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update squiggle.c to avoid cache sharing

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This commit is contained in:
NunoSempere 2024-01-12 17:02:10 +01:00
parent b208879e45
commit 3a9a290ba8
3 changed files with 13 additions and 8 deletions
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2
README.md
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@ -25,7 +25,7 @@ The name of this repository is a pun on two meanings of "time to": "how much tim
| Language | Time | Lines of code |
|-----------------------------|-----------|---------------|
| C | 5.6ms | 252 |
| squiggle.c | 10.5ms | 29* |
| squiggle.c | 8.2ms | 29* |
| Nim | 40.8ms | 84 |
| Lua (LuaJIT) | 69.9ms | 82 |
| OCaml (flambda) | 187.9ms | 123 |

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squiggle.c/samples
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19
squiggle.c/squiggle_c/squiggle_more.c
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@ -11,10 +11,13 @@
/* Parallel sampler */
#define CACHE_LINE_SIZE 64
typedef struct seed_cache_box_t {
uint64_t* seed;
uint64_t seed;
char padding[CACHE_LINE_SIZE - sizeof(uint64_t*)];
} seed_cache_box;
// This avoid false sharing. Dealing with this shaves ~2ms.
// This avoids "false sharing", i.e., different threads competing for the same cache line
// It's possible dealing with this shaves ~2ms
// However, it's possible it doesn't, since pointers aren't changed, just their contents (and the location of their contents doesn't necessarily have to be close, since they are malloc'ed sepately)
// Still, I thought it was interesting
void sampler_parallel(double (*sampler)(uint64_t* seed), double* results, int n_threads, int n_samples)
{
@ -40,11 +43,10 @@ void sampler_parallel(double (*sampler)(uint64_t* seed), double* results, int n_
seed_cache_box* cache_box = (seed_cache_box*) malloc(sizeof(seed_cache_box) * (size_t)n_threads);
srand(1);
for (int i = 0; i < n_threads; i++) {
cache_box[i].seed = malloc(sizeof(uint64_t*));
// Constraints:
// - xorshift can't start with 0
// - the seeds should be reasonably separated and not correlated
*(cache_box[i].seed) = (uint64_t)rand() * (UINT64_MAX / RAND_MAX);
cache_box[i].seed = (uint64_t)rand() * (UINT64_MAX / RAND_MAX);
// printf("#%ld: %lu\n",i, *seeds[i]);
// Other initializations tried:
@ -58,21 +60,24 @@ void sampler_parallel(double (*sampler)(uint64_t* seed), double* results, int n_
{
#pragma omp for
for (i = 0; i < n_threads; i++) {
int quotient = n_samples / n_threads;
int lower_bound_inclusive = i * quotient;
int upper_bound_not_inclusive = ((i + 1) * quotient); // note the < in the for loop below,
for (int j = lower_bound_inclusive; j < upper_bound_not_inclusive; j++) {
results[j] = sampler(cache_box[i].seed);
results[j] = sampler(&(cache_box[i].seed));
// Could also result in inefficient cache stuff, but hopefully not too often
}
}
}
for (int j = divisor_multiple; j < n_samples; j++) {
results[j] = sampler(cache_box[0].seed);
results[j] = sampler(&(cache_box[0].seed));
// we can just reuse a seed, this isn't problematic because we are not doing multithreading
}
/*
for (int i = 0; i < n_threads; i++) {
free(cache_box[i].seed);
}
*/
free(cache_box);
}