/****************************************************************************
*
* cuda-anneal.cu - ANNEAL cellular automaton
*
* Copyright (C) 2017--2022 by Moreno Marzolla
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* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
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/***
% HPC - ANNEAL cellular automaton
% Moreno Marzolla
% Last updated: 2022-11-24
The ANNEAL Callular Automaton (also known as _twisted majority rule_)
is a simple two-dimensional, binary CA defined on a grid of size $W
\times H$. Cyclic boundary conditions are assumed, so that each cell
has eight neighbors. Two cells are adjacent if they share a side or a
corner.
The automaton evolves at discrete time steps $t = 0, 1, \ldots$. The
state of a cell $x$ at time $t + 1$ depends on its state at time $t$,
and on the state of its neighbors at time $t$. Specifically, let $B_x$
be the number of cells in state 1 within the neighborhood of size $3
\times 3$ centered on $x$ (including $x$). Then, if $B_x = 4$ or $B_x
\geq 6$ the new state of $x$ is 1, otherwise the new state of $x$ is
0. See Figure 1 for some examples.
![Figure 1: Computation of the new state of the central cell of a
block of size $3 \times 3$](cuda-anneal1.svg)
To simulate synchrnonous, concurrent updates of all cells, two domains
must be used. The state of a cell is read from the "current" domain,
and new values are written to the "next" domain. "Current" and "next"
are exchanged at the end of each step.
The initial states are chosen at random with uniform
probability. Figure 2 shows the evolution of a grid of size $256
\times 256$ after 10, 100 and 1024 steps. We observe the emergence of
"blobs" that grow over time, with the exception of small "specks".
![Figure 2: Evolution of the _ANNEAL_ CA](anneal-demo.png)
I made a short YouTube video to show the evolution of the automaton
over time:
The program [cuda-anneal.cu](cuda-anneal.cu) computes the evolution of
the _ANNEAL_ CA after $K$ iterations. The final state is written to a
file. The goal of this exercise is to modify the program to use the
GPU to update the domain.
Some suggestions:
- Start writing a version that does _not_ use shared memory. Transform
the `copy_top_bottom()`, `copy_left_right()` and `step()` functions
into kernels. The size of the thread blocks that copy the sides of
the domain will be different from the size of the domain that
computes the evolution of the automaton (see the next points).
- Use a 1D array of threads to copy the ghost cells.
`copy_top_bottom()` requires $(W + 2)$ threads, and
`copy_left_right()` requires $(H + 2)$ threads.
- To compute new states, organize the threads in two-dimensional
blocks of size $32 \times 32$.
- In the `step()` kernel, each thread computes the new state of a cell
$(i, j)$. We are working with an extended domain with ghost
rows/columns, hence the "true" (non-ghost) cells are those with
coordinates $1 \leq i \leq H$, $1 \leq j \leq W$. Therefore, each
thread will compute $i, j$ as:
```C
const int i = 1 + threadIdx.y + blockIdx.y * blockDim.y;
const int j = 1 + threadIdx.x + blockIdx.x * blockDim.x;
```
Before making any computation, each thread must verify that $1 \leq i
\leq H$, $1 \leq j \leq W$, so that excess threads are
deactivated.
## Using local memory
This program _might_ benefit from the use of shared memory, since each
cell is read 9 times by 9 different threads in the same
block. However, you might observe no improvement on modern GPUs, since
they have on-board caches. In fact, you might actually observe that
using shared memory _decreases_ the performance of the computation,
since there is not enough data reuse to compensate for the overhead of
managing the shared memory. Despite this, it is a useful exercise to
use local memory anyway, to see how it can be done.
Let us assume that thead blocks have size $\mathit{BLKDIM} \times
\mathit{BLKDIM}$, where _BLKDIM = 32_ divides $W$ and $H$. Each block
uses a local buffer `buf[BLKDIM+2][BLKDIM+2]` which includes a ghost
area. The buffer is filled with data from the current domain, and the
new cell states are computed using the data from the buffer.
Each thread is mapped to cell $(gi, gj)$ in the global domain, and to
a copy of the same cell at coordinates $(li, lj)$ in the local
buffer. The coordinates can be computes as follows:
```C
const int gi = 1 + threadIdx.y + blockIdx.y * blockDim.y;
const int gj = 1 + threadIdx.x + blockIdx.x * blockDim.x;
const int li = 1 + threadIdx.y;
const int lj = 1 + threadIdx.x;
```
![Figure 3: Copying data from global to shared memory](cuda-anneal3.svg)
There are several ways to fill the ghost area, all of them rather
cumbersome and potentially inefficient. The solution proposed below is
one of them; other possibilities exist.
We use blocks of size $\mathit{BLKDIM} \times
\mathit{BLKDIM}$. Filling the central part of the local domain (i.e.,
everything excluding the ghost area) is done with a single instruction
executed by all threads:
```C
buf[li][lj] = *IDX(cur, ext_width, gi, gj);
```
where `ext_width = (W + 2)` is the width of the domain including the
ghost area.
![Figure 4: Active threads while filling the shared memory](cuda-anneal4.svg)
Filling the ghost area can be done as follows (see Figure 4):
1. The upper and lower rows are delegated to the threads of the first
row (i.e., those with $li = 1$);
2. The left and right columns are delegated to the threads of the
first column (i.e., those with $lj = 1$);
3. The corners are delegated to the top left thread with $(li, lj) =
(1, 1)$.
