(Optional) Do it yourself
OverviewTeaching: 0 min
Exercises: 0 minQuestions
What is the best way to write parallel code from serial?Objectives
Work on a second example, possibly your own research code.
Going from Serial to Parallel
Choose a working serial code to work on. It can be your own research code, or the example below. Choose a function or a compact region of the code and write a parallel version of it.
Follow the same steps as in lesson 9:
- Identify parallel and serial regions. Decide if the potential speedup is worth the effort.
- Decide how to split the data among a number of ranks. Think about how much data needs to be communicated and how often.
- Set up tests. Write a program that runs the function with different parameters and checks that it’s running correctly.
- Write the parallel code. When using MPI, this means writing the function that will run on a single rank and communicate with the others. If you can, split this into multiple parts for different kinds on input.
- Profile when the program runs correctly.
- Optimise, making sure the tests still succeed. Goto: 5.
Serial code for the Ising Model
The Ising model describes the behaviour of a ferromagnetic material. The material is formed of atoms arranged in a 2 dimensional square lattice. Each atom has a magnetic field pointing either up or down. The atoms only interact with their immediate neighbours. When neighbouring atoms point in different directions, they increase the energy of the system a bit. Similarly when they point to different directions, they decrease the energy by a bit.
At zero temperature the system would just fall to the state with lowest energy, but with a nonzero temperature it will sample states with different energies. Increasing energy is still alway less likely than decreasing it. Precisely, the probability of changing the direction of an atom is:
Where is the change in energy. The code runs multiple iterations of a loop over all atoms in the lattice and attempts to flip the direction of each one at a time. After enough iterations, the system should find a natural equilibrium for the given temperature.
The code reads parameters from a file called
parameter. It needs to contain two lines for two parameters,
iter. For example:
beta 0.5 iter 100
Start from a working serial code.
Write a parallel implementation for each function or parallel region.
Connect the parallel regions with a minimal amount of communication.
Continuously compare with the working serial code.