diff --git a/otherlibs/memtrace/memprof_tracer.ml b/otherlibs/memtrace/memprof_tracer.ml
index a967ae50ed7d..a6bdcf1c3dc6 100644
--- a/otherlibs/memtrace/memprof_tracer.ml
+++ b/otherlibs/memtrace/memprof_tracer.ml
@@ -32,8 +32,7 @@ let default_report_exn e =
 
 let start ?(report_exn=default_report_exn) ~sampling_rate trace =
   let s = { trace; locked = false; stopped = false; failed = false; report_exn } in
-  let tracker : (_,_) Gc.Memprof.tracker = {
-    alloc_minor = (fun info ->
+  let minor_alloc_callback = (fun (info : Gc.Memprof.allocation) ->
       if lock_tracer s then begin
         match Trace.Writer.put_alloc_with_raw_backtrace trace (Trace.Timestamp.now ())
                 ~length:info.size
@@ -43,8 +42,9 @@ let start ?(report_exn=default_report_exn) ~sampling_rate trace =
         with
         | r -> unlock_tracer s; Some r
         | exception e -> mark_failed s e; None
-      end else None);
-    alloc_major = (fun info ->
+      end else None)
+  in
+  let major_alloc_callback = (fun (info : Gc.Memprof.allocation) ->
       if lock_tracer s then begin
         match Trace.Writer.put_alloc_with_raw_backtrace trace (Trace.Timestamp.now ())
                 ~length:info.size
@@ -54,27 +54,36 @@ let start ?(report_exn=default_report_exn) ~sampling_rate trace =
         with
         | r -> unlock_tracer s; Some r
         | exception e -> mark_failed s e; None
-      end else None);
-    promote = (fun id ->
+      end else None)
+  in
+  let promote_callback = (fun id ->
       if lock_tracer s then
         match Trace.Writer.put_promote trace (Trace.Timestamp.now ()) id with
         | () -> unlock_tracer s; Some id
         | exception e -> mark_failed s e; None
-      else None);
-    dealloc_minor = (fun id ->
+      else None)
+  in
+  let minor_dealloc_callback = (fun id ->
       if lock_tracer s then
         match Trace.Writer.put_collect trace (Trace.Timestamp.now ()) id with
         | () -> unlock_tracer s
-        | exception e -> mark_failed s e);
-    dealloc_major = (fun id ->
+        | exception e -> mark_failed s e)
+  in
+  let major_dealloc_callback = (fun id ->
       if lock_tracer s then
         match Trace.Writer.put_collect trace (Trace.Timestamp.now ()) id with
         | () -> unlock_tracer s
-        | exception e -> mark_failed s e) } in
+        | exception e -> mark_failed s e)
+  in
   Gc.Memprof.start
     ~sampling_rate
     ~callstack_size:max_int
-    tracker;
+    ~minor_alloc_callback
+    ~major_alloc_callback
+    ~promote_callback
+    ~minor_dealloc_callback
+    ~major_dealloc_callback
+    ();
   s
 
 let stop s =
diff --git a/otherlibs/memtrace/vendor b/otherlibs/memtrace/vendor
index dc5ed19173fd..095570a616ec 100644
--- a/otherlibs/memtrace/vendor
+++ b/otherlibs/memtrace/vendor
@@ -1,2 +1,7 @@
 This a vendored version of the `src` dir of:
 git@github.com:janestreet/memtrace.git#6c7ed830b3535bcd77be698a5d556fe4a54974ba
+
+Additional patches:
+- Use the 5 arguments interface of Gc.Memprof.start instead of the record one,
+  + see 2805c598a94b3081f887eaf6403b8de4582cb653 for the commit that changed the contents of this directory
+  + see ff6b20098f7f51758d78546af4bb8747cdc8c9f4 for the commit introducing the record representation
diff --git a/runtime/memprof.c b/runtime/memprof.c
index e71f06657859..bacb4a1eeef7 100644
--- a/runtime/memprof.c
+++ b/runtime/memprof.c
@@ -867,19 +867,17 @@ void caml_memprof_shutdown(void) {
   callstack_buffer_len = 0;
 }
 
-CAMLprim value caml_memprof_start(value lv, value szv, value tracker)
+CAMLprim value caml_memprof_start(value lv, value szv,
+                                  value cb_alloc_minor, value cb_alloc_major,
+                                  value cb_promote,
+                                  value cb_dealloc_minor,
+                                  value cb_dealloc_major)
 {
-  CAMLparam3(lv, szv, tracker);
-
+  CAMLparam5(lv, szv, cb_alloc_minor, cb_alloc_major, cb_promote);
+  CAMLxparam2(cb_dealloc_minor, cb_dealloc_major);
   double l = Double_val(lv);
   intnat sz = Long_val(szv);
 
