dict.c: add dictGetSomeKeys(), specialized for eviction.

src/dict.c

@@ -664,7 +664,10 @@ dictEntry *dictGetRandomKey(dict *d)
     return he;
 }
 
-/* This is a version of dictGetRandomKey() that is modified in order to
+/* XXX: This is going to be removed soon and SPOP internals
+ *      reimplemented.
+ *
+ * This is a version of dictGetRandomKey() that is modified in order to
  * return multiple entries by jumping at a random place of the hash table
  * and scanning linearly for entries.
  *
@@ -735,6 +738,95 @@ unsigned int dictGetRandomKeys(dict *d, dictEntry **des, unsigned int count) {
     return stored; /* Never reached. */
 }
 
+
+/* This function samples the dictionary to return a few keys from random
+ * locations.
+ *
+ * It does not guarantee to return all the keys specified in 'count', nor
+ * it does guarantee to return non-duplicated elements, however it will make
+ * some effort to do both things.
+ *
+ * Returned pointers to hash table entries are stored into 'des' that
+ * points to an array of dictEntry pointers. The array must have room for
+ * at least 'count' elements, that is the argument we pass to the function
+ * to tell how many random elements we need.
+ *
+ * The function returns the number of items stored into 'des', that may
+ * be less than 'count' if the hash table has less than 'count' elements
+ * inside, or if not enough elements were found in a reasonable amount of
+ * steps.
+ *
+ * Note that this function is not suitable when you need a good distribution
+ * of the returned items, but only when you need to "sample" a given number
+ * of continuous elements to run some kind of algorithm or to produce
+ * statistics. However the function is much faster than dictGetRandomKey()
+ * at producing N elements. */
+unsigned int dictGetSomeKeys(dict *d, dictEntry **des, unsigned int count) {
+    unsigned int j; /* internal hash table id, 0 or 1. */
+    unsigned int tables; /* 1 or 2 tables? */
+    unsigned int stored = 0, maxsizemask;
+    unsigned int maxsteps;
+
+    if (dictSize(d) < count) count = dictSize(d);
+    maxsteps = count*10;
+
+    /* Try to do a rehashing work proportional to 'count'. */
+    for (j = 0; j < count; j++) {
+        if (dictIsRehashing(d))
+            _dictRehashStep(d);
+        else
+            break;
+    }
+
+    tables = dictIsRehashing(d) ? 2 : 1;
+    maxsizemask = d->ht[0].sizemask;
+    if (tables > 1 && maxsizemask < d->ht[1].sizemask)
+        maxsizemask = d->ht[1].sizemask;
+
+    /* Pick a random point inside the larger table. */
+    unsigned int i = random() & maxsizemask;
+    unsigned int emptylen = 0; /* Continuous empty entries so far. */
+    while(stored < count && maxsteps--) {
+        for (j = 0; j < tables; j++) {
+            /* Invariant of the dict.c rehashing: up to the indexes already
+             * visited in ht[0] during the rehashing, there are no populated
+             * buckets, so we can skip ht[0] for indexes between 0 and idx-1. */
+            if (tables == 2 && j == 0 && i < d->rehashidx) {
+                /* Moreover, if we are currently out of range in the second
+                 * table, there will be no elements in both tables up to
+                 * the current rehashing index, so we jump if possible.
+                 * (this happens when going from big to small table). */
+                if (i >= d->ht[1].size) i = d->rehashidx;
+                continue;
+            }
+            if (i >= d->ht[j].size) continue; /* Out of range for this table. */
+            dictEntry *he = d->ht[j].table[i];
+
+            /* Count contiguous empty buckets, and jump to other
+             * locations if they reach 'count' (with a minimum of 5). */
+            if (he == NULL) {
+                emptylen++;
+                if (emptylen >= 5 && emptylen > count) {
+                    i = random() & maxsizemask;
+                    emptylen = 0;
+                }
+            } else {
+                while (he) {
+                    /* Collect all the elements of the buckets found non
+                     * empty while iterating. */
+                    *des = he;
+                    des++;
+                    he = he->next;
+                    stored++;
+                    if (stored == count) return stored;
+                }
+            }
+        }
+        i = (i+1) & maxsizemask;
+    }
+    return stored;
+}
+
 /* Function to reverse bits. Algorithm from:
  * http://graphics.stanford.edu/~seander/bithacks.html#ReverseParallel */
 static unsigned long rev(unsigned long v) {

src/dict.h

@@ -164,6 +164,7 @@ dictIterator *dictGetSafeIterator(dict *d);
 dictEntry *dictNext(dictIterator *iter);
 void dictReleaseIterator(dictIterator *iter);
 dictEntry *dictGetRandomKey(dict *d);
+unsigned int dictGetSomeKeys(dict *d, dictEntry **des, unsigned int count);
 unsigned int dictGetRandomKeys(dict *d, dictEntry **des, unsigned int count);
 void dictPrintStats(dict *d);
 unsigned int dictGenHashFunction(const void *key, int len);

src/redis.c

@@ -3148,7 +3148,7 @@ void evictionPoolPopulate(dict *sampledict, dict *keydict, struct evictionPoolEn
     }
 
 #if 1 /* Use bulk get by default. */
-    count = dictGetRandomKeys(sampledict,samples,server.maxmemory_samples);
+    count = dictGetSomeKeys(sampledict,samples,server.maxmemory_samples);
 #else
     count = server.maxmemory_samples;
     for (j = 0; j < count; j++) samples[j] = dictGetRandomKey(sampledict);