Computer Science 400
Parallel Processing and High Performance Computing

Fall 2017, Siena College

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In this lab, we will begin to consider another major paradigm for parallelism: multithreading. So far, we have been working with message passing. In that case, each process has its own private address space. That is, each process has its own variables and any information to be exchanged among processes needs to be done with explicit message passing. Multithreading usually allows for the use of shared memory. Many operating systems provide support for threads, and a standard interface has been developed: POSIX Threads or pthreads.

You may work alone or with a partner on this lab.

Getting Set Up

You will receive an email with the link to follow to set up your GitHub repository pthreads-yourgitname for this Lab. One member of the group should follow the link to set up the repository on GitHub, then that person should email the instructor with the other group members' GitHub usernames so they can be granted access. This will allow all members of the group to clone the repository and commit and push changes to the origin on GitHub. At least one group member should make a clone of the repository to begin work.

Readings

Before starting, read through Pacheco Sections 4.1 and 4.2.

pthreads Basics

The basic idea is that we can create and destroy threads of execution in a program, on the fly, during its execution. These threads can then be executed in parallel by the operating system scheduler. If we have multiple processors, we should be able to achieve a speedup over the single-threaded equivalent.

We start with a look at a pthreads "Hello, world" program, which is in the pthreadhello directory of your repository for this lab.

The most basic functionality involves the creation and destruction of threads:

• pthread_create(3THR) - This creates a new thread. It takes 4 arguments. The first is a pointer to a variable of type pthread_t. Upon return, this contains a thread identifier that may be used later in a call to pthread_join(). The second is a pointer to a pthread_attr_t structure that specifies thread creation attributes. In the pthreadhello program, we pass in NULL, which will request the system default attributes. The third argument is a pointer to a function that will be called when the thread is started. This function must take a single parameter of type void * and return void *. The fourth parameter is the pointer that will be passed as the argument to the thread function.
• pthread_exit(3THR) - This causes the calling thread to exit. This is called implicitly if the thread function called during the thread creation returns. Its argument is a return status value, which can be retrieved by pthread_join().
• pthread_join(3THR) - This causes the calling thread to block (wait) until the thread with the identifier passed as the first argument to pthread_join() has exited. The second argument is a pointer to a location where the return status passed to pthread_exit() can be stored. In the pthreadhello program, we pass in NULL, and hence ignore the value.

Prototypes for pthread functions are in pthread.h and programs need to link with libpthread.a (use -lpthread at link time).

Any global variables in your program are accessible to all threads. Local variables are directly accessible only to the thread in which they were created, though the memory can be shared by passing a pointer as part of the last argument to pthread_create().

A slightly more interesting example is in proctree_threads.

This example builds a "tree" of threads to a depth given on the command line. It includes calls to pthread_self(). This function returns the thread identifier of the calling thread.

Try it out and study the code to make sure you understand how it works.

Multithreaded Pi

• Your program should take two command-line parameters: the number of threads and the number of random points to be generated by each of those threads.
• Store the number of points to generate in each thread in a global variable so your main function can set it, and your thread function can access it.
• Pass a pointer to an element of your array of per-thread "in circle" counts to each pthread_create call. That thread will be responsible for accumulating its own count in that variable. Then the main thread will have all of those values in its array when the threads have completed.

Submitting

Your submission requires that all required deliverables are committed and pushed to the master for your repository on GitHub.

Grading

This assignment is worth 35 points, which are distributed as follows: