Limit remote batch size (#724)

When processing a remote batch, the system will process every single message that was available at the start of processing.
This can lead to a long pause time if there have been a considerable number of frees to this thread.

This commit introduces a new mechanism to only process messages up to a limit of 1MiB. The limit is configurable using CMake.

Choosing too small a limit can cause freeing to never catch up with the incoming messages.

CMakeLists.txt

@@ -206,6 +206,10 @@ else()
   target_compile_definitions(snmalloc INTERFACE SNMALLOC_USE_WAIT_ON_ADDRESS=0)
 endif()
 
+if(SNMALLOC_REMOTE_BATCH_PROCESS_SIZE)
+  target_compile_definitions(snmalloc INTERFACE SNMALLOC_REMOTE_BATCH_PROCESS_SIZE=${SNMALLOC_REMOTE_BATCH_PROCESS_SIZE})
+endif()
+
 if(SNMALLOC_USE_SELF_VENDORED_STL)
   target_compile_definitions(snmalloc INTERFACE SNMALLOC_USE_SELF_VENDORED_STL=1)
 else()

src/snmalloc/ds/allocconfig.h

@@ -182,6 +182,16 @@ namespace snmalloc
 #endif
     ;
 
+  // Stop processing remote batch when we reach this amount of deallocations in
+  // bytes
+  static constexpr int64_t REMOTE_BATCH_LIMIT =
+#ifdef SNMALLOC_REMOTE_BATCH_PROCESS_SIZE
+    SNMALLOC_REMOTE_BATCH_PROCESS_SIZE
+#else
+    1 * 1024 * 1024
+#endif
+    ;
+
   // Used to configure when the backend should use thread local buddies.
   // This only basically is used to disable some buddy allocators on small
   // fixed heap scenarios like OpenEnclave.

src/snmalloc/mem/corealloc.h

@@ -510,17 +510,18 @@ namespace snmalloc
     handle_message_queue_inner(Action action, Args... args)
     {
       bool need_post = false;
+      size_t bytes_freed = 0;
       auto local_state = backend_state_ptr();
       auto domesticate = [local_state](freelist::QueuePtr p)
                            SNMALLOC_FAST_PATH_LAMBDA {
                              return capptr_domesticate<Config>(local_state, p);
                            };
-      auto cb = [this, domesticate, &need_post](
+      auto cb = [this, domesticate, &need_post, &bytes_freed](
                   capptr::Alloc<RemoteMessage> msg) SNMALLOC_FAST_PATH_LAMBDA {
         auto& entry =
           Config::Backend::get_metaentry(snmalloc::address_cast(msg));
-        handle_dealloc_remote(entry, msg, need_post, domesticate);
-        return true;
+        handle_dealloc_remote(entry, msg, need_post, domesticate, bytes_freed);
+        return bytes_freed < REMOTE_BATCH_LIMIT;
       };
 
 #ifdef SNMALLOC_TRACING
@@ -563,7 +564,8 @@ namespace snmalloc
       const PagemapEntry& entry,
       capptr::Alloc<RemoteMessage> msg,
       bool& need_post,
-      Domesticator_queue domesticate)
+      Domesticator_queue domesticate,
+      size_t& bytes_returned)
     {
       // TODO this needs to not double count stats
       // TODO this needs to not double revoke if using MTE
@@ -573,8 +575,8 @@ namespace snmalloc
       {
         auto meta = entry.get_slab_metadata();
 
-        auto unreturned =
-          dealloc_local_objects_fast(msg, entry, meta, entropy, domesticate);
+        auto unreturned = dealloc_local_objects_fast(
+          msg, entry, meta, entropy, domesticate, bytes_returned);
 
         /*
          * dealloc_local_objects_fast has updated the free list but not updated
@@ -777,7 +779,8 @@ namespace snmalloc
       const PagemapEntry& entry,
       BackendSlabMetadata* meta,
       LocalEntropy& entropy,
-      Domesticator domesticate)
+      Domesticator domesticate,
+      size_t& bytes_freed)
     {
       SNMALLOC_ASSERT(!meta->is_unused());
 
@@ -795,6 +798,8 @@ namespace snmalloc
         meta->as_key_tweak(),
         domesticate);
 
+      bytes_freed = objsize * length;
+
       // Update the head and the next pointer in the free list.
       meta->free_queue.append_segment(
         curr,
@@ -940,14 +945,18 @@ namespace snmalloc
                              return capptr_domesticate<Config>(local_state, p);
                            };
 
+      size_t bytes_flushed = 0; // Not currently used.
+
       if (destroy_queue)
       {
-        auto cb = [this, domesticate](capptr::Alloc<RemoteMessage> m) {
-          bool need_post = true; // Always going to post, so ignore.
-          const PagemapEntry& entry =
-            Config::Backend::get_metaentry(snmalloc::address_cast(m));
-          handle_dealloc_remote(entry, m, need_post, domesticate);
-        };
+        auto cb =
+          [this, domesticate, &bytes_flushed](capptr::Alloc<RemoteMessage> m) {
+            bool need_post = true; // Always going to post, so ignore.
+            const PagemapEntry& entry =
+              Config::Backend::get_metaentry(snmalloc::address_cast(m));
+            handle_dealloc_remote(
+              entry, m, need_post, domesticate, bytes_flushed);
+          };
 
         message_queue().destroy_and_iterate(domesticate, cb);
       }