You might be tempted to collapse steps 1 and 2 into a single step that
is carried out, e.g., by the threads of the first row. This could
work, but it would be difficult to generalize the program to domains
whose sides $W, H$ are not multiple of _BLKDIM_.
In practice, you may use the following schema:
```C
if ( li == 1 ) {
"fill buf[0][lj] and buf[BLKDIM+1][lj]"
}
if ( lj == 1 ) {
"fill buf[li][0] and buf[li][BLKDIM+1]"
}
if ( li == 1 && lj == 1 ) {
"fill buf[0][0]"
"fill buf[0][BLKDIM+1]"
"fill buf[BLKDIM+1][0]"
"fill buf[BLKDIM+1][BLKDIM+1]"
}
```
Handling the ghost area is more difficult if $W, H$ are not multiple
of _BLKDIM_. Deactivating threads outside the domain is not enough:
you need to modify the code that fills the ghost area.
To compile without using shared memory:
nvcc cuda-anneal.cu -o cuda-anneal
To generate an image after every step:
nvcc -DDUMPALL cuda-anneal.cu -o cuda-anneal
You can make an AVI / MPEG-4 animation using:
ffmpeg -y -i "cuda-anneal-%06d.pbm" -vcodec mpeg4 cuda-anneal.avi
To compile with shared memory:
nvcc -DUSE_SHARED cuda-anneal.cu -o cuda-anneal-shared
To execute:
./cuda-anneal [steps [W [H]]]
Example:
./cuda-anneal 64
## References
- Tommaso Toffoli, Norman Margolus, _Cellular Automata Machines: a new
environment for modeling_, MIT Press, 1987, ISBN 9780262526319.
[PDF](https://people.csail.mit.edu/nhm/cam-book.pdf) from Norman
Margolus home page.
## Files
- [cuda-anneal.cu](cuda-anneal.cu)
- [hpc.h](hpc.h)
- [Animation of the ANNEAL CA on YouTube](https://youtu.be/TSHWSjICCxs)
***/
#include "hpc.h"
#include
#include
#include
typedef unsigned char cell_t;
/* The following function makes indexing of the 2D domain
easier. Instead of writing, e.g., grid[i*ext_width + j] you write
IDX(grid, ext_width, i, j) to get a pointer to grid[i][j]. This
function assumes that the size of the CA grid is
(ext_width*ext_height), where the first and last rows/columns are
ghost cells. */
cell_t* IDX(cell_t *grid, int ext_width, int i, int j)
{
return (grid + i*ext_width + j);
}
int d_min(int a, int b)
{
return (a**= 6 || nblack == 4);
}
}
}
/* Initialize the current grid `cur` with alive cells with density
`p`. */
void init( cell_t *cur, int ext_width, int ext_height, float p )
{
int i, j;
const int LEFT = 1;
const int RIGHT = ext_width - 2;
const int TOP = 1;
const int BOTTOM = ext_height - 2;
srand(1234); /* initialize PRND */
for (i=TOP; i <= BOTTOM; i++) {
for (j=LEFT; j <= RIGHT; j++) {
*IDX(cur, ext_width, i, j) = (((float)rand())/RAND_MAX < p);
}
}
}
/* Write `cur` to a PBM (Portable Bitmap) file whose name is derived
from the step number `stepno`. */
void write_pbm( cell_t *cur, int ext_width, int ext_height, int stepno )
{
int i, j;
char fname[128];
FILE *f;
const int LEFT = 1;
const int RIGHT = ext_width - 2;
const int TOP = 1;
const int BOTTOM = ext_height - 2;
snprintf(fname, sizeof(fname), "cuda-anneal-%06d.pbm", stepno);
if ((f = fopen(fname, "w")) == NULL) {
fprintf(stderr, "Cannot open %s for writing\n", fname);
exit(EXIT_FAILURE);
}
fprintf(f, "P1\n");
fprintf(f, "# produced by cuda-anneal.cu\n");
fprintf(f, "%d %d\n", ext_width-2, ext_height-2);
for (i=LEFT; i<=RIGHT; i++) {
for (j=TOP; j<=BOTTOM; j++) {
fprintf(f, "%d ", *IDX(cur, ext_width, i, j));
}
fprintf(f, "\n");
}
fclose(f);
}
int main( int argc, char* argv[] )
{
cell_t *cur, *next;
int s, nsteps = 64, width = 512, height = 512;
const int MAXN = 2048;
if ( argc > 4 ) {
fprintf(stderr, "Usage: %s [nsteps [W [H]]]\n", argv[0]);
return EXIT_FAILURE;
}
if ( argc > 1 ) {
nsteps = atoi(argv[1]);
}
if ( argc > 2 ) {
width = height = atoi(argv[2]);
}
if ( argc > 3 ) {
height = atoi(argv[3]);
}
if ( width > MAXN || height > MAXN ) { /* maximum image size is MAXN */
fprintf(stderr, "FATAL: the maximum allowed grid size is %d\n", MAXN);
return EXIT_FAILURE;
}
const int ext_width = width + 2;
const int ext_height = height + 2;
const size_t ext_size = ext_width * ext_height * sizeof(cell_t);
fprintf(stderr, "Anneal CA: steps=%d size=%d x %d\n", nsteps, width, height);
cur = (cell_t*)malloc(ext_size); assert(cur != NULL);
next = (cell_t*)malloc(ext_size); assert(next != NULL);
init(cur, ext_width, ext_height, 0.5);
const double tstart = hpc_gettime();
for (s=0; s**