Modify heuristic for adding new slabs. (#429)

If there is only one slab remaining, then we probabalisticly allocator a
new one. If a slab is barely in use, then this could cause us to
effectively double the number of slabs in use.

This commit checks if the remaining slab has enough remaining elements
to provide randomisation.

src/mem/corealloc.h

@@ -689,11 +689,16 @@ namespace snmalloc
       {
 #ifdef SNMALLOC_CHECK_CLIENT
         // Occassionally don't use the last list.
-        if (
-          SNMALLOC_UNLIKELY(alloc_classes[sizeclass].length == 1) &&
-          (entropy.next_bit() == 0))
+        if (SNMALLOC_UNLIKELY(alloc_classes[sizeclass].length == 1))
         {
-          return small_alloc_slow<zero_mem>(sizeclass, fast_free_list);
+          // If the slab has a lot of free space, then we shouldn't allocate a
+          // new slab.
+          auto min = alloc_classes[sizeclass]
+                       .available.peek()
+                       ->free_queue.min_list_length();
+          if ((min * 2) < threshold_for_waking_slab(sizeclass))
+            if (entropy.next_bit() == 0)
+              return small_alloc_slow<zero_mem>(sizeclass, fast_free_list);
         }
 #endif