-  value cb_alloc_minor = Field(tracker, 0);
-  value cb_alloc_major = Field(tracker, 1);
-  value cb_promote = Field(tracker, 2);
-  value cb_dealloc_minor = Field(tracker, 3);
-  value cb_dealloc_major = Field(tracker, 4);
-
   if (started) caml_failwith("Gc.Memprof.start: already started.");
 
   if (sz < 0 || !(l >= 0.) || l > 1.) /* Checks that [l] is not NAN. */
@@ -914,10 +912,11 @@ CAMLprim value caml_memprof_start(value lv, value szv, value tracker)
   CAMLreturn(Val_unit);
 }
 
-/* not used anymore, the prototype is only kept until the next bootstrap */
 CAMLprim value caml_memprof_start_byt(value* argv, int argn)
 {
-  return(Val_unit);
+  CAMLassert(argn == 7);
+  return caml_memprof_start(argv[0], argv[1], argv[2], argv[3],
+                            argv[4], argv[5], argv[6]);
 }
 
 CAMLprim value caml_memprof_stop(value unit)
diff --git a/stdlib/gc.ml b/stdlib/gc.ml
index 52433ca43235..771dea51b8b2 100644
--- a/stdlib/gc.ml
+++ b/stdlib/gc.ml
@@ -127,31 +127,27 @@ module Memprof =
         unmarshalled : bool;
         callstack : Printexc.raw_backtrace }
 
-    type ('minor, 'major) tracker = {
-      alloc_minor: allocation -> 'minor option;
-      alloc_major: allocation -> 'major option;
-      promote: 'minor -> 'major option;
-      dealloc_minor: 'minor -> unit;
-      dealloc_major: 'major -> unit;
-    }
-
-    let null_tracker = {
-      alloc_minor = (fun _ -> None);
-      alloc_major = (fun _ -> None);
-      promote = (fun _ -> None);
-      dealloc_minor = (fun _ -> ());
-      dealloc_major = (fun _ -> ());
-    }
-
     external c_start :
-      float -> int -> ('minor, 'major) tracker -> unit
-      = "caml_memprof_start"
+      float -> int ->
+      (allocation -> 'minor option) ->
+      (allocation -> 'major option) ->
+      ('minor -> 'major option) ->
+      ('minor -> unit) ->
+      ('major -> unit) ->
+      unit
+      = "caml_memprof_start_byt" "caml_memprof_start"
 
     let start
       ~sampling_rate
       ?(callstack_size = max_int)
-      tracker =
-      c_start sampling_rate callstack_size tracker
+      ?(minor_alloc_callback = fun _ -> None)
+      ?(major_alloc_callback = fun _ -> None)
+      ?(promote_callback = fun _ -> None)
+      ?(minor_dealloc_callback = fun _ -> ())
+      ?(major_dealloc_callback = fun _ -> ()) () =
+      c_start sampling_rate callstack_size minor_alloc_callback
+              major_alloc_callback promote_callback minor_dealloc_callback
+              major_dealloc_callback
 
     external stop : unit -> unit = "caml_memprof_stop"
   end
diff --git a/stdlib/gc.mli b/stdlib/gc.mli
index 77682456f7da..fb84421bd9f2 100644
--- a/stdlib/gc.mli
+++ b/stdlib/gc.mli
@@ -458,32 +458,15 @@ module Memprof :
        type of records passed to the callback triggered by the
        sampling of an allocation. *)
 
-    type ('minor, 'major) tracker = {
-      alloc_minor: allocation -> 'minor option;
-      alloc_major: allocation -> 'major option;
-      promote: 'minor -> 'major option;
-      dealloc_minor: 'minor -> unit;
-      dealloc_major: 'major -> unit;
-    }
-    (**
-       A [('minor, 'major) tracker] describes how memprof should track
-       sampled blocks over their lifetime, keeping a user-defined piece
-       of metadata for each of them: ['minor] is the type of metadata
-       to keep for minor blocks, and ['major] the type of metadata
-       for major blocks.
-
-       If an allocation-tracking or promotion-tracking function returns [None],
-       memprof stops tracking the corresponding value.
-     *)
-
-    val null_tracker: ('minor, 'major) tracker
-    (** Default callbacks simply return [None] or [()] *)
-
     val start :
       sampling_rate:float ->
       ?callstack_size:int ->
-      ('minor, 'major) tracker ->
-      unit
+      ?minor_alloc_callback:(allocation -> 'minor option) ->
+      ?major_alloc_callback:(allocation -> 'major option) ->
+      ?promote_callback:('minor -> 'major option) ->
+      ?minor_dealloc_callback:('minor -> unit) ->
+      ?major_dealloc_callback:('major -> unit) ->
+      unit -> unit
     (** Start the sampling with the given parameters. Fails if
        sampling is already active.
 
@@ -495,10 +478,14 @@ module Memprof :
        The parameter [callstack_size] is the length of the callstack
        recorded at every sample. Its default is [max_int].
 
-       The parameter [tracker] determines how to track sampled blocks
-       over their lifetime in the minor and major heap.
+       The parameters *[_callback] are functions called when an event
+       occurs on a sampled block. If such a callback returns [None],
+       then the tracking of this particular block is cancelled. If
+       they return [Some v], then the value [v] will be passed to the
+       next callback for this block. Default callbacks simply return
+       [None] or [()].
 
