diff --git a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler_test.ml b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler_test.ml
index 0a4a847403f..259a24ee260 100644
--- a/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler_test.ml
+++ b/ocaml/libs/xapi-stdext/lib/xapi-stdext-threads/scheduler_test.ml
@@ -14,90 +14,106 @@
 
 module Scheduler = Xapi_stdext_threads_scheduler.Scheduler
 
+let calibrated_ratio () =
+  let expected = Mtime.Span.(100 * ms |> to_float_ns) in
+  let elapsed = Mtime_clock.counter () in
+  (* Add a 10% leeway to the ratio calculated *)
+  Thread.delay 0.11 ;
+  let actual = Mtime_clock.count elapsed |> Mtime.Span.to_float_ns in
+  let ratio = actual /. expected in
+  Alcotest.(check bool) (Printf.sprintf "ratio is %f" ratio) true true ;
+  ratio
+
 let started = Atomic.make false
 
 let start_schedule () =
   if not (Atomic.exchange started true) then
     Thread.create Scheduler.loop () |> ignore
 
-let send event data = Event.(send event data |> sync)
+let send event data () = Event.(send event data |> sync)
 
 let receive event = Event.(receive event |> sync)
 
-let elapsed_ms cnt =
-  let elapsed_ns = Mtime_clock.count cnt |> Mtime.Span.to_uint64_ns in
-  Int64.(div elapsed_ns 1000000L |> to_int)
+let is_less ratio a b =
+  let a =
+    Mtime.Span.to_float_ns a
+    |> Float.mul ratio
+    |> Int64.of_float
+    |> Mtime.Span.of_uint64_ns
+  in
+  Mtime.Span.is_shorter ~than:a b
 
-let is_less = Alcotest.(testable (pp int)) Stdlib.( > )
+let mtime_span () =
+  let cmp = is_less (calibrated_ratio ()) in
+  Alcotest.(testable Mtime.Span.pp) cmp
 
 let test_single () =
   let finished = Event.new_channel () in
-  Scheduler.add_to_queue "one" Scheduler.OneShot 0.001 (fun () ->
-      send finished true
-  ) ;
+  Scheduler.add_to_queue "one" Scheduler.OneShot 0.001 (send finished true) ;
   start_schedule () ;
   Alcotest.(check bool) "result" true (receive finished)
 
-let test_remove_self () =
+let test_remove_self mtime_span () =
   let which = Event.new_channel () in
   Scheduler.add_to_queue "self" (Scheduler.Periodic 0.001) 0.001 (fun () ->
       (* this should remove the periodic scheduling *)
       Scheduler.remove_from_queue "self" ;
       (* add an operation to stop the test *)
-      Scheduler.add_to_queue "stop" Scheduler.OneShot 0.1 (fun () ->
-          send which "stop"
-      ) ;
-      send which "self"
+      Scheduler.add_to_queue "stop" Scheduler.OneShot 0.1 (send which "stop") ;
+      send which "self" ()
   ) ;
   start_schedule () ;
-  let cnt = Mtime_clock.counter () in
+
+  let from_wait_to_receive = Mtime_clock.counter () in
   Alcotest.(check string) "same event name" "self" (receive which) ;
   Alcotest.(check string) "same event name" "stop" (receive which) ;
-  let elapsed_ms = elapsed_ms cnt in
-  Alcotest.check is_less "small time" 300 elapsed_ms
 
-let test_empty () =
+  let elapsed = Mtime_clock.count from_wait_to_receive in
+  let expected = Mtime.Span.(300 * ms) in
+  Alcotest.check mtime_span "small time" expected elapsed
+
+let test_empty mtime_span () =
   let finished = Event.new_channel () in
-  Scheduler.add_to_queue "one" Scheduler.OneShot 0.001 (fun () ->
-      send finished true
-  ) ;
+  Scheduler.add_to_queue "one" Scheduler.OneShot 0.001 (send finished true) ;
   start_schedule () ;
   Alcotest.(check bool) "finished" true (receive finished) ;
   (* wait loop to go to wait with no work to do *)
   Thread.delay 0.1 ;
-  Scheduler.add_to_queue "two" Scheduler.OneShot 0.001 (fun () ->
-      send finished true
-  ) ;
-  let cnt = Mtime_clock.counter () in
+  Scheduler.add_to_queue "two" Scheduler.OneShot 0.001 (send finished true) ;
+
+  let from_wait_to_receive = Mtime_clock.counter () in
   Alcotest.(check bool) "finished" true (receive finished) ;
-  let elapsed_ms = elapsed_ms cnt in
-  Alcotest.check is_less "small time" 100 elapsed_ms
 
-let test_wakeup () =
+  let elapsed = Mtime_clock.count from_wait_to_receive in
+  let expected = Mtime.Span.(100 * ms) in
+  Alcotest.check mtime_span "small time" expected elapsed
+
+let test_wakeup mtime_span () =
   let which = Event.new_channel () in
   (* schedule a long event *)
-  Scheduler.add_to_queue "long" Scheduler.OneShot 2.0 (fun () ->
-      send which "long"
-  ) ;
+  Scheduler.add_to_queue "long" Scheduler.OneShot 2.0 (send which "long") ;
   start_schedule () ;
   (* wait loop to go to wait with no work to do *)
   Thread.delay 0.1 ;
-  let cnt = Mtime_clock.counter () in
+
   (* schedule a quick event, should wake up the loop *)
-  Scheduler.add_to_queue "quick" Scheduler.OneShot 0.1 (fun () ->
-      send which "quick"
-  ) ;
+  Scheduler.add_to_queue "quick" Scheduler.OneShot 0.1 (send which "quick") ;
+
+  let from_wait_to_receive_quick = Mtime_clock.counter () in
   Alcotest.(check string) "same event name" "quick" (receive which) ;
+
   Scheduler.remove_from_queue "long" ;
-  let elapsed_ms = elapsed_ms cnt in
-  Alcotest.check is_less "small time" 150 elapsed_ms
+  let elapsed = Mtime_clock.count from_wait_to_receive_quick in
+  let expected = Mtime.Span.(100 * ms) in
+  Alcotest.check mtime_span "small time" expected elapsed
 
 let tests =
+  let mtime_span = mtime_span () in
   [
     ("test_single", `Quick, test_single)
-  ; ("test_remove_self", `Quick, test_remove_self)
-  ; ("test_empty", `Quick, test_empty)
-  ; ("test_wakeup", `Quick, test_wakeup)
+  ; ("test_remove_self", `Quick, test_remove_self mtime_span)
+  ; ("test_empty", `Quick, test_empty mtime_span)
+  ; ("test_wakeup", `Quick, test_wakeup mtime_span)
   ]
 
 let () = Alcotest.run "Scheduler" [("generic", tests)]