diff --git a/benchs/fib_rec.ml b/benchs/fib_rec.ml
index b0f1623d..25291e8c 100644
--- a/benchs/fib_rec.ml
+++ b/benchs/fib_rec.ml
@@ -1,4 +1,7 @@
 open Moonpool
+module Trace = Trace_core
+
+let ( let@ ) = ( @@ )
 
 let rec fib_direct x =
   if x <= 1 then
@@ -10,7 +13,7 @@ let cutoff = ref 20
 
 let rec fib ~on x : int Fut.t =
   if x <= !cutoff then
-    Fut.spawn ~on (fun () -> fib_direct x)
+    Fut.spawn ~name:"fib" ~on (fun () -> fib_direct x)
   else
     let open Fut.Infix in
     let+ t1 = fib ~on (x - 1) and+ t2 = fib ~on (x - 2) in
@@ -27,14 +30,14 @@ let fib_fj ~on x : int Fut.t =
       n1 + n2
     )
   in
-  Fut.spawn ~on (fun () -> fib_rec x)
+  Fut.spawn ~name:"fib" ~on (fun () -> fib_rec x)
 
 let fib_await ~on x : int Fut.t =
   let rec fib_rec x : int Fut.t =
     if x <= !cutoff then
-      Fut.spawn ~on (fun () -> fib_direct x)
+      Fut.spawn ~name:"fib" ~on (fun () -> fib_direct x)
     else
-      Fut.spawn ~on (fun () ->
+      Fut.spawn ~name:"fib" ~on (fun () ->
           let n1 = fib_rec (x - 1) in
           let n2 = fib_rec (x - 2) in
           let n1 = Fut.await n1 in
@@ -66,6 +69,7 @@ let str_of_int_opt = function
   | Some i -> Printf.sprintf "Some %d" i
 
 let run ~psize ~n ~seq ~dl ~fj ~await ~niter ~kind () : unit =
+  let@ _sp = Trace.with_span ~__FILE__ ~__LINE__ "fib.run" in
   let pool = lazy (create_pool ~kind ~psize ()) in
   let dl_pool =
     lazy
@@ -108,6 +112,7 @@ let run ~psize ~n ~seq ~dl ~fj ~await ~niter ~kind () : unit =
     Ws_pool.shutdown (Lazy.force pool)
 
 let () =
+  let@ () = Trace_tef.with_setup () in
   let n = ref 40 in
   let psize = ref None in
   let seq = ref false in
diff --git a/benchs/pi.ml b/benchs/pi.ml
index c8ef57b5..7e0dfd91 100644
--- a/benchs/pi.ml
+++ b/benchs/pi.ml
@@ -7,6 +7,7 @@ let j = ref 0
 let spf = Printf.sprintf
 
 let run_sequential (num_steps : int) : float =
+  let@ _sp = Trace.with_span ~__FILE__ ~__LINE__ "pi.seq" in
   let step = 1. /. float num_steps in
   let sum = ref 0. in
   for i = 0 to num_steps - 1 do
@@ -42,6 +43,11 @@ let run_par1 ~kind (num_steps : int) : float =
 
   (* one chunk of the work *)
   let run_task _idx_task : unit =
+    let@ _sp =
+      Trace.with_span ~__FILE__ ~__LINE__ "pi.slice" ~data:(fun () ->
+          [ "i", `Int _idx_task ])
+    in
+
     let sum = ref 0. in
     let i = ref 0 in
     while !i < num_steps do
@@ -69,7 +75,7 @@ let run_fork_join ~kind num_steps : float =
   let step = 1. /. float num_steps in
   let global_sum = Lock.create 0. in
 
-  Ws_pool.run_wait_block pool (fun () ->
+  Ws_pool.run_wait_block ~name:"pi.fj" pool (fun () ->
       Fork_join.for_
         ~chunk_size:(3 + (num_steps / num_tasks))
         num_steps
@@ -99,6 +105,9 @@ type mode =
 
 let () =
   let@ () = Trace_tef.with_setup () in
+  Trace.set_thread_name "main";
+  let@ _sp = Trace.with_span ~__FILE__ ~__LINE__ "main" in
+
   let mode = ref Sequential in
   let n = ref 1000 in
   let time = ref false in
diff --git a/test/effect-based/t_fork_join_heavy.ml b/test/effect-based/t_fork_join_heavy.ml
index a981bee1..bacb1d18 100644
--- a/test/effect-based/t_fork_join_heavy.ml
+++ b/test/effect-based/t_fork_join_heavy.ml
@@ -33,10 +33,12 @@ let run ~min () =
       let l1, l2 =
         Fork_join.both
           (fun () ->
+            let@ _sp = Trace.with_span ~__FILE__ ~__LINE__ "fj.left" in
             Fork_join.map_list ~chunk_size
               (Fork_join.map_list ~chunk_size neg)
               l)
           (fun () ->
+            let@ _sp = Trace.with_span ~__FILE__ ~__LINE__ "fj.right" in
             Fork_join.map_list ~chunk_size
               (Fork_join.map_list ~chunk_size neg)
               ref_l1)