-       Sampling is temporarily disabled when calling a callback
+       The sampling is temporarily disabled when calling a callback
        for the current thread. So they do not need to be reentrant if
        the program is single-threaded. However, if threads are used,
        it is possible that a context switch occurs during a callback,
diff --git a/testsuite/tests/statmemprof/arrays_in_major.ml b/testsuite/tests/statmemprof/arrays_in_major.ml
index f3c5b8a68f79..f213828670c8 100644
--- a/testsuite/tests/statmemprof/arrays_in_major.ml
+++ b/testsuite/tests/statmemprof/arrays_in_major.ml
@@ -17,12 +17,12 @@ let[@inline never] allocate_arrays lo hi cnt keep =
 
 let check_nosample () =
   Printf.printf "check_nosample\n%!";
-  let alloc _ =
+  let cb _ =
     Printf.printf "Callback called with sampling_rate = 0\n";
     assert(false)
   in
-  start ~callstack_size:10 ~sampling_rate:0.
-    { null_tracker with alloc_minor = alloc; alloc_major = alloc; };
+  start ~callstack_size:10 ~minor_alloc_callback:cb ~major_alloc_callback:cb
+        ~sampling_rate:0. ();
   allocate_arrays 300 3000 1 false;
   stop ()
 
@@ -36,26 +36,25 @@ let check_counts_full_major force_promote =
   let npromote = ref 0 in
   let ndealloc_minor = ref 0 in
   let ndealloc_major = ref 0 in
-  start ~callstack_size:10 ~sampling_rate:0.01
-    {
-      alloc_minor = (fun _ ->
-        if not !enable then None
-        else Some (incr nalloc_minor)
-      );
-      alloc_major = (fun _ ->
-        if not !enable then None
-        else Some (incr nalloc_major)
-      );
-      promote = (fun _ ->
-        Some (incr npromote)
-      );
-      dealloc_minor = (fun _ ->
-        incr ndealloc_minor
-      );
-      dealloc_major = (fun _ ->
-        incr ndealloc_major
-      );
-    };
+  start ~callstack_size:10
+        ~minor_alloc_callback:(fun _ ->
+          if !enable then begin
+              incr nalloc_minor;
+              Some ()
+          end else
+            None)
+        ~major_alloc_callback:(fun _ ->
+           if !enable then begin
+             incr nalloc_major;
+             Some ()
+           end else
+             None)
+        ~promote_callback:(fun _ ->
+           incr npromote;
+           Some ())
+        ~minor_dealloc_callback:(fun _ -> incr ndealloc_minor)
+        ~major_dealloc_callback:(fun _ -> incr ndealloc_major)
+        ~sampling_rate:0.01 ();
   allocate_arrays 300 3000 1 true;
   enable := false;
   assert (!ndealloc_minor = 0 && !ndealloc_major = 0);
@@ -92,14 +91,11 @@ let check_no_nested () =
     ()
   in
   let cb' _ = cb (); Some () in
-  start ~callstack_size:10 ~sampling_rate:1.
-    {
-      alloc_minor = cb';
-      alloc_major = cb';
-      promote = cb';
-      dealloc_minor = cb;
-      dealloc_major = cb;
-    };
+  start ~callstack_size:10
+        ~minor_alloc_callback:cb' ~major_alloc_callback:cb'
+        ~promote_callback:cb' ~minor_dealloc_callback:cb
+        ~major_dealloc_callback:cb
+        ~sampling_rate:1. ();
   allocate_arrays 300 300 100 false;
   stop ()
 
@@ -108,16 +104,15 @@ let () = check_no_nested ()
 let check_distrib lo hi cnt rate =
   Printf.printf "check_distrib %d %d %d %f\n%!" lo hi cnt rate;
   let smp = ref 0 in
-  start ~callstack_size:10 ~sampling_rate:rate
-    { null_tracker with
-      alloc_major = (fun info ->
-        assert (info.size >= lo && info.size <= hi);
-        assert (info.n_samples > 0);
-        assert (not info.unmarshalled);
-        smp := !smp + info.n_samples;
-        None
-      );
-    };
+  start ~callstack_size:10
+        ~major_alloc_callback:(fun info ->
+           assert (info.size >= lo && info.size <= hi);
+           assert (info.n_samples > 0);
+           assert (not info.unmarshalled);
+           smp := !smp + info.n_samples;
+           None
+        )
+        ~sampling_rate:rate ();
   allocate_arrays lo hi cnt false;
   stop ();
 
diff --git a/testsuite/tests/statmemprof/arrays_in_minor.ml b/testsuite/tests/statmemprof/arrays_in_minor.ml
index ec6131f19b24..4fb2013430d1 100644
--- a/testsuite/tests/statmemprof/arrays_in_minor.ml
+++ b/testsuite/tests/statmemprof/arrays_in_minor.ml
@@ -25,12 +25,12 @@ let[@inline never] allocate_arrays lo hi cnt keep =
 
 let check_nosample () =
   Printf.printf "check_nosample\n%!";
-  let alloc _ =
+  let cb _ =
     Printf.printf "Callback called with sampling_rate = 0\n";
     assert(false)
   in
-  start ~callstack_size:10 ~sampling_rate:0.
-    { null_tracker with alloc_minor = alloc; alloc_major = alloc };
+  start ~callstack_size:10 ~minor_alloc_callback:cb ~major_alloc_callback:cb
+        ~sampling_rate:0. ();
   allocate_arrays 1 250 100 false;
   stop ()
 
