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#define _GNU_SOURCE
/* What a small commit's sync costs by where its head goes, on one device:
* a commit of SIZE bytes written over zeros written earlier (no new size or
* blocks, so no journal commit), 40 times per way. The ways run round-robin
* so device drift reaches all of them alike.
* probe DIRECTORY [SIZE] -> one JSON object per way (milliseconds). */
#include <fcntl.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <time.h>
#include <unistd.h>
static double now_ms(void) {
struct timespec t;
clock_gettime(CLOCK_MONOTONIC, &t);
return t.tv_sec * 1e3 + t.tv_nsec / 1e6;
}
static int order(const void *a, const void *b) {
double x = *(const double *)a, y = *(const double *)b;
return x < y ? -1 : x > y;
}
enum { WAYS = 3, ROUNDS = 40, HEAD = 128, DATA = 8192 };
static const char *names[WAYS] = {
"bytes, head in a slot at the start, fdatasync",
"head just before the bytes, fdatasync",
"bytes only, fdatasync",
};
int main(int argc, char **argv) {
const char *directory = argc > 1 ? argv[1] : "/var/tmp";
size_t size = argc > 2 ? (size_t)atoi(argv[2]) : 8192;
char path[WAYS][512];
int fd[WAYS];
off_t end[WAYS];
char *bytes = malloc(size), head[HEAD];
if (!bytes) return 1;
memset(bytes, 'r', size);
for (int w = 0; w < WAYS; ++w) {
snprintf(path[w], sizeof(path[w]), "%s/pure-record-probe-%d", directory, w);
fd[w] = open(path[w], O_RDWR | O_CREAT | O_TRUNC, 0600);
if (fd[w] < 0) return 1;
size_t zeros_size = DATA + (size_t)(ROUNDS + 2) * (size + HEAD);
char *zeros = calloc(1, zeros_size);
if (!zeros || pwrite(fd[w], zeros, zeros_size, 0) != (ssize_t)zeros_size) return 1;
free(zeros);
fsync(fd[w]);
end[w] = DATA;
}
double t[WAYS][ROUNDS];
for (int r = 0; r < ROUNDS; ++r)
for (int w = 0; w < WAYS; ++w) {
memset(head, 'h' + r % 2, sizeof(head));
double start = now_ms();
if (w == 1 && pwrite(fd[w], head, HEAD, end[w]) != HEAD) return 1;
off_t at = end[w] + (w == 1 ? HEAD : 0);
if (pwrite(fd[w], bytes, size, at) != (ssize_t)size) return 1;
end[w] = at + (off_t)size;
if (w == 0 && pwrite(fd[w], head, HEAD, (r % 2) * 4096) != HEAD) return 1;
fdatasync(fd[w]);
t[w][r] = now_ms() - start;
}
for (int w = 0; w < WAYS; ++w) {
qsort(t[w], ROUNDS, sizeof(double), order);
printf("{\"way\":\"%s\",\"bytes\":%zu,\"p50_ms\":%.2f,\"p90_ms\":%.2f,\"max_ms\":%.2f}\n", names[w], size,
t[w][ROUNDS / 2], t[w][ROUNDS * 9 / 10], t[w][ROUNDS - 1]);
close(fd[w]);
unlink(path[w]);
}
free(bytes);
return 0;
}
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