SF patch #729395: Dictionary tuning

* Increase dictionary growth rate resulting in more sparse dictionaries,
  fewer lookup collisions, increased memory use, and better cache
  performance.  For dicts with over 50k entries, keep the current
  growth rate in case an application is suffering from tight memory
  constraints.

* Set the most common case (no resize) to fall-through the test.

Objects/dictobject.c

@@ -531,17 +531,23 @@ PyDict_SetItem(register PyObject *op, PyObject *key, PyObject *value)
 	Py_INCREF(value);
 	Py_INCREF(key);
 	insertdict(mp, key, hash, value);
-	/* If we added a key, we can safely resize.  Otherwise skip this!
-	 * If fill >= 2/3 size, adjust size.  Normally, this doubles the
-	 * size, but it's also possible for the dict to shrink (if ma_fill is
-	 * much larger than ma_used, meaning a lot of dict keys have been
-	 * deleted).
+	/* If we added a key, we can safely resize.  Otherwise just return!
+	 * If fill >= 2/3 size, adjust size.  Normally, this doubles or
+	 * quaduples the size, but it's also possible for the dict to shrink
+	 * (if ma_fill is much larger than ma_used, meaning a lot of dict 
+	 * keys have been * deleted).
+	 * 
+	 * Quadrupling the size improves average dictionary sparseness
+	 * (reducing collisions) at the cost of some memory and iteration
+	 * speed (which loops over every possible entry).  It also halves
+	 * the number of expensive resize operations in a growing dictionary.
+	 * 
+	 * Very large dictionaries (over 50K items) use doubling instead.  
+	 * This may help applications with severe memory constraints.
 	 */
-	if (mp->ma_used > n_used && mp->ma_fill*3 >= (mp->ma_mask+1)*2) {
-		if (dictresize(mp, mp->ma_used*2) != 0)
-			return -1;
-	}
-	return 0;
+	if (!(mp->ma_used > n_used && mp->ma_fill*3 >= (mp->ma_mask+1)*2))
+		return 0;
+	return dictresize(mp, mp->ma_used*(mp->ma_used>50000 ? 2 : 4));
 }
 
 int