@@ -44,28 +44,23 @@ let check_counts_full_major force_promote =
   let npromote = ref 0 in
   let ndealloc_minor = ref 0 in
   let ndealloc_major = ref 0 in
-  start ~callstack_size:10 ~sampling_rate:0.01
-    {
-      alloc_minor = (fun info ->
-        if !enable then begin
-          incr nalloc_minor; if !nalloc_minor mod 100 = 0 then Gc.minor ();
-          Some (ref 42)
-        end else begin
-          allocate_arrays 1 250 1 true;
-          None
-        end);
-      alloc_major = (fun _ -> assert false);
-      promote = (fun k ->
-        assert (!k = 42 && !promotes_allowed);
-        incr npromote; if !npromote mod 1097 = 0 then Gc.minor ();
-        Some (ref 17));
-      dealloc_minor = (fun k ->
-        assert (!k = 42);
-        incr ndealloc_minor);
-      dealloc_major = (fun r ->
-        assert (!r = 17);
-        incr ndealloc_major);
-    };
+  start ~callstack_size:10
+        ~minor_alloc_callback:(fun info ->
+          if !enable then begin
+            incr nalloc_minor; if !nalloc_minor mod 100 = 0 then Gc.minor ();
+            Some (ref 42)
+          end else begin
+            allocate_arrays 1 250 1 true;
+            None
+          end)
+        ~major_alloc_callback:(fun _ -> assert false)
+        ~promote_callback:(fun k ->
+           assert (!k = 42 && !promotes_allowed);
+           incr npromote; if !npromote mod 1097 = 0 then Gc.minor ();
+           Some (ref 17))
+        ~minor_dealloc_callback:(fun k -> assert (!k = 42); incr ndealloc_minor)
+        ~major_dealloc_callback:(fun r -> assert (!r = 17); incr ndealloc_major)
+        ~sampling_rate:0.01 ();
   allocate_arrays 1 250 100 true;
   enable := false;
   assert (!ndealloc_minor = 0 && !ndealloc_major = 0);
@@ -105,14 +100,11 @@ let check_no_nested () =
     ()
   in
   let cb' _ = cb (); Some () in
-  start ~callstack_size:10 ~sampling_rate:1.
-    {
-      alloc_minor = cb';
-      alloc_major = cb';
-      promote = cb';
-      dealloc_minor = cb;
-      dealloc_major = cb;
-    };
+  start ~callstack_size:10
+        ~minor_alloc_callback:cb' ~major_alloc_callback:cb'
+        ~promote_callback:cb' ~minor_dealloc_callback:cb
+        ~major_dealloc_callback:cb
+        ~sampling_rate:1. ();
   allocate_arrays 1 250 5 false;
   stop ()
 
@@ -121,17 +113,16 @@ let () = check_no_nested ()
 let check_distrib lo hi cnt rate =
   Printf.printf "check_distrib %d %d %d %f\n%!" lo hi cnt rate;
   let smp = ref 0 in
-  start ~callstack_size:10 ~sampling_rate:rate
-    { null_tracker with
-      alloc_major = (fun _ -> assert false);
-      alloc_minor = (fun info ->
-        assert (info.size >= lo && info.size <= hi);
-        assert (info.n_samples > 0);
-        assert (not info.unmarshalled);
-        smp := !smp + info.n_samples;
-        None
-      );
-    };
+  start ~callstack_size:10
+        ~major_alloc_callback:(fun _ -> assert false)
+        ~minor_alloc_callback:(fun info ->
+          assert (info.size >= lo && info.size <= hi);
+          assert (info.n_samples > 0);
+          assert (not info.unmarshalled);
+          smp := !smp + info.n_samples;
+          None
+        )
+        ~sampling_rate:rate ();
   allocate_arrays lo hi cnt false;
   stop ();
 
diff --git a/testsuite/tests/statmemprof/blocking_in_callback.ml b/testsuite/tests/statmemprof/blocking_in_callback.ml
index d5e8d2ce175b..b1562ba74f18 100644
--- a/testsuite/tests/statmemprof/blocking_in_callback.ml
+++ b/testsuite/tests/statmemprof/blocking_in_callback.ml
@@ -59,8 +59,11 @@ let rec go () =
 
