Tracking: first conversion from hashing to key names.

src/server.c

@@ -4221,7 +4221,7 @@ sds genRedisInfoString(const char *section) {
             "active_defrag_misses:%lld\r\n"
             "active_defrag_key_hits:%lld\r\n"
             "active_defrag_key_misses:%lld\r\n"
-            "tracking_used_slots:%lld\r\n",
+            "tracking_tracked_keys:%lld\r\n",
             server.stat_numconnections,
             server.stat_numcommands,
             getInstantaneousMetric(STATS_METRIC_COMMAND),
@@ -4249,7 +4249,7 @@ sds genRedisInfoString(const char *section) {
             server.stat_active_defrag_misses,
             server.stat_active_defrag_key_hits,
             server.stat_active_defrag_key_misses,
-            trackingGetUsedSlots());
+            (unsigned long long) trackingGetTotalItems());
     }
 
     /* Replication */

src/server.h

@@ -1654,7 +1654,7 @@ void trackingRememberKeys(client *c);
 void trackingInvalidateKey(robj *keyobj);
 void trackingInvalidateKeysOnFlush(int dbid);
 void trackingLimitUsedSlots(void);
-unsigned long long trackingGetUsedSlots(void);
+uint64_t trackingGetTotalItems(void);
 
 /* List data type */
 void listTypeTryConversion(robj *subject, robj *value);

src/tracking.c

@@ -30,37 +30,22 @@
 
 #include "server.h"
 
-/* The tracking table is constituted by 2^24 radix trees (each tree, and the
- * table itself, are allocated in a lazy way only when needed) tracking
- * clients that may have certain keys in their local, client side, cache.
- *
- * Keys are grouped into 2^24 slots, in a way similar to Redis Cluster hash
- * slots, however here the function we use is crc64, taking the least
- * significant 24 bits of the output.
+/* The tracking table is constituted by a radix tree of keys, each pointing
+ * to a radix tree of client IDs, used to track the clients that may have
+ * certain keys in their local, client side, cache.
  *
  * When a client enables tracking with "CLIENT TRACKING on", each key served to
- * the client is hashed to one of such slots, and Redis will remember what
- * client may have keys about such slot. Later, when a key in a given slot is
- * modified, all the clients that may have local copies of keys in that slot
- * will receive an invalidation message. There is no distinction of database
- * number: a single table is used.
+ * the client is remembered in the table mapping the keys to the client IDs.
+ * Later, when a key is modified, all the clients that may have local copy
+ * of such key will receive an invalidation message.
  *
  * Clients will normally take frequently requested objects in memory, removing
- * them when invalidation messages are received. A strategy clients may use is
- * to just cache objects in a dictionary, associating to each cached object
- * some incremental epoch, or just a timestamp. When invalidation messages are
- * received clients may store, in a different table, the timestamp (or epoch)
- * of the invalidation of such given slot: later when accessing objects, the
- * eviction of stale objects may be performed in a lazy way by checking if the
- * cached object timestamp is older than the invalidation timestamp for such
- * objects.
- *
- * The output of the 24 bit hash function is very large (more than 16 million
- * possible slots), so clients that may want to use less resources may only
- * use the most significant bits instead of the full 24 bits. */
-#define TRACKING_TABLE_SIZE (1<<24)
-rax **TrackingTable = NULL;
-unsigned long TrackingTableUsedSlots = 0;
+ * them when invalidation messages are received. */
+rax *TrackingTable = NULL;
+uint64_t TrackingTableTotalItems = 0; /* Total number of IDs stored across
+                                         the whole tracking table. This givesn
+                                         an hint about the total memory we
+                                         are using server side for CSC. */
 robj *TrackingChannelName;
 
 /* Remove the tracking state from the client 'c'. Note that there is not much
@@ -90,7 +75,7 @@ void enableTracking(client *c, uint64_t redirect_to) {
     c->client_tracking_redirection = redirect_to;
     server.tracking_clients++;
     if (TrackingTable == NULL) {
-        TrackingTable = zcalloc(sizeof(rax*) * TRACKING_TABLE_SIZE);
+        TrackingTable = raxNew();
         TrackingChannelName = createStringObject("__redis__:invalidate",20);
     }
 }
@@ -108,19 +93,20 @@ void trackingRememberKeys(client *c) {
     for(int j = 0; j < numkeys; j++) {
         int idx = keys[j];
         sds sdskey = c->argv[idx]->ptr;
-        uint64_t hash = crc64(0,
-            (unsigned char*)sdskey,sdslen(sdskey))&(TRACKING_TABLE_SIZE-1);
-        if (TrackingTable[hash] == NULL) {
-            TrackingTable[hash] = raxNew();
-            TrackingTableUsedSlots++;
+        rax *ids = raxFind(TrackingTable,(unsigned char*)sdskey,sdslen(sdskey));
+        if (ids == raxNotFound) {
+            ids = raxNew();
+            int inserted = raxTryInsert(TrackingTable,(unsigned char*)sdskey,
+                                        sdslen(sdskey),ids, NULL);
+            serverAssert(inserted == 1);
         }
-        raxTryInsert(TrackingTable[hash],
-            (unsigned char*)&c->id,sizeof(c->id),NULL,NULL);
+        if (raxTryInsert(ids,(unsigned char*)&c->id,sizeof(c->id),NULL,NULL))
+            TrackingTableTotalItems++;
     }
     getKeysFreeResult(keys);
 }
 
