Use FlatPageMap on OS with lazy commit

If the operating system will allocate private pages on demand for the
pagemap then use the FlatPageMap by default as it generates better code
for deallocation.

src/mem/alloc.h

@@ -46,8 +46,10 @@ namespace snmalloc
 // Use flat map is under a single node.
 #  define SNMALLOC_MAX_FLATPAGEMAP_SIZE PAGEMAP_NODE_SIZE
 #endif
-  static constexpr bool USE_FLATPAGEMAP = SNMALLOC_MAX_FLATPAGEMAP_SIZE >=
-    sizeof(FlatPagemap<SUPERSLAB_BITS, uint8_t>);
+  static constexpr bool USE_FLATPAGEMAP =
+    (Pal::pal_features & PalFeatures::LazyCommit) ||
+    (SNMALLOC_MAX_FLATPAGEMAP_SIZE >=
+     sizeof(FlatPagemap<SUPERSLAB_BITS, uint8_t>));
 
   using SuperslabPagemap = std::conditional_t<
     USE_FLATPAGEMAP,
@@ -1152,6 +1154,8 @@ namespace snmalloc
     SNMALLOC_SLOW_PATH void small_dealloc_offseted_slow(
       Superslab* super, void* p, sizeclass_t sizeclass)
     {
+      handle_message_queue();
+
       bool was_full = super->is_full();
       SlabList* sl = &small_classes[sizeclass];
       Slab* slab = Slab::get(p);

src/pal/pal_apple.h

@@ -22,7 +22,7 @@ namespace snmalloc
      * Bitmap of PalFeatures flags indicating the optional features that this
      * PAL supports.
      */
-    static constexpr uint64_t pal_features = 0;
+    static constexpr uint64_t pal_features = LazyCommit;
     static void error(const char* const str)
     {
       puts(str);

src/pal/pal_consts.h

@@ -25,7 +25,13 @@ namespace snmalloc
      * a size and alignment.  A PAL that does *not* support it must expose a
      * `request()` method that takes only a size.
      */
-    AlignedAllocation = (1 << 1)
+    AlignedAllocation = (1 << 1),
+    /**
+     * This PAL natively supports lazy commit of pages. This means have large
+     * allocations and not touching them does not up memory usage.  This is
+     * exposed in the P
+     */
+    LazyCommit = (1 << 2),
   };
   /**
    * Flag indicating whether requested memory should be zeroed.

src/pal/pal_freebsd.h

@@ -17,7 +17,7 @@ namespace snmalloc
      * Bitmap of PalFeatures flags indicating the optional features that this
      * PAL supports.
      */
-    static constexpr uint64_t pal_features = AlignedAllocation;
+    static constexpr uint64_t pal_features = AlignedAllocation | LazyCommit;
     static void error(const char* const str)
     {
       puts(str);

src/pal/pal_linux.h

@@ -18,7 +18,7 @@ namespace snmalloc
      * Bitmap of PalFeatures flags indicating the optional features that this
      * PAL supports.
      */
-    static constexpr uint64_t pal_features = 0;
+    static constexpr uint64_t pal_features = LazyCommit;
     static void error(const char* const str)
     {
       puts(str);