backpressure.diff

diff --git i/src/libponyc/codegen/codegen.c w/src/libponyc/codegen/codegen.c
index 0aa8de7fb..484f3948a 100644
--- i/src/libponyc/codegen/codegen.c
+++ w/src/libponyc/codegen/codegen.c
@@ -336,12 +336,13 @@ static void init_runtime(compile_t* c)
 #  endif
 #endif
 
-  // void pony_sendv(i8*, __object*, $message*, $message*)
+  // void pony_sendv(i8*, __object*, $message*, $message*, i1)
   params[0] = c->void_ptr;
   params[1] = c->object_ptr;
   params[2] = c->msg_ptr;
   params[3] = c->msg_ptr;
-  type = LLVMFunctionType(c->void_type, params, 4, false);
+  params[4] = c->i1;
+  type = LLVMFunctionType(c->void_type, params, 5, false);
   value = LLVMAddFunction(c->module, "pony_sendv", type);
 #if PONY_LLVM >= 309
   LLVMAddAttributeAtIndex(value, LLVMAttributeFunctionIndex, nounwind_attr);
@@ -354,12 +355,13 @@ static void init_runtime(compile_t* c)
 #  endif
 #endif
 
-  // void pony_sendv_single(i8*, __object*, $message*, $message*)
+  // void pony_sendv_single(i8*, __object*, $message*, $message*, i1)
   params[0] = c->void_ptr;
   params[1] = c->object_ptr;
   params[2] = c->msg_ptr;
   params[3] = c->msg_ptr;
-  type = LLVMFunctionType(c->void_type, params, 4, false);
+  params[4] = c->i1;
+  type = LLVMFunctionType(c->void_type, params, 5, false);
   value = LLVMAddFunction(c->module, "pony_sendv_single", type);
 #if PONY_LLVM >= 309
   LLVMAddAttributeAtIndex(value, LLVMAttributeFunctionIndex, nounwind_attr);
diff --git i/src/libponyc/codegen/gencall.c w/src/libponyc/codegen/gencall.c
index d29fbe384..71047a39a 100644
--- i/src/libponyc/codegen/gencall.c
+++ w/src/libponyc/codegen/gencall.c
@@ -536,7 +536,7 @@ void gen_send_message(compile_t* c, reach_method_t* m, LLVMValueRef args[],
     arg_ast = ast_sibling(arg_ast);
   }
 
-  LLVMValueRef msg_args[4];
+  LLVMValueRef msg_args[5];
 
   msg_args[0] = LLVMConstInt(c->i32, ponyint_pool_index(msg_size), false);
   msg_args[1] = LLVMConstInt(c->i32, m->vtable_index, false);
@@ -595,12 +595,13 @@ void gen_send_message(compile_t* c, reach_method_t* m, LLVMValueRef args[],
   msg_args[1] = LLVMBuildBitCast(c->builder, args[0], c->object_ptr, "");
   msg_args[2] = msg;
   msg_args[3] = msg;
+  msg_args[4] = LLVMConstInt(c->i1, 1, false);
   LLVMValueRef send;
 
   if(ast_id(m->r_fun) == TK_NEW)
-    send = gencall_runtime(c, "pony_sendv_single", msg_args, 4, "");
+    send = gencall_runtime(c, "pony_sendv_single", msg_args, 5, "");
   else
-    send = gencall_runtime(c, "pony_sendv", msg_args, 4, "");
+    send = gencall_runtime(c, "pony_sendv", msg_args, 5, "");
 
   LLVMSetMetadataStr(send, "pony.msgsend", md);
 
diff --git i/src/libponyc/codegen/genexe.c w/src/libponyc/codegen/genexe.c
index 748d4660f..a646280fc 100644
--- i/src/libponyc/codegen/genexe.c
+++ w/src/libponyc/codegen/genexe.c
@@ -47,7 +47,7 @@ LLVMValueRef gen_main(compile_t* c, reach_type_t* t_main, reach_type_t* t_env)
 
   codegen_startfun(c, func, NULL, NULL, false);
 
-  LLVMValueRef args[4];
+  LLVMValueRef args[5];
   args[0] = LLVMGetParam(func, 0);
   LLVMSetValueName(args[0], "argc");
 
@@ -121,7 +121,8 @@ LLVMValueRef gen_main(compile_t* c, reach_type_t* t_main, reach_type_t* t_env)
   args[1] = main_actor;
   args[2] = msg;
   args[3] = msg;
-  gencall_runtime(c, "pony_sendv_single", args, 4, "");
+  args[4] = LLVMConstInt(c->i1, 1, false);
+  gencall_runtime(c, "pony_sendv_single", args, 5, "");
 