-void sendTrackingMessage(client *c, long long hash) {
+void sendTrackingMessage(client *c, char *keyname, size_t keylen) {
     int using_redirection = 0;
     if (c->client_tracking_redirection) {
         client *redir = lookupClientByID(c->client_tracking_redirection);
@@ -146,49 +132,44 @@ void sendTrackingMessage(client *c, long long hash) {
     if (c->resp > 2) {
         addReplyPushLen(c,2);
         addReplyBulkCBuffer(c,"invalidate",10);
-        addReplyLongLong(c,hash);
+        addReplyBulkCBuffer(c,keyname,keylen);
     } else if (using_redirection && c->flags & CLIENT_PUBSUB) {
-        robj *msg = createStringObjectFromLongLong(hash);
-        addReplyPubsubMessage(c,TrackingChannelName,msg);
-        decrRefCount(msg);
+        /* We use a static object to speedup things, however we assume
+         * that addReplyPubsubMessage() will not take a reference. */
+        robj keyobj;
+        initStaticStringObject(keyobj,keyname);
+        addReplyPubsubMessage(c,TrackingChannelName,&keyobj);
+        serverAssert(keyobj.refcount == 1);
     }
 }
 
-/* Invalidates a caching slot: this is actually the low level implementation
- * of the API that Redis calls externally, that is trackingInvalidateKey(). */
-void trackingInvalidateSlot(uint64_t slot) {
-    if (TrackingTable == NULL || TrackingTable[slot] == NULL) return;
+/* This function is called from signalModifiedKey() or other places in Redis
+ * when a key changes value. In the context of keys tracking, our task here is
+ * to send a notification to every client that may have keys about such caching
+ * slot. */
+void trackingInvalidateKey(robj *keyobj) {
+    if (TrackingTable == NULL) return;
+    sds sdskey = keyobj->ptr;
+    rax *ids = raxFind(TrackingTable,(unsigned char*)sdskey,sdslen(sdskey));
+    if (ids == raxNotFound) return;;
 
     raxIterator ri;
-    raxStart(&ri,TrackingTable[slot]);
+    raxStart(&ri,ids);
     raxSeek(&ri,"^",NULL,0);
     while(raxNext(&ri)) {
         uint64_t id;
         memcpy(&id,ri.key,sizeof(id));
         client *c = lookupClientByID(id);
         if (c == NULL || !(c->flags & CLIENT_TRACKING)) continue;
-        sendTrackingMessage(c,slot);
+        sendTrackingMessage(c,sdskey,sdslen(sdskey));
     }
     raxStop(&ri);
 
     /* Free the tracking table: we'll create the radix tree and populate it
      * again if more keys will be modified in this caching slot. */
-    raxFree(TrackingTable[slot]);
-    TrackingTable[slot] = NULL;
-    TrackingTableUsedSlots--;
-}
-
-/* This function is called from signalModifiedKey() or other places in Redis
- * when a key changes value. In the context of keys tracking, our task here is
- * to send a notification to every client that may have keys about such caching
- * slot. */
-void trackingInvalidateKey(robj *keyobj) {
-    if (TrackingTable == NULL || TrackingTableUsedSlots == 0) return;
-
-    sds sdskey = keyobj->ptr;
-    uint64_t hash = crc64(0,
-        (unsigned char*)sdskey,sdslen(sdskey))&(TRACKING_TABLE_SIZE-1);
-    trackingInvalidateSlot(hash);
+    TrackingTableTotalItems -= raxSize(ids);
+    raxFree(ids);
+    raxRemove(TrackingTable,(unsigned char*)sdskey,sdslen(sdskey),NULL);
 }
 
 /* This function is called when one or all the Redis databases are flushed
@@ -205,6 +186,10 @@ void trackingInvalidateKey(robj *keyobj) {
  * we just send the invalidation message to all the clients, but don't
  * flush the table: it will slowly get garbage collected as more keys
  * are modified in the used caching slots. */
+void freeTrackingRadixTree(void *rt) {
+    raxFree(rt);
+}
+
 void trackingInvalidateKeysOnFlush(int dbid) {
     if (server.tracking_clients) {
         listNode *ln;
@@ -213,84 +198,69 @@ void trackingInvalidateKeysOnFlush(int dbid) {
         while ((ln = listNext(&li)) != NULL) {
             client *c = listNodeValue(ln);
             if (c->flags & CLIENT_TRACKING) {
-                sendTrackingMessage(c,-1);
+                sendTrackingMessage(c,"",1);
             }
         }
     }
 
