element is the variable the pointer points to

This is useful for splitting up problems into parts

so each processor can work on one part of the array

Threads

A thread is like a “virtual CPU”

Multiple threads execute within the same process

A process is an address space: in other words, a memory space
Therefore, multiple threads can access and modify the same data
Need to be careful!

Analogy: children playing with blocks

Children are the threads
Blocks are the data
Piles/structures of blocks are data structures
2 or more children working on a structure at the same time: bad
Children should wait their turn to work on a structure

Making threads wait their turn is called synchronization

Pthreads

Pthreads = “POSIX Threads”

A standard API for writing multithreaded programs on Unix

Also supported by Cygwin under windows.

Brief overview of API:

pthread_create: Parent thread calls to start a new child thread.
pthread_exit: Called by a thread when it is done.
pthread_join: Called by a parent thread to wait for a child thread to exit.

Important data type: pthread_t

Serves as an identifier for a thread.
pthread_self function: returns the thread identifier of the currently-executing thread

Each child thread executes a start function. The start function is analogous to the main function in a C/C++ program (or the main method in a Java program).

[Review synopsis of each API function.]

[Write a hello world using pthreads.]

Embarrassing Parallelism

Problems that are extremely easy to solve using parallel computation are called embarrassingly parallel.

Typical structure:

Main process divides up problem into equal sized chunks.
Main process starts a worker process to process each chunk. The workers do not communicate with each other.
Main process waits for workers to finish.
Main process combines worker solutions into a single global solution.

Example application: computing the sum of an array of 16 bit integers.

Question: how would you parallelize this computation?

Example pthreads program: hello_pthread,c

Lab 6

Parallelize a sequential program to compute the sum of an array of 16 bit integer values.