# HPC - "Rule 30" Cellular Automaton

Last updated: 2022-11-23

The goal of this exercise is to implement the Rule 30 Cellular Automaton in CUDA.

The Rule 30 CA is a 1D cellular aotmaton that consists of an array x[N] of $$N$$ integers that can be either 0 or 1. The state of the CA evolves at discrete time steps: the new state of a cell depends on its current state, and on the current state of the two neighbors. We assume cyclic boundary conditions, so that the neighbors of $$x[0]$$ are $$x[N-1]$$ and $$x[1]$$ and the neighbors of $$x[N-1]$$ are $$x[N-2]$$ and $$x[0]$$ (Figure 1).

Given the current values $$pqr$$ of three adjacent cells, the new value $$q'$$ of the middle cell is computed according to Table 1.

 Current configuration $$pqr$$ ■■■ ■■□ ■□■ ■□□ □■■ □■□ □□■ □□□ New state $$q'$$ of the central cell □ □ □ ■ ■ ■ ■ □

The sequence □□□■■■■□ = 00011110 on the second row is the binary representation of decimal 30, from which the name ("Rule 30 CA"); more details can be found here.

The file cuda-rule30.cu contains a serial program that computes the evolution of the Rule 30 CA, assuming an initial condition where all cells are 0 except the central one. The program accepts two optional command line parameters: the domain size $$N$$ and the number of steps nsteps to simulate. At the end, the program produces the image rule30.pbm shown in Figure 2 of size $$N \times \textit{nsteps}$$.

Each row of the image represents the state of the automaton at a specific time step (1 = black, 0 = white). Time moves from top to bottom: the first line is the initial state (time 0), the second line is the state at time 1, and so on.

Interestingly, the pattern shown in Figure 2 is similar to the pattern on the Conus textile shell, a highly poisonous marine mollusk which can be found in tropical seas (Figure 3).

The goal of this exercise is to write a parallel version where the computation of the new states are performed by CUDA threads. In particular, the rule30() function should be turned into a kernel. Assume that the domain size $$N$$ is a multiple of the number of threads per block (BLKDIM).

I suggest that you start with a version that does not use shared memory; this first version should be easily derived from the provided serial code.

Since each domain cell is read three times by three different threads within the same block, the computation might benefit from the use of shared memory.

Note: The use shared memory could produce minor improvements on modern GPUs, or even make the program slower. The reason is that there is little data reuse, and modern GPUs are equipped with caches that work reasonably well in these kind of computations. Nevertheless, it is useful to practice with shared memory, so this exercise should be considered as it is: an exercise.

To use shared memory, refer to the simple example of 1D stencil computation that we have seen during the class; in this case, the radius of the stencil is one, i.e., the new state of each cell depends on the state of a cell and the state of the two neighbors. Be careful, since in this exercise we are assuming a cyclic domain, whereas in the stencil computation discussed in the class we did not.

Looking at Figure 2, you might proceed as follows:

• d_cur[] is the current state on GPU memory.

• We create a kernel, say fill_ghost(...) that fills the ghost area of d_cur[]. The kernel will be executed by a single thread only, since just two values need to be copied, and therefore will be executed as fill_ghost<<<1, 1>>>(...)

• We create another kernel that computes the new state of the domain, given the current state. To this aim, we use 1D blocks and grid. Each block defined a __shared__ array buf[BLKDIM+2]; we need BLKDIM+2 elements since we need to include ghost cells in each partition in order to be able to compute the new states of all cells.

• Each thread computes the "local" index lindex in the buf[] array, and a "global" index gindex in the d_cur[] array, of the element it is associated with. Care should be taken, since both the local and global domains have ghost cells. Therefore, indices should be computed as:

  const int lindex = 1 + threadIdx.x;
const int gindex = 1 + threadIdx.x + blockIdx.x * blockDim.x;
• Each thread copies one element from global to shared memory:

  buf[lindex] = cur[gindex];
• The first thread of each block also fills the ghost area of the shared array buf[]:

  if (0 == threadIdx.x) {
buf[0] = cur[gindex-1];
buf[BLKDIM + 1] = cur[gindex + BLKDIM];
}

To generate the output image, the new domain should be transferred back to host memory after each iteration. Then, d_cur and d_next must be exchanged before starting the next iteration.

To compile:

    nvcc cuda-rule30.cu -o cuda-rule30

To execute:

    ./cuda-rule30 [width [steps]]

Example:

    ./cuda-rule30 1024 1024

The output is stored to the file cuda-rule30.pbm