From 5e90a99dfdcc0a16f52213370cda29779571e36c Mon Sep 17 00:00:00 2001
From: eunhhu <necpition205@gmail.com>
Date: Sun, 27 Sep 2026 13:57:51 +0100
Subject: Measure whether sharding owners by key would add writes (084)

It would not, on this device. One file's syncs or four at once, the SD card
completes the same 510-690 commits per second, the latency growing with the
number of files. The scoreboard app with two owners, each with a store of
its own, took 23% fewer durable posts than one owner (median 16,230 against
20,974 a second) with 2-3x the p99: syncs queue, and each carries half the
events. One owner's group commit stays; cores serve reads, and writes grow
by making the sync itself cheaper.
---
 research/experiments/013-parallel-syncs/probe.c | 96 +++++++++++++++++++++++++
 1 file changed, 96 insertions(+)
 create mode 100644 research/experiments/013-parallel-syncs/probe.c

(limited to 'research/experiments/013-parallel-syncs/probe.c')

diff --git a/research/experiments/013-parallel-syncs/probe.c b/research/experiments/013-parallel-syncs/probe.c
new file mode 100644
index 0000000..d7f4974
--- /dev/null
+++ b/research/experiments/013-parallel-syncs/probe.c
@@ -0,0 +1,96 @@
+#define _GNU_SOURCE
+/* Whether one device syncs several files at once: THREADS threads, each with
+ * a file of its own, each commit SIZE bytes written over zeros written
+ * earlier (no new size or blocks, so no journal commit) and fdatasync, for
+ * SECONDS. Sharding apps by key gives each owner a store of its own, so this
+ * is what separate owners would get from the device.
+ * probe DIRECTORY THREADS SECONDS [SIZE] -> one JSON object. */
+#include <fcntl.h>
+#include <pthread.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+#include <time.h>
+#include <unistd.h>
+
+enum { MOST = 20000 };
+typedef struct {
+    int fd;
+    size_t size;
+    double until, times[MOST];
+    int count;
+} Syncer;
+
+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;
+}
+
+static void *commit_all(void *argument) {
+    Syncer *s = argument;
+    char *bytes = malloc(s->size);
+    memset(bytes, 'c', s->size);
+    off_t at = 0;
+    while (s->count < MOST && now_ms() < s->until) {
+        double start = now_ms();
+        if (pwrite(s->fd, bytes, s->size, at) != (ssize_t)s->size || fdatasync(s->fd)) break;
+        at += (off_t)s->size;
+        s->times[s->count++] = now_ms() - start;
+    }
+    free(bytes);
+    return NULL;
+}
+
+int main(int argc, char **argv) {
+    if (argc < 4) return 2;
+    const char *directory = argv[1];
+    int threads = atoi(argv[2]);
+    double seconds = atof(argv[3]);
+    size_t size = argc > 4 ? (size_t)atoi(argv[4]) : 4096;
+    if (threads < 1 || threads > 16) return 2;
+    Syncer *syncers = calloc((size_t)threads, sizeof(*syncers));
+    char paths[16][512];
+    size_t zeros_size = (size_t)MOST * size;
+    char *zeros = calloc(1, zeros_size);
+    for (int i = 0; i < threads; ++i) {
+        snprintf(paths[i], sizeof(paths[i]), "%s/pure-sync-probe-%d", directory, i);
+        syncers[i].fd = open(paths[i], O_RDWR | O_CREAT | O_TRUNC, 0600);
+        if (syncers[i].fd < 0 || pwrite(syncers[i].fd, zeros, zeros_size, 0) != (ssize_t)zeros_size) return 1;
+        fsync(syncers[i].fd);
+        syncers[i].size = size;
+    }
+    free(zeros);
+    pthread_t running[16];
+    double start = now_ms();
+    for (int i = 0; i < threads; ++i) {
+        syncers[i].until = start + seconds * 1e3;
+        pthread_create(&running[i], NULL, commit_all, &syncers[i]);
+    }
+    int total = 0;
+    for (int i = 0; i < threads; ++i) {
+        pthread_join(running[i], NULL);
+        total += syncers[i].count;
+    }
+    double elapsed = now_ms() - start;
+    double *all = malloc((size_t)total * sizeof(*all));
+    int n = 0;
+    for (int i = 0; i < threads; ++i) {
+        memcpy(all + n, syncers[i].times, (size_t)syncers[i].count * sizeof(double));
+        n += syncers[i].count;
+        close(syncers[i].fd);
+        unlink(paths[i]);
+    }
+    qsort(all, (size_t)n, sizeof(double), order);
+    printf("{\"threads\":%d,\"bytes\":%zu,\"commits\":%d,\"commits_per_second\":%.1f,\"p50_ms\":%.2f,\"p90_ms\":%.2f,"
+           "\"p99_ms\":%.2f}\n", threads, size, n, n / (elapsed / 1e3), n ? all[n / 2] : 0, n ? all[n * 9 / 10] : 0,
+           n ? all[n * 99 / 100] : 0);
+    free(all);
+    free(syncers);
+    return 0;
+}
-- 
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