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Support non power of two OMP_NUM_THREADS #2

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a87cd38
Support non power of two OMP_NUM_THREADS
hholst80 Mar 28, 2025
b8dba53
nice(2) to set privileged processs run level
hholst80 Mar 29, 2025
8f509ac
Allocate and align with PAGE_SIZE
hholst80 Mar 29, 2025
214dd9b
Add helpful out-of-band information to stderr
hholst80 Mar 29, 2025
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28 changes: 21 additions & 7 deletions main.c
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Original file line number Diff line number Diff line change
Expand Up @@ -7,6 +7,7 @@
// bandwidth as advertised by the intel specs: 23.8 GiB/s (http://goo.gl/r8Aab)

#include <assert.h>
#include <errno.h>
#include <math.h>
#ifdef WITH_OPENMP
#include <omp.h>
Expand All @@ -23,12 +24,18 @@
#define SAMPLES 5
#define TIMES 5
#define BYTES_PER_GB (1024*1024*1024LL)
#define SIZE (1*BYTES_PER_GB)
#define PAGE_SIZE (1<<12)
#define MAX_SIZE (1*BYTES_PER_GB)
#ifndef PAGE_SIZE
# define PAGE_SIZE (1<<12)
#endif
// NOTE(hholst): Use to set higher priority to avoid background or UI to interfere too much.
#define RENICE (-10)

static unsigned long long SIZE = MAX_SIZE;

// This must be at least 32 byte aligned to make some AVX instructions happy.
// Have PAGE_SIZE buffering so we don't have to do math for prefetching.
char array[SIZE + PAGE_SIZE] __attribute__((aligned (32)));
char array[MAX_SIZE] __attribute__((aligned (PAGE_SIZE)));

// Compute the bandwidth in GiB/s.
static inline double to_bw(size_t bytes, double secs) {
Expand Down Expand Up @@ -102,9 +109,14 @@ void timeit(void (*function)(void*, size_t), char* name) {

int main() {
memset(array, 0xFF, SIZE); // un-ZFOD the page.
* ((uint64_t *) &array[SIZE]) = 0;

// TODO(awreece) iopl(0) and cli/sti?
#ifdef RENICE
if (errno = 0, nice(RENICE) < 0 && errno != 0) {
perror("warning: failed to set process priority level");
}
#endif

fprintf(stderr, "# Single-core performance. Threads: 1\n\n");

timefun(read_memory_rep_lodsq);
timefun(read_memory_loop);
Expand All @@ -129,9 +141,11 @@ int main() {
timefun(write_memory_memset);

#ifdef WITH_OPENMP

fprintf(stderr, "\n# Multi-core performance. Threads: %i\n\n", omp_get_max_threads());
unsigned long long npages_per_thread = (MAX_SIZE / omp_get_max_threads()) / PAGE_SIZE;
SIZE = PAGE_SIZE * npages_per_thread * omp_get_max_threads();
// fprintf(stderr, "OMP SIZE: %llu\n", SIZE);
memset(array, 0xFF, SIZE); // un-ZFOD the page.
* ((uint64_t *) &array[SIZE]) = 0;

timefunp(read_memory_rep_lodsq);
timefunp(read_memory_loop);
Expand Down