   // Start the runtime.
   args[0] = LLVMConstInt(c->i32, 0, false);
diff --git i/src/libponyrt/actor/actor.c w/src/libponyrt/actor/actor.c
index 21905e1a3..0898b5f71 100644
--- i/src/libponyrt/actor/actor.c
+++ w/src/libponyrt/actor/actor.c
@@ -417,7 +417,7 @@ PONY_API pony_msg_t* pony_alloc_msg_size(size_t size, uint32_t id)
 }
 
 PONY_API void pony_sendv(pony_ctx_t* ctx, pony_actor_t* to, pony_msg_t* first,
-  pony_msg_t* last)
+  pony_msg_t* last, bool has_app_msg)
 {
   // The function takes a prebuilt chain instead of varargs because the latter
   // is expensive and very hard to optimise.
@@ -437,7 +437,8 @@ PONY_API void pony_sendv(pony_ctx_t* ctx, pony_actor_t* to, pony_msg_t* first,
     DTRACE2(ACTOR_MSG_SEND, (uintptr_t)ctx->scheduler, last->id);
   }
 
-  ponyint_maybe_mute(ctx, to, first, last);
+  if(has_app_msg)
+    ponyint_maybe_mute(ctx, to);
 
   if(ponyint_messageq_push(&to->q, first, last))
   {
@@ -449,7 +450,7 @@ PONY_API void pony_sendv(pony_ctx_t* ctx, pony_actor_t* to, pony_msg_t* first,
 }
 
 PONY_API void pony_sendv_single(pony_ctx_t* ctx, pony_actor_t* to,
-  pony_msg_t* first, pony_msg_t* last)
+  pony_msg_t* first, pony_msg_t* last, bool has_app_msg)
 {
   // The function takes a prebuilt chain instead of varargs because the latter
   // is expensive and very hard to optimise.
@@ -469,7 +470,8 @@ PONY_API void pony_sendv_single(pony_ctx_t* ctx, pony_actor_t* to,
     DTRACE2(ACTOR_MSG_SEND, (uintptr_t)ctx->scheduler, last->id);
   }
 
-  ponyint_maybe_mute(ctx, to, first, last);
+  if(has_app_msg)
+    ponyint_maybe_mute(ctx, to);
 
   if(ponyint_messageq_push_single(&to->q, first, last))
   {
@@ -482,8 +484,7 @@ PONY_API void pony_sendv_single(pony_ctx_t* ctx, pony_actor_t* to,
   }
 }
 
-void ponyint_maybe_mute(pony_ctx_t* ctx, pony_actor_t* to, pony_msg_t* first,
-  pony_msg_t* last)
+void ponyint_maybe_mute(pony_ctx_t* ctx, pony_actor_t* to)
 {
   if(ctx->current != NULL)
   {
@@ -497,29 +498,7 @@ void ponyint_maybe_mute(pony_ctx_t* ctx, pony_actor_t* to, pony_msg_t* first,
        (atomic_load_explicit(&to->muted, memory_order_relaxed) > 0)) &&
       !has_flag(ctx->current, FLAG_OVERLOADED) &&
       ctx->current != to)
-    {
-      if (first == last)
-      {
-        if(first->id <= ACTORMSG_APPLICATION_START)
-        {
-          ponyint_sched_mute(ctx, ctx->current, to);
-          return;
-        }
-      }
-
-      pony_msg_t* m = first;
-
-      while(m != last)
-      {
-        if(m->id <= ACTORMSG_APPLICATION_START)
-        {
-          ponyint_sched_mute(ctx, ctx->current, to);
-          return;
-        }
-
-        m = atomic_load_explicit(&m->next, memory_order_relaxed);
-      }
-    }
+      ponyint_sched_mute(ctx, ctx->current, to);
   }
 }
 
@@ -532,7 +511,7 @@ PONY_API void pony_chain(pony_msg_t* prev, pony_msg_t* next)
 PONY_API void pony_send(pony_ctx_t* ctx, pony_actor_t* to, uint32_t id)
 {
   pony_msg_t* m = pony_alloc_msg(POOL_INDEX(sizeof(pony_msg_t)), id);
-  pony_sendv(ctx, to, m, m);
+  pony_sendv(ctx, to, m, m, id <= ACTORMSG_APPLICATION_START);
 }
 
 PONY_API void pony_sendp(pony_ctx_t* ctx, pony_actor_t* to, uint32_t id,
@@ -542,7 +521,7 @@ PONY_API void pony_sendp(pony_ctx_t* ctx, pony_actor_t* to, uint32_t id,
     POOL_INDEX(sizeof(pony_msgp_t)), id);
   m->p = p;
 