     /* In case of FLUSHALL, reclaim all the memory used by tracking. */
     if (dbid == -1 && TrackingTable) {
-        for (int j = 0; j < TRACKING_TABLE_SIZE && TrackingTableUsedSlots > 0; j++) {
-            if (TrackingTable[j] != NULL) {
-                raxFree(TrackingTable[j]);
-                TrackingTable[j] = NULL;
-                TrackingTableUsedSlots--;
-            }
-        }
-
-        /* If there are no clients with tracking enabled, we can even
-         * reclaim the memory used by the table itself. The code assumes
-         * the table is allocated only if there is at least one client alive
-         * with tracking enabled. */
-        if (server.tracking_clients == 0) {
-            zfree(TrackingTable);
-            TrackingTable = NULL;
-        }
+        raxFreeWithCallback(TrackingTable,freeTrackingRadixTree);
+        TrackingTableTotalItems = 0;
     }
 }
 
 /* Tracking forces Redis to remember information about which client may have
- * keys about certian caching slots. In workloads where there are a lot of
- * reads, but keys are hardly modified, the amount of information we have
- * to remember server side could be a lot: for each 16 millions of caching
- * slots we may end with a radix tree containing many entries.
+ * certain keys. In workloads where there are a lot of reads, but keys are
+ * hardly modified, the amount of information we have to remember server side
+ * could be a lot, with the number of keys being totally not bound.
  *
- * So Redis allows the user to configure a maximum fill rate for the
+ * So Redis allows the user to configure a maximum number of keys for the
  * invalidation table. This function makes sure that we don't go over the
  * specified fill rate: if we are over, we can just evict informations about
- * random caching slots, and send invalidation messages to clients like if
- * the key was modified. */
+ * a random key, and send invalidation messages to clients like if the key was
+ * modified. */
 void trackingLimitUsedSlots(void) {
     static unsigned int timeout_counter = 0;
-
+    if (TrackingTable == NULL) return;
     if (server.tracking_table_max_fill == 0) return; /* No limits set. */
-    unsigned int max_slots =
-        (TRACKING_TABLE_SIZE/100) * server.tracking_table_max_fill;
-    if (TrackingTableUsedSlots <= max_slots) {
+    size_t max_keys = server.tracking_table_max_fill;
+    if (raxSize(TrackingTable) <= max_keys) {
         timeout_counter = 0;
         return; /* Limit not reached. */
     }
 
-    /* We have to invalidate a few slots to reach the limit again. The effort
+    /* We have to invalidate a few keys to reach the limit again. The effort
      * we do here is proportional to the number of times we entered this
      * function and found that we are still over the limit. */
     int effort = 100 * (timeout_counter+1);
 
-    /* Let's start at a random position, and perform linear probing, in order
-     * to improve cache locality. However once we are able to find an used
-     * slot, jump again randomly, in order to avoid creating big holes in the
-     * table (that will make this funciton use more resourced later). */
+    /* We just remove one key after another by using a random walk. */
+    raxIterator ri;
+    raxStart(&ri,TrackingTable);
     while(effort > 0) {
-        unsigned int idx = rand() % TRACKING_TABLE_SIZE;
-        do {
-            effort--;
-            idx = (idx+1) % TRACKING_TABLE_SIZE;
-            if (TrackingTable[idx] != NULL) {
-                trackingInvalidateSlot(idx);
-                if (TrackingTableUsedSlots <= max_slots) {
-                    timeout_counter = 0;
-                    return; /* Return ASAP: we are again under the limit. */
-                } else {
-                    break; /* Jump to next random position. */
-                }
-            }
-        } while(effort > 0);
+        effort--;
+        raxSeek(&ri,"^",NULL,0);
+        raxRandomWalk(&ri,0);
+        rax *ids = ri.data;
+        TrackingTableTotalItems -= raxSize(ids);
+        raxFree(ids);
+        raxRemove(TrackingTable,ri.key,ri.key_len,NULL);
+        if (raxSize(TrackingTable) <= max_keys) {
+            timeout_counter = 0;
+            raxStop(&ri);
+            return; /* Return ASAP: we are again under the limit. */
+        }
     }
+
+    /* If we reach this point, we were not able to go under the configured
+     * limit using the maximum effort we had for this run. */
+    raxStop(&ri);
     timeout_counter++;
 }
 
 /* This is just used in order to access the amount of used slots in the
  * tracking table. */
-unsigned long long trackingGetUsedSlots(void) {
-    return TrackingTableUsedSlots;
+uint64_t trackingGetTotalItems(void) {
+    return TrackingTableTotalItems;
 }