 let () =
   let t = Thread.create go () in
-  Gc.Memprof.(start ~callstack_size:10 ~sampling_rate:1.
-    { null_tracker with alloc_minor = minor_alloc_callback; });
+  Gc.Memprof.start
+    ~callstack_size:10
+    ~minor_alloc_callback
+    ~major_alloc_callback:(fun _ -> None)
+    ~sampling_rate:1. ();
   Mutex.unlock mut;
   go ();
   Thread.join t;
diff --git a/testsuite/tests/statmemprof/callstacks.ml b/testsuite/tests/statmemprof/callstacks.ml
index c1064806f6e5..c921577ab89b 100644
--- a/testsuite/tests/statmemprof/callstacks.ml
+++ b/testsuite/tests/statmemprof/callstacks.ml
@@ -71,17 +71,16 @@ let allocators =
 let test alloc =
   Printf.printf "-----------\n%!";
   let callstack = ref None in
-  start ~callstack_size:10 ~sampling_rate:1.
-    { null_tracker with
-      alloc_minor = (fun info ->
-         callstack := Some info.callstack;
-         None
-      );
-      alloc_major = (fun info ->
-         callstack := Some info.callstack;
-         None
-      );
-    };
+  start ~callstack_size:10
+        ~minor_alloc_callback:(fun info ->
+           callstack := Some info.callstack;
+           None
+        )
+        ~major_alloc_callback:(fun info ->
+           callstack := Some info.callstack;
+           None
+        )
+        ~sampling_rate:1. ();
   alloc ();
   stop ();
   match !callstack with
diff --git a/testsuite/tests/statmemprof/callstacks.reference b/testsuite/tests/statmemprof/callstacks.reference
index bd0239d435c0..d5707751e659 100644
--- a/testsuite/tests/statmemprof/callstacks.reference
+++ b/testsuite/tests/statmemprof/callstacks.reference
@@ -1,74 +1,74 @@
 -----------
 Raised by primitive operation at file "callstacks.ml", line 11, characters 30-53
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "callstacks.ml", line 14, characters 30-76
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "callstacks.ml", line 19, characters 12-66
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "callstacks.ml", line 22, characters 30-60
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "callstacks.ml", line 25, characters 30-55
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "callstacks.ml", line 29, characters 12-62
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "callstacks.ml", line 32, characters 22-27
 Called from file "callstacks.ml", line 34, characters 30-65
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "callstacks.ml", line 37, characters 30-69
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "callstacks.ml", line 40, characters 30-73
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "callstacks.ml", line 44, characters 30-43
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "callstacks.ml", line 47, characters 28-33
 Called from file "callstacks.ml", line 49, characters 30-47
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "marshal.ml", line 61, characters 9-35
 Called from file "callstacks.ml", line 55, characters 12-87
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "callstacks.ml", line 58, characters 30-59
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
 -----------
 Raised by primitive operation at file "callstacks.ml", line 61, characters 37-43
 Called from file "callstacks.ml", line 63, characters 30-49
-Called from file "callstacks.ml", line 85, characters 2-10
+Called from file "callstacks.ml", line 84, characters 2-10
 Called from file "list.ml", line 110, characters 12-15
-Called from file "callstacks.ml", line 92, characters 2-27
+Called from file "callstacks.ml", line 91, characters 2-27
diff --git a/testsuite/tests/statmemprof/comballoc.byte.reference b/testsuite/tests/statmemprof/comballoc.byte.reference
index afbd1ed547c6..cb17fd093379 100644
--- a/testsuite/tests/statmemprof/comballoc.byte.reference
+++ b/testsuite/tests/statmemprof/comballoc.byte.reference
@@ -1,49 +1,49 @@
 2: 0.42 false
 Raised by primitive operation at file "comballoc.ml", line 16, characters 2-19
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 3: 0.42 false
 Raised by primitive operation at file "comballoc.ml", line 16, characters 6-18
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 4: 0.42 true
 Raised by primitive operation at file "comballoc.ml", line 13, characters 11-20
 Called from file "comballoc.ml", line 16, characters 13-17
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 2: 0.01 false
 Raised by primitive operation at file "comballoc.ml", line 16, characters 2-19
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 3: 0.01 false
 Raised by primitive operation at file "comballoc.ml", line 16, characters 6-18
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 4: 0.01 true
 Raised by primitive operation at file "comballoc.ml", line 13, characters 11-20
 Called from file "comballoc.ml", line 16, characters 13-17
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 2: 0.83 false
 Raised by primitive operation at file "comballoc.ml", line 16, characters 2-19
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 3: 0.83 false
 Raised by primitive operation at file "comballoc.ml", line 16, characters 6-18
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 4: 0.83 true
 Raised by primitive operation at file "comballoc.ml", line 13, characters 11-20
 Called from file "comballoc.ml", line 16, characters 13-17
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 OK
diff --git a/testsuite/tests/statmemprof/comballoc.ml b/testsuite/tests/statmemprof/comballoc.ml
index f2998594415c..9d135f47c7b4 100644
--- a/testsuite/tests/statmemprof/comballoc.ml
+++ b/testsuite/tests/statmemprof/comballoc.ml
@@ -20,21 +20,20 @@ let test sampling_rate =
   let deallocs = Array.make 257 0 in
   let promotes = Array.make 257 0 in
   let callstacks = Array.make 257 None in
-  start ~callstack_size:10  ~sampling_rate
-    { null_tracker with
-      alloc_minor = (fun info ->
-        allocs.(info.size) <- allocs.(info.size) + info.n_samples;
-        begin match callstacks.(info.size) with
-        | None -> callstacks.(info.size) <- Some info.callstack
-        | Some s -> assert (s = info.callstack)
-        end;
-        Some (info.size, info.n_samples));
-      dealloc_minor = (fun (sz,n) ->
-        deallocs.(sz) <- deallocs.(sz) + n);
-      promote = (fun (sz,n) ->
-        promotes.(sz) <- promotes.(sz) + n;
-        None);
-    };
+  start ~callstack_size:10
+    ~minor_alloc_callback:(fun info ->
+      allocs.(info.size) <- allocs.(info.size) + info.n_samples;
+      begin match callstacks.(info.size) with
+      | None -> callstacks.(info.size) <- Some info.callstack
+      | Some s -> assert (s = info.callstack)
+      end;
+      Some (info.size, info.n_samples))
+    ~minor_dealloc_callback:(fun (sz,n) ->
+      deallocs.(sz) <- deallocs.(sz) + n)
+    ~promote_callback:(fun (sz,n) ->
+      promotes.(sz) <- promotes.(sz) + n;
+      None)
+    ~sampling_rate ();
   let iter = 100_000 in
   let arr = Array.make iter (0,0,0,0) in
   for i = 0 to Array.length arr - 1 do
@@ -74,17 +73,17 @@ let () =
 let no_callback_after_stop trigger =
   let stopped = ref false in
   let cnt = ref 0 in
-  start ~callstack_size:0 ~sampling_rate:1.
-    { null_tracker with
-      alloc_minor = (fun info ->
-        assert(not !stopped);
-        incr cnt;
-        if !cnt > trigger then begin
-          stop ();
-          stopped := true
-        end;
-        None);
-    };
+  start ~callstack_size:0
+    ~minor_alloc_callback:(fun info ->
+      assert(not !stopped);
+      incr cnt;
+      if !cnt > trigger then begin
+        stop ();
+        stopped := true
+      end;
+      None
+    )
+    ~sampling_rate:1. ();
   for i = 0 to 1000 do ignore (Sys.opaque_identity f i) done;
   assert !stopped
 
