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-rw-r--r--utils/lru/lfu-simulation.c158
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;
+}
+