1. Implementing Processes and Threads CS550 Operating Systems

2. pthread_create See last class's notes and check the man page. Recall: there are four parameters that you pass in. Each has a different meaning – The thread identifier within the function where the thread is created – The thread attributes – The thread name, and – The thread parameter.

3. pthread_exit The thread exits and returns the value passed into this function

4. pthread_join Waits for a thread to complete in the calling thread. This should be called for every thread that is created aside from the main thread.

5. Thread Memory Remember that threads share the same process memory. Variables declared outside the scope of the thread (outside the thread's curly brackets) are accessible (and shared between) within every thread. Variables declared inside the scope of the thread (within the curly brackets of the function representing the thread) are local to the thread.

6. Variables for Process X +-------------------------------------+ | static variables declared outside | | of main, and outside threads | | | +----------+ +-------+ +-------+ | | | main | |thread1| |thread2| | | | x: int | |y: int | |y: int | | | | y: int | +-------+ +-------+ | | +----------+ | +-------------------------------------+ Notice that there are 3 variables called y - one in main, thread1, and thread2, but there is only one copy of the static variables.

7. Variables for Process Y +-------------------------------------+ | static variables declared outside | | of main, and outside threads | | | +----------+ +-------+ +-------+ | | | main | |thread1| |thread2| | | | x: int | |y: int | |y: int | | | | y: int | +-------+ +-------+ | | +----------+ | +-------------------------------------+ Remember that processes do not share the same memory.

8. Processes X and Y The static variables for process X and process Y are different even if they have the same names. Processes do not share static or local variables, even if they were created using the same set of code.

9. Interprocess Communication As we mentioned before, interprocess communication can occur by many different means: shared memory, pipes, sockets, etc. These tools allow for data transfer between running programs (processes), but they are often not portable. Shared memory, for example, acts almost like a shared file. Processes can read and write data to and from the file.

10. Interprocess Communication Pipes may allow for multidirectional communication between processes on the same machine. Sockets may allow for multidirectional communication between processes on the same or networked machines. We will investigate these in lab assignments, but to reduce some of the complexity, we will look at MPI in depth MPI allows us to abstract away from reliance upon specific systems for shared memory and socket toolkits.

11. MPI Message Passing MPI - the message passing interface allows for communication (IPC) between running processes, even those using the same source code.

12. Using MPI Processes use MPI by using #include "mpi.h" or depending upon the system and MPI stack. MPI is started in a program using: MPI_Init(&argc, &argv); and ended with: MPI_Finalize(); These function almost like curly brackets to start and end the program.

13. Using MPI Anything between the MPI_Init and MPI_Finalize statements runs in as many processes that are requested by " mpirun " at the command line. For example, on littlefe : mpirun -np 12 -machinefile machines-openmpi prog1.exe Runs 12 processes using the executable code from prog1.exe.

14. Identifying Processes in MPI The MPI_Comm_rank and MPI_Comm_size functions get the rank (process identifier) and number of processes (the value 12 after -np, on the previous slide). These were previously reviewed in class.

15. MPI Message Passing Messages are passed in MPI using MPI_Send and MPI_Recv MPI_Send - sends a message of a given size with a given type to a process with a specific rank. MPI_Recv - receives a message of a maximum size with a given type from a process with a specific rank. MPI_COMM_WORLD - the "world" in which the processes exist. This is a constant.

16. Sending and Receiving Messages MPI_Send and MPI_Recv have the following parameters: MPI_Send( pointer to message, message size, message type, process rank to send to, message tag or id, MPI_COMM_WORLD) MPI_Recv( pointer to variable used to receive, maximum recv size, message type, process rank to receive from, message tag or id, MPI_COMM_WORLD, MPI_STATUS_IGNORE)