diff --git a/testsuite/tests/statmemprof/comballoc.opt.reference b/testsuite/tests/statmemprof/comballoc.opt.reference
index 3c7eb6c09af1..073747c99391 100644
--- a/testsuite/tests/statmemprof/comballoc.opt.reference
+++ b/testsuite/tests/statmemprof/comballoc.opt.reference
@@ -1,49 +1,49 @@
 2: 0.42 false
 Raised by primitive operation at file "comballoc.ml", line 16, characters 2-19
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 3: 0.42 false
 Raised by primitive operation at file "comballoc.ml", line 16, characters 6-18
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 4: 0.42 true
 Raised by primitive operation at file "comballoc.ml" (inlined), line 13, characters 11-20
 Called from file "comballoc.ml", line 16, characters 13-17
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 2: 0.01 false
 Raised by primitive operation at file "comballoc.ml", line 16, characters 2-19
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 3: 0.01 false
 Raised by primitive operation at file "comballoc.ml", line 16, characters 6-18
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 4: 0.01 true
 Raised by primitive operation at file "comballoc.ml" (inlined), line 13, characters 11-20
 Called from file "comballoc.ml", line 16, characters 13-17
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 2: 0.83 false
 Raised by primitive operation at file "comballoc.ml", line 16, characters 2-19
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 3: 0.83 false
 Raised by primitive operation at file "comballoc.ml", line 16, characters 6-18
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 4: 0.83 true
 Raised by primitive operation at file "comballoc.ml" (inlined), line 13, characters 11-20
 Called from file "comballoc.ml", line 16, characters 13-17
-Called from file "comballoc.ml", line 41, characters 25-48
+Called from file "comballoc.ml", line 40, characters 25-48
 Called from file "list.ml", line 110, characters 12-15
-Called from file "comballoc.ml", line 71, characters 2-35
+Called from file "comballoc.ml", line 70, characters 2-35
 OK
diff --git a/testsuite/tests/statmemprof/exception_callback.ml b/testsuite/tests/statmemprof/exception_callback.ml
index 55dd5e555cbb..3280c778b009 100644
--- a/testsuite/tests/statmemprof/exception_callback.ml
+++ b/testsuite/tests/statmemprof/exception_callback.ml
@@ -4,12 +4,6 @@
 
 open Gc.Memprof
 
-let alloc_tracker on_alloc =
-  { null_tracker with
-    alloc_minor = (fun info -> on_alloc info; None);
-    alloc_major = (fun info -> on_alloc info; None);
-  }
-
 (* We don't want to print the backtrace. We just want to make sure the
    exception is printed.
    This also makes sure [Printexc] is loaded, otherwise we don't use
@@ -17,7 +11,9 @@ let alloc_tracker on_alloc =
 let _ = Printexc.record_backtrace false
 
