1 files changed, 158 insertions, 0 deletions
diff --git a/utils/lru/lfu-simulation.c b/utils/lru/lfu-simulation.c
new file mode 100644
index 0000000..60105e5
--- /dev/null
+++ b/utils/lru/lfu-simulation.c
@@ -0,0 +1,158 @@
+#include <stdio.h>
+#include <time.h>
+#include <stdint.h>
+#include <stdlib.h>
+
+int decr_every = 1;
+int keyspace_size = 1000000;
+time_t switch_after = 30; /* Switch access pattern after N seconds. */
+
+struct entry {
+    /* Field that the LFU Redis implementation will have (we have
+     * 24 bits of total space in the object->lru field). */
+    uint8_t counter;    /* Logarithmic counter. */
+    uint16_t decrtime;  /* (Reduced precision) time of last decrement. */
+
+    /* Fields only useful for visualization. */
+    uint64_t hits;      /* Number of real accesses. */
+    time_t ctime;       /* Key creation time. */
+};
+
+#define to_16bit_minutes(x) ((x/60) & 65535)
+#define LFU_INIT_VAL 5
+
+/* Compute the difference in minutes between two 16 bit minutes times
+ * obtained with to_16bit_minutes(). Since they can wrap around if
+ * we detect the overflow we account for it as if the counter wrapped
+ * a single time. */
+uint16_t minutes_diff(uint16_t now, uint16_t prev) {
+    if (now >= prev) return now-prev;
+    return 65535-prev+now;
+}
+
+/* Increment a counter logarithmically: the greatest is its value, the
+ * less likely is that the counter is really incremented.
+ * The maximum value of the counter is saturated at 255. */
+uint8_t log_incr(uint8_t counter) {
+    if (counter == 255) return counter;
+    double r = (double)rand()/RAND_MAX;
+    double baseval = counter-LFU_INIT_VAL;
+    if (baseval < 0) baseval = 0;
+    double limit = 1.0/(baseval*10+1);
+    if (r < limit) counter++;
+    return counter;
+}
+
+/* Simulate an access to an entry. */
+void access_entry(struct entry *e) {
+    e->counter = log_incr(e->counter);
+    e->hits++;
+}
+
+/* Return the entry LFU value and as a side effect decrement the
+ * entry value if the decrement time was reached. */
+uint8_t scan_entry(struct entry *e) {
+    if (minutes_diff(to_16bit_minutes(time(NULL)),e->decrtime)
+        >= decr_every)
+    {
+        if (e->counter) {
+            if (e->counter > LFU_INIT_VAL*2) {
+                e->counter /= 2;
+            } else {
+                e->counter--;
+            }
+        }
+        e->decrtime = to_16bit_minutes(time(NULL));
+    }
+    return e->counter;
+}
+
+/* Print the entry info. */
+void show_entry(long pos, struct entry *e) {
+    char *tag = "normal       ";
+
+    if (pos >= 10 && pos <= 14) tag = "new no access";
+    if (pos >= 15 && pos <= 19) tag = "new accessed ";
+    if (pos >= keyspace_size -5) tag= "old no access";
+
+    printf("%ld] <%s> frequency:%d decrtime:%d [%lu hits | age:%ld sec]\n",
+        pos, tag, e->counter, e->decrtime, (unsigned long)e->hits,
+            time(NULL) - e->ctime);
+}
+
+int main(void) {
+    time_t start = time(NULL);
+    time_t new_entry_time = start;
+    time_t display_time = start;
+    struct entry *entries = malloc(sizeof(*entries)*keyspace_size);
+    long j;
+
+    /* Initialize. */
+    for (j = 0; j < keyspace_size; j++) {
+        entries[j].counter = LFU_INIT_VAL;
+        entries[j].decrtime = to_16bit_minutes(start);
+        entries[j].hits = 0;
+        entries[j].ctime = time(NULL);
+    }
+
+    while(1) {
+        time_t now = time(NULL);
+        long idx;
+
+        /* Scan N random entries (simulates the eviction under maxmemory). */
+        for (j = 0; j < 3; j++) {
+            scan_entry(entries+(rand()%keyspace_size));
+        }
+
+        /* Access a random entry: use a power-law access pattern up to
+         * 'switch_after' seconds. Then revert to flat access pattern. */
+        if (now-start < switch_after) {
+            /* Power law. */
+            idx = 1;
+            while((rand() % 21) != 0 && idx < keyspace_size) idx *= 2;
+            if (idx > keyspace_size) idx = keyspace_size;
+            idx = rand() % idx;
+        } else {
+            /* Flat. */
+            idx = rand() % keyspace_size;
+        }
+
+        /* Never access entries between position 10 and 14, so that
+         * we simulate what happens to new entries that are never
+         * accessed VS new entries which are accessed in positions
+         * 15-19.
+         *
+         * Also never access last 5 entry, so that we have keys which
+         * are never recreated (old), and never accessed. */
+        if ((idx < 10 || idx > 14) && (idx < keyspace_size-5))
+            access_entry(entries+idx);
+
+        /* Simulate the addition of new entries at positions between
+         * 10 and 19, a random one every 10 seconds. */
+        if (new_entry_time <= now) {
+            idx = 10+(rand()%10);
+            entries[idx].counter = LFU_INIT_VAL;
+            entries[idx].decrtime = to_16bit_minutes(time(NULL));
+            entries[idx].hits = 0;
+            entries[idx].ctime = time(NULL);
+            new_entry_time = now+10;
+        }
+
+        /* Show the first 20 entries and the last 20 entries. */
+        if (display_time != now) {
+            printf("=============================\n");
+            printf("Current minutes time: %d\n", (int)to_16bit_minutes(now));
+            printf("Access method: %s\n",
+                (now-start < switch_after) ? "power-law" : "flat");
+
+            for (j = 0; j < 20; j++)
+                show_entry(j,entries+j);
+
+            for (j = keyspace_size-20; j < keyspace_size; j++)
+                show_entry(j,entries+j);
+            display_time = now;
+        }
+    }
+    return 0;
+}
+