From ce077a6d2c2c06349d44b69cab341d3c650eec1b Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marek=20Vavru=C5=A1a?= <marek.vavrusa@nic.cz>
Date: Mon, 13 Jul 2015 18:22:50 +0200
Subject: [PATCH] generic/lru: count updates in slot for item-slot stickiness
---
lib/generic/lru.h | 20 ++++++++++++++++----
1 file changed, 16 insertions(+), 4 deletions(-)
diff --git a/lib/generic/lru.h b/lib/generic/lru.h
index b9ab19ac1..572b4f1ed 100644
--- a/lib/generic/lru.h
+++ b/lib/generic/lru.h
@@ -17,8 +17,10 @@
* @file lru.h
* @brief LRU-like cache.
*
- * @note This is a naive LRU implementation, if value exists it is treated as old
- * if the key collides. This may be improved with double hashing or hopscotch.
+ * @note This is a naive LRU implementation with a simple slot stickiness counting.
+ * Each write access increases stickiness on success, and decreases on collision.
+ * A slot is freed if the stickiness decreases to zero. This makes it less likely,
+ * that often-updated entries are jousted out of cache.
*
* # Example usage:
*
@@ -48,7 +50,9 @@
* }
* char *enemies[] = {"goro", "raiden", "subzero", "scorpion"};
* for (int i = 0; i < 4; ++i) {
- * *lru_set(&lru, enemies[i], strlen(enemies[i])) = i;
+ * int *val = lru_set(&lru, enemies[i], strlen(enemies[i]));
+ * if (val)
+ * *val = i;
* }
*
* // We're done
@@ -68,6 +72,7 @@
#define lru_slot_struct \
char *key; /**< Slot key */ \
uint32_t len; /**< Slot length */ \
+ uint32_t refs; /**< Slot importance (#writes - #collisions) */ \
/** @brief Slot header. */
struct lru_slot {
lru_slot_struct
@@ -142,8 +147,14 @@ static inline void *lru_slot_set(struct lru_hash_base *lru, const char *key, uin
}
uint32_t id = hash(key, len) % lru->size;
struct lru_slot *slot = lru_slot_at(lru, id);
- if (!lru_slot_match(slot, key, len)) {
+ if (lru_slot_match(slot, key, len)) {
+ slot->refs += 1; /* Increase slot significance */
+ } else {
if (slot->key) {
+ slot->refs -= 1; /* Decrease slot significance */
+ if (slot->refs > 0) {
+ return NULL; /* Couldn't joust former key. */
+ }
lru->evictions += 1;
free(slot->key);
if (lru->evict) {
@@ -157,6 +168,7 @@ static inline void *lru_slot_set(struct lru_hash_base *lru, const char *key, uin
}
memcpy(slot->key, key, len);
slot->len = len;
+ slot->refs = 1;
}
return lru_slot_val(slot, offset);
}
--
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