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/ds/seqset.h

@@ -159,5 +159,13 @@ namespace snmalloc
         v.end = &(item->next);
       }
     }
+
+    /**
+     * Peek at next element in the set.
+     */
+    SNMALLOC_FAST_PATH const T* peek()
+    {
+      return v.head;
+    }
   };
 } // namespace snmalloc

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
 

src/mem/freelist.h

@@ -740,6 +740,20 @@ namespace snmalloc
         UNUSED(domesticate);
 #endif
       }
+
+      /**
+       * Returns length of the shorter free list.
+       *
+       * This method is only usable if the free list is adding randomisation
+       * as that is when it has two lists.
+       */
+      template<bool RANDOM_ = RANDOM>
+      [[nodiscard]] std::enable_if_t<RANDOM_, size_t> min_list_length() const
+      {
+        static_assert(RANDOM_ == RANDOM, "Don't set SFINAE parameter!");
+
+        return length[0] < length[1] ? length[0] : length[1];
+      }
     };
   } // namespace freelist
 } // namespace snmalloc