Decay Range

Add a backend range that delays returning memory to the next level.  This reduces the pressure on the backend global allocator.

Author: mjp41

src/snmalloc/backend/meta_protected_range.h

@@ -55,6 +55,7 @@ namespace snmalloc
       LogRange<3>,
       GlobalRange,
       CommitRange<PAL>,
+      DecayRange<PAL, Pagemap>,
       StatsRange>;
 
     // Controls the padding around the meta-data range.

src/snmalloc/backend/standard_range.h

@@ -37,8 +37,14 @@ namespace snmalloc
       LogRange<2>,
       GlobalRange>;
 
-    // Track stats of the committed memory
-    using Stats = Pipe<GlobalR, CommitRange<PAL>, StatsRange>;
+    // Decay range caches deallocated memory and gradually releases it
+    // back to the parent, avoiding expensive repeated decommit/recommit
+    // cycles for transient allocation patterns.
+    using DecayR = Pipe<GlobalR, CommitRange<PAL>, DecayRange<PAL, Pagemap>>;
+
+    // Track stats of the memory handed out (outside decay so stats
+    // methods are directly visible to StatsCombiner).
+    using Stats = Pipe<DecayR, StatsRange>;
 
   private:
     static constexpr size_t page_size_bits =

src/snmalloc/backend_helpers/backend_helpers.h

@@ -5,6 +5,7 @@
 #include "buddy.h"
 #include "commitrange.h"
 #include "commonconfig.h"
+#include "decayrange.h"
 #include "defaultpagemapentry.h"
 #include "empty_range.h"
 #include "globalrange.h"