-  pony_sendv(ctx, to, &m->msg, &m->msg);
+  pony_sendv(ctx, to, &m->msg, &m->msg, id <= ACTORMSG_APPLICATION_START);
 }
 
 PONY_API void pony_sendi(pony_ctx_t* ctx, pony_actor_t* to, uint32_t id,
@@ -552,7 +531,7 @@ PONY_API void pony_sendi(pony_ctx_t* ctx, pony_actor_t* to, uint32_t id,
     POOL_INDEX(sizeof(pony_msgi_t)), id);
   m->i = i;
 
-  pony_sendv(ctx, to, &m->msg, &m->msg);
+  pony_sendv(ctx, to, &m->msg, &m->msg, id <= ACTORMSG_APPLICATION_START);
 }
 
 #ifdef USE_ACTOR_CONTINUATIONS
diff --git i/src/libponyrt/actor/actor.h w/src/libponyrt/actor/actor.h
index 828f1621d..fe3a4aba7 100644
--- i/src/libponyrt/actor/actor.h
+++ w/src/libponyrt/actor/actor.h
@@ -61,8 +61,7 @@ void ponyint_actor_setoverloaded(pony_actor_t* actor);
 
 void ponyint_actor_unsetoverloaded(pony_ctx_t* ctx, pony_actor_t* actor);
 
-void ponyint_maybe_mute(pony_ctx_t* ctx, pony_actor_t* to, pony_msg_t* first,
-  pony_msg_t* last);
+void ponyint_maybe_mute(pony_ctx_t* ctx, pony_actor_t* to);
 
 PONY_API void ponyint_destroy(pony_actor_t* actor);
 
diff --git i/src/libponyrt/asio/event.c w/src/libponyrt/asio/event.c
index 280900f87..4c9aba895 100644
--- i/src/libponyrt/asio/event.c
+++ w/src/libponyrt/asio/event.c
@@ -118,5 +118,8 @@ PONY_API void pony_asio_event_send(asio_event_t* ev, uint32_t flags,
   pony_register_thread();
 #endif
 
-  pony_sendv(pony_ctx(), ev->owner, &m->msg, &m->msg);
+  // ASIO messages technically are application messages, but since they have no
+  // sender they aren't covered by backpressure. We pass false for an early
+  // bailout in the backpressure code.
+  pony_sendv(pony_ctx(), ev->owner, &m->msg, &m->msg, false);
 }
diff --git i/src/libponyrt/gc/cycle.c w/src/libponyrt/gc/cycle.c
index e289e1c62..90f12f578 100644
--- i/src/libponyrt/gc/cycle.c
+++ w/src/libponyrt/gc/cycle.c
@@ -922,7 +922,7 @@ void ponyint_cycle_block(pony_ctx_t* ctx, pony_actor_t* actor, gc_t* gc)
   m->delta = ponyint_gc_delta(gc);
   pony_assert(gc->delta == NULL);
 
-  pony_sendv(ctx, cycle_detector, &m->msg, &m->msg);
+  pony_sendv(ctx, cycle_detector, &m->msg, &m->msg, false);
 }
 
 void ponyint_cycle_unblock(pony_ctx_t* ctx, pony_actor_t* actor)
diff --git i/src/libponyrt/pony.h w/src/libponyrt/pony.h
index 2a6aa9e60..81c8dbfd3 100644
--- i/src/libponyrt/pony.h
+++ w/src/libponyrt/pony.h
@@ -175,9 +175,11 @@ PONY_API pony_msg_t* pony_alloc_msg_size(size_t size, uint32_t id);
  *
  * The first and last messages must either be the same message or the two ends
  * of a chain built with calls to pony_chain.
+ *
+ * has_app_msg should be true if there are application messages in the chain.
  */
 PONY_API void pony_sendv(pony_ctx_t* ctx, pony_actor_t* to, pony_msg_t* first,
-  pony_msg_t* last);
+  pony_msg_t* last, bool has_app_msg);
 
 /** Single producer version of pony_sendv.
  *
@@ -188,9 +190,11 @@ PONY_API void pony_sendv(pony_ctx_t* ctx, pony_actor_t* to, pony_msg_t* first,
  *
  * The first and last messages must either be the same message or the two ends
  * of a chain built with calls to pony_chain.
+ *
+ * has_app_msg should be true if there are application messages in the chain.
  */
 PONY_API void pony_sendv_single(pony_ctx_t* ctx, pony_actor_t* to,
-  pony_msg_t* first, pony_msg_t* last);
+  pony_msg_t* first, pony_msg_t* last, bool has_app_msg);
 
 /** Chain two messages together.
  *