 let _ =
-  start ~callstack_size:10 ~sampling_rate:1.
-    (alloc_tracker (fun _ -> failwith "callback failed"));
+  start ~callstack_size:10
+        ~minor_alloc_callback:(fun _ -> assert false)
+        ~major_alloc_callback:(fun _ -> assert false)
+        ~sampling_rate:1. ();
   ignore (Sys.opaque_identity (Array.make 200 0));
   stop ()
diff --git a/testsuite/tests/statmemprof/exception_callback.reference b/testsuite/tests/statmemprof/exception_callback.reference
index 6371f8249e29..b389da5b1fd2 100644
--- a/testsuite/tests/statmemprof/exception_callback.reference
+++ b/testsuite/tests/statmemprof/exception_callback.reference
@@ -1 +1 @@
-Fatal error: exception Failure("callback failed")
+Fatal error: exception File "exception_callback.ml", line 15, characters 40-46: Assertion failed
diff --git a/testsuite/tests/statmemprof/exception_callback_minor.ml b/testsuite/tests/statmemprof/exception_callback_minor.ml
index f51412327643..0b1f59ff50bf 100644
--- a/testsuite/tests/statmemprof/exception_callback_minor.ml
+++ b/testsuite/tests/statmemprof/exception_callback_minor.ml
@@ -11,10 +11,9 @@ open Gc.Memprof
 let _ = Printexc.record_backtrace false
 
 let _ =
-  start ~callstack_size:10 ~sampling_rate:1.
-    { null_tracker with
-      alloc_minor = (fun _ -> assert false);
-      alloc_major = (fun _ -> assert false);
-    };
+  start ~callstack_size:10
+        ~minor_alloc_callback:(fun _ -> assert false)
+        ~major_alloc_callback:(fun _ -> assert false)
+        ~sampling_rate:1. ();
   ignore (Sys.opaque_identity (ref (ref 42)));
   stop ()
diff --git a/testsuite/tests/statmemprof/exception_callback_minor.reference b/testsuite/tests/statmemprof/exception_callback_minor.reference
index af75fbbe9b2b..63e3b54836cc 100644
--- a/testsuite/tests/statmemprof/exception_callback_minor.reference
+++ b/testsuite/tests/statmemprof/exception_callback_minor.reference
@@ -1 +1 @@
-Fatal error: exception File "exception_callback_minor.ml", line 16, characters 30-36: Assertion failed
+Fatal error: exception File "exception_callback_minor.ml", line 15, characters 40-46: Assertion failed
diff --git a/testsuite/tests/statmemprof/intern.ml b/testsuite/tests/statmemprof/intern.ml
index 5a5ff558e383..2a0173f8967d 100644
--- a/testsuite/tests/statmemprof/intern.ml
+++ b/testsuite/tests/statmemprof/intern.ml
@@ -7,12 +7,6 @@
 
 open Gc.Memprof
 
-let alloc_tracker on_alloc =
-  { null_tracker with
-    alloc_minor = (fun info -> on_alloc info; None);
-    alloc_major = (fun info -> on_alloc info; None);
-  }
-
 type t = I of int | II of int * int | Cons of t
 let rec t_of_len = function
   | len when len <= 1 -> assert false
@@ -41,11 +35,12 @@ let[@inline never] do_intern lo hi cnt keep =
 let check_nosample () =
   Printf.printf "check_nosample\n%!";
   precompute_marshalled_data 2 3000;
-  let fail_on_alloc _ =
+  let cb _ =
     Printf.printf "Callback called with sampling_rate = 0\n";
     assert(false)
   in
-  start ~callstack_size:10 ~sampling_rate:0. (alloc_tracker fail_on_alloc);
+  start ~callstack_size:10 ~minor_alloc_callback:cb ~major_alloc_callback:cb
+        ~sampling_rate:0. ();
   do_intern 2 3000 1 false;
   stop ()
 
@@ -60,26 +55,25 @@ let check_counts_full_major force_promote =
   let npromote = ref 0 in
   let ndealloc_minor = ref 0 in
   let ndealloc_major = ref 0 in
-  start ~callstack_size:10 ~sampling_rate:0.01
-    {
-      alloc_minor = (fun _ ->
-        if not !enable then None
-        else Some (incr nalloc_minor)
-      );
-      alloc_major = (fun _ ->
-        if not !enable then None
-        else Some (incr nalloc_major)
-      );
-      promote = (fun _ ->
-        Some (incr npromote)
-      );
-      dealloc_minor = (fun _ ->
-        incr ndealloc_minor
-      );
-      dealloc_major = (fun _ ->
-        incr ndealloc_major
-      );
-    };
+  start ~callstack_size:10
+        ~minor_alloc_callback:(fun _ ->
+          if !enable then begin
+              incr nalloc_minor;
+              Some ()
+          end else
+            None)
+        ~major_alloc_callback:(fun _ ->
+           if !enable then begin
+             incr nalloc_major;
+             Some ()
+           end else
+             None)
+        ~promote_callback:(fun _ ->
+           incr npromote;
+           Some ())
+        ~minor_dealloc_callback:(fun _ -> incr ndealloc_minor)
+        ~major_dealloc_callback:(fun _ -> incr ndealloc_major)
+        ~sampling_rate:0.01 ();
   do_intern 2 3000 1 true;
   enable := false;
   assert (!ndealloc_minor = 0 && !ndealloc_major = 0);
@@ -117,14 +111,11 @@ let check_no_nested () =
     ()
   in
   let cb' _ = cb (); Some () in
-  start ~callstack_size:10 ~sampling_rate:1.
-    {
-      alloc_minor = cb';
-      alloc_major = cb';
-      promote = cb';
-      dealloc_minor = cb;
-      dealloc_major = cb;
-    };
+  start ~callstack_size:10
+        ~minor_alloc_callback:cb' ~major_alloc_callback:cb'
+        ~promote_callback:cb' ~minor_dealloc_callback:cb
+        ~major_dealloc_callback:cb
+        ~sampling_rate:1. ();
   do_intern 100 200 1 false;
   stop ()
 
