From a8e2d0849e2a40f8bec14cd1b7ff170c17f772c3 Mon Sep 17 00:00:00 2001
From: antirez <antirez@gmail.com>
Date: Mon, 18 Jul 2016 13:49:31 +0200
Subject: [PATCH] LFU: Initial naive eviction cycle.

It is possible to get better results by using the pool like in the LRU
case. Also from tests during the morning I believe the current
implementation has issues in the frequency decay function that should
decrease the counter at periodic intervals.
---
 src/db.c     |  9 ++++++++-
 src/evict.c  | 36 +++++++++++++++++++++++++++++++++---
 src/object.c |  8 ++++++++
 3 files changed, 49 insertions(+), 4 deletions(-)

diff --git a/src/db.c b/src/db.c
index b00227d81..d33c810b3 100644
--- a/src/db.c
+++ b/src/db.c
@@ -175,7 +175,14 @@ void dbOverwrite(redisDb *db, robj *key, robj *val) {
     dictEntry *de = dictFind(db->dict,key->ptr);
 
     serverAssertWithInfo(NULL,key,de != NULL);
-    dictReplace(db->dict, key->ptr, val);
+    if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
+        robj *old = dictGetVal(de);
+        int saved_lru = old->lru;
+        dictReplace(db->dict, key->ptr, val);
+        val->lru = saved_lru;
+    } else {
+        dictReplace(db->dict, key->ptr, val);
+    }
 }
 
 /* High level Set operation. This function can be used in order to set
diff --git a/src/evict.c b/src/evict.c
index 753a2ac81..b48892b41 100644
--- a/src/evict.c
+++ b/src/evict.c
@@ -294,6 +294,7 @@ unsigned long LFUDecrAndReturn(robj *o) {
     if (LFUTimeElapsed(ldt) > LFU_DECR_INTERVAL && counter) {
         if (counter > LFU_INIT_VAL*2) {
             counter /= 2;
+            if (counter < LFU_INIT_VAL*2) counter = LFU_INIT_VAL*2;
         } else {
             counter--;
         }
@@ -360,9 +361,7 @@ int freeMemoryIfNeeded(void) {
         dict *dict;
         dictEntry *de;
 
-        if (server.maxmemory_policy == MAXMEMORY_ALLKEYS_LRU ||
-            server.maxmemory_policy == MAXMEMORY_VOLATILE_LRU)
-        {
+        if (server.maxmemory_policy & MAXMEMORY_FLAG_LRU) {
             struct evictionPoolEntry *pool = EvictionPoolLRU;
 
             while(bestkey == NULL) {
@@ -470,6 +469,37 @@ int freeMemoryIfNeeded(void) {
             }
         }
 
+        /* allkeys-lfu and volatile-lfu */
+        else if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
+            long bestfreq = 0; /* Initialized to avoid warning. */
+
+            for (i = 0; i < server.dbnum; i++) {
+                j = (++next_db) % server.dbnum;
+                db = server.db+j;
+                dict = (server.maxmemory_policy == MAXMEMORY_ALLKEYS_LFU) ?
+                        db->dict : db->expires;
+                if (dictSize(dict) != 0) {
+                    for (k = 0; k < server.maxmemory_samples; k++) {
+                        sds thiskey;
+                        long thisfreq;
+
+                        de = dictGetRandomKey(dict);
+                        thiskey = dictGetKey(de);
+                        robj *o = dictFetchValue(db->dict,thiskey);
+                        thisfreq = LFUDecrAndReturn(o);
+
+                        /* Keys with a smaller access frequency are
+                         * better candidates for deletion */
+                        if (bestkey == NULL || thisfreq < bestfreq) {
+                            bestkey = thiskey;
+                            bestfreq = thisfreq;
+                            bestdbid = j;
+                        }
+                    }
+                }
+            }
+        }
+
         /* Finally remove the selected key. */
         if (bestkey) {
             db = server.db+bestdbid;
diff --git a/src/object.c b/src/object.c
index 8f19f60f2..c64d810b0 100644
--- a/src/object.c
+++ b/src/object.c
@@ -722,10 +722,18 @@ void objectCommand(client *c) {
     } else if (!strcasecmp(c->argv[1]->ptr,"idletime") && c->argc == 3) {
         if ((o = objectCommandLookupOrReply(c,c->argv[2],shared.nullbulk))
                 == NULL) return;
+        if (server.maxmemory_policy & MAXMEMORY_FLAG_LFU) {
+            addReplyError(c,"An LFU maxmemory policy is selected, idle time not tracked. Please note that when switching between policies at runtime LRU and LFU data will take some time to adjust.");
+            return;
+        }
         addReplyLongLong(c,estimateObjectIdleTime(o)/1000);
     } else if (!strcasecmp(c->argv[1]->ptr,"freq") && c->argc == 3) {
         if ((o = objectCommandLookupOrReply(c,c->argv[2],shared.nullbulk))
                 == NULL) return;
+        if (server.maxmemory_policy & MAXMEMORY_FLAG_LRU) {
+            addReplyError(c,"An LRU maxmemory policy is selected, access frequency not tracked. Please note that when switching between policies at runtime LRU and LFU data will take some time to adjust.");
+            return;
+        }
         addReplyLongLong(c,o->lru&255);
     } else {
         addReplyError(c,"Syntax error. Try OBJECT (refcount|encoding|idletime|freq)");