diff --git a/lib_eio/core/eio__core.ml b/lib_eio/core/eio__core.ml
index 2c8f20536..3418d944d 100644
--- a/lib_eio/core/eio__core.ml
+++ b/lib_eio/core/eio__core.ml
@@ -7,6 +7,7 @@ module Private = struct
   module Suspend = Suspend
   module Cells = Cells
   module Broadcast = Broadcast
+  module Single_waiter = Single_waiter
   module Trace = Trace
   module Fiber_context = Cancel.Fiber_context
   module Debug = Debug
diff --git a/lib_eio/core/eio__core.mli b/lib_eio/core/eio__core.mli
index 65e2f41e4..734d9f1c7 100644
--- a/lib_eio/core/eio__core.mli
+++ b/lib_eio/core/eio__core.mli
@@ -606,6 +606,7 @@ module Private : sig
 
   module Cells = Cells
   module Broadcast = Broadcast
+  module Single_waiter = Single_waiter
 
   (** Every fiber has an associated context. *)
   module Fiber_context : sig
diff --git a/lib_eio_linux/low_level.ml b/lib_eio_linux/low_level.ml
index 2db0f2fe6..6967fb80a 100644
--- a/lib_eio_linux/low_level.ml
+++ b/lib_eio_linux/low_level.ml
@@ -207,11 +207,34 @@ let write ?file_offset:off fd buf len =
     raise @@ Err.wrap (Uring.error_of_errno res) "write" ""
   )
 
-let alloc_fixed () = Effect.perform Sched.Alloc
-
-let alloc_fixed_or_wait () = Effect.perform Sched.Alloc_or_wait
-
-let free_fixed buf = Effect.perform (Sched.Free buf)
+let alloc_fixed () =
+  let s = Sched.get () in
+  match s.mem with
+  | None -> None
+  | Some mem ->
+    match Uring.Region.alloc mem with
+    | buf -> Some buf
+    | exception Uring.Region.No_space -> None
+
+let alloc_fixed_or_wait () =
+  let s = Sched.get () in
+  match s.mem with
+  | None -> failwith "No fixed buffer available"
+  | Some mem ->
+    match Uring.Region.alloc mem with
+    | buf -> buf
+    | exception Uring.Region.No_space ->
+      let id = Eio.Private.Trace.mint_id () in
+      let trigger = Eio.Private.Single_waiter.create () in
+      Queue.push trigger s.mem_q;
+      (* todo: remove protect; but needs to remove from queue on cancel *)
+      Eio.Private.Single_waiter.await_protect trigger "alloc_fixed_or_wait" id
+
+let free_fixed buf =
+  let s = Sched.get () in
+  match Queue.take_opt s.mem_q with
+  | None -> Uring.Region.free buf
+  | Some k -> Eio.Private.Single_waiter.wake k (Ok buf)
 
 let splice src ~dst ~len =
   Fd.use_exn "splice-src" src @@ fun src ->
diff --git a/lib_eio_linux/sched.ml b/lib_eio_linux/sched.ml
index 4d5f1a901..7d9e4b27b 100644
--- a/lib_eio_linux/sched.ml
+++ b/lib_eio_linux/sched.ml
@@ -50,7 +50,7 @@ type t = {
   uring: io_job Uring.t;
   mem: Uring.Region.t option;
   io_q: (t -> unit) Queue.t;     (* waiting for room on [uring] *)
-  mem_q : Uring.Region.chunk Suspended.t Queue.t;
+  mem_q : Uring.Region.chunk Eio.Private.Single_waiter.t Queue.t;
 
   (* The queue of runnable fibers ready to be resumed. Note: other domains can also add work items here. *)
   run_q : runnable Lf_queue.t;
@@ -74,9 +74,9 @@ type t = {
 type _ Effect.t +=
   | Enter : (t -> 'a Suspended.t -> unit) -> 'a Effect.t
   | Cancel : io_job Uring.job -> unit Effect.t
-  | Alloc : Uring.Region.chunk option Effect.t
-  | Alloc_or_wait : Uring.Region.chunk Effect.t
-  | Free : Uring.Region.chunk -> unit Effect.t
+  | Get : t Effect.t
+
+let get () = Effect.perform Get
 
 let wake_buffer =
   let b = Bytes.create 8 in
@@ -339,21 +339,6 @@ and complete_rw_req st ({len; cur_off; action; _} as req) res =
   | _, Exactly len -> Suspended.continue action len
   | n, Upto _ -> Suspended.continue action n
 
-let alloc_buf_or_wait st k =
-  match st.mem with
-  | None -> Suspended.discontinue k (Failure "No fixed buffer available")
-  | Some mem ->
-    match Uring.Region.alloc mem with
-    | buf -> Suspended.continue k buf
-    | exception Uring.Region.No_space ->
-      Queue.push k st.mem_q;
-      schedule st
-
-let free_buf st buf =
-  match Queue.take_opt st.mem_q with
-  | None -> Uring.Region.free buf
-  | Some k -> enqueue_thread st k buf
-
 let rec enqueue_poll_add fd poll_mask st action =
   Trace.log "poll_add";
   let retry = with_cancel_hook ~action st (fun () ->
@@ -411,8 +396,9 @@ let run ~extra_effects st main arg =
             Fiber_context.destroy fiber;
             Printexc.raise_with_backtrace ex (Printexc.get_raw_backtrace ())
           );
-        effc = fun (type a) (e : a Effect.t) ->
+        effc = fun (type a) (e : a Effect.t) : ((a, _) continuation -> _) option ->
           match e with
+          | Get -> Some (fun k -> continue k st)
           | Enter fn -> Some (fun k ->
               match Fiber_context.get_error fiber with
               | Some e -> discontinue k e
@@ -467,22 +453,6 @@ let run ~extra_effects st main arg =
               Eio_unix.Private.Thread_pool.submit st.thread_pool ~ctx:fiber ~enqueue fn;
               schedule st
             )
-          | Alloc -> Some (fun k ->
-              match st.mem with
-              | None -> continue k None
-              | Some mem ->
-                match Uring.Region.alloc mem with
-                | buf -> continue k (Some buf)
-                | exception Uring.Region.No_space -> continue k None
-            )
-          | Alloc_or_wait -> Some (fun k ->
-              let k = { Suspended.k; fiber } in
-              alloc_buf_or_wait st k
-            )
-          | Free buf -> Some (fun k ->
-              free_buf st buf;
-              continue k ()
-            )
           | e -> extra_effects.effc e
       }
   in