@@ -134,7 +125,7 @@ let check_distrib lo hi cnt rate =
   Printf.printf "check_distrib %d %d %d %f\n%!" lo hi cnt rate;
   precompute_marshalled_data lo hi;
   let smp = ref 0 in
-  let alloc info =
+  let cb info =
     (* We also allocate the list constructor in the minor heap,
        so we filter that out. *)
     if info.unmarshalled then begin
@@ -142,8 +133,10 @@ let check_distrib lo hi cnt rate =
       assert (info.n_samples > 0);
       smp := !smp + info.n_samples
     end;
+    None
   in
-  start ~callstack_size:10 ~sampling_rate:rate (alloc_tracker alloc);
+  start ~callstack_size:10 ~major_alloc_callback:cb ~minor_alloc_callback:cb
+        ~sampling_rate:rate ();
   do_intern lo hi cnt false;
   stop ();
 
diff --git a/testsuite/tests/statmemprof/lists_in_minor.ml b/testsuite/tests/statmemprof/lists_in_minor.ml
index 7a3736a21f7f..8aaae3e545f4 100644
--- a/testsuite/tests/statmemprof/lists_in_minor.ml
+++ b/testsuite/tests/statmemprof/lists_in_minor.ml
@@ -19,16 +19,15 @@ let[@inline never] allocate_lists len cnt =
 let check_distrib len cnt rate =
   Printf.printf "check_distrib %d %d %f\n%!" len cnt rate;
   let smp = ref 0 in
-  start ~callstack_size:10 ~sampling_rate:rate
-    { null_tracker with
-      alloc_major = (fun _ -> assert false);
-      alloc_minor = (fun info ->
-        assert (info.size = 2);
-        assert (info.n_samples > 0);
-        assert (not info.unmarshalled);
-        smp := !smp + info.n_samples;
-        None);
-    };
+  start ~callstack_size:10
+        ~major_alloc_callback:(fun _ -> assert false)
+        ~minor_alloc_callback:(fun info ->
+          assert (info.size = 2);
+          assert (info.n_samples > 0);
+          assert (not info.unmarshalled);
+          smp := !smp + info.n_samples;
+          None)
+        ~sampling_rate:rate ();
   allocate_lists len cnt;
   stop ();
 
diff --git a/testsuite/tests/statmemprof/minor_no_postpone.ml b/testsuite/tests/statmemprof/minor_no_postpone.ml
index 9d9ecd7917ef..cd5f0e7298ae 100644
--- a/testsuite/tests/statmemprof/minor_no_postpone.ml
+++ b/testsuite/tests/statmemprof/minor_no_postpone.ml
@@ -4,19 +4,15 @@
 
 open Gc.Memprof
 
-let notify_minor ref_ok ref_done =
-  { null_tracker with
-    alloc_minor = (fun _ ->
-      assert !ref_ok;
-      ref_done := true;
-      None);
-  }
-
 let () =
   let callback_ok = ref true in
   let callback_done = ref false in
-  start ~callstack_size:0 ~sampling_rate:1.
-    (notify_minor callback_ok callback_done);
+  start ~callstack_size:0
+        ~minor_alloc_callback:(fun _ ->
+          assert !callback_ok;
+          callback_done := true;
+          None)
+        ~sampling_rate:1. ();
   ignore (Sys.opaque_identity (ref 0));
   assert(!callback_done);
   callback_ok := false;
@@ -27,8 +23,12 @@ external alloc_stub : unit -> unit ref = "alloc_stub"
 let () =
   let callback_ok = ref false in
   let callback_done = ref false in
-  start ~callstack_size:0 ~sampling_rate:1.
-    (notify_minor callback_ok callback_done);
+  start ~callstack_size:0
+        ~minor_alloc_callback:(fun _ ->
+          assert !callback_ok;
+          callback_done := true;
+          None)
+        ~sampling_rate:1. ();
   ignore (Sys.opaque_identity (alloc_stub ()));
   assert(not !callback_done);
   callback_ok := true;