erlang · RobinMorisset · Sep 11, 2025
diff --git a/erts/doc/src/erlang_system_info.md b/erts/doc/src/erlang_system_info.md
@@ -55,6 +55,7 @@ order to make it easier to navigate.
   [`port_limit`](`m:erlang#system_info_port_limit`),
   [`process_count`](`m:erlang#system_info_process_count`),
   [`process_limit`](`m:erlang#system_info_process_limit`)
+  [`bif_timer_count`](`m:erlang#system_info_bif_timer_count`),
 
 - [`System Time`](`m:erlang#system_info/1-system-time`) -
   [`end_time`](`m:erlang#system_info_end_time`),
@@ -397,6 +398,13 @@ Returns information about the current system (emulator) limits as specified by `
   given as an integer. This limit can be configured at startup by using
   command-line flag [`+P`](erl_cmd.md#+P) in `erl(1)`.
 
+- `bif_timer_count`{: #system_info_bif_timer_count } - Returns the number of bif
+  timers currently existing at the local node. Bif timers are those created by `erlang:send_after`
+  and `erlang:start_timer`, but not those implicitly created by receive statements with timeouts.
+  The value is given as an integer.
+
+  Since OTP 29.0
+
 ## System Time
 
 Returns information about the current system (emulator) time as specified by `Item`:

diff --git a/erts/emulator/beam/atom.names b/erts/emulator/beam/atom.names
@@ -733,6 +733,7 @@ atom threads
 atom time_offset
 atom timeout
 atom timeout_value
+atom bif_timer_count
 atom Times='*'
 atom timestamp
 atom total

diff --git a/erts/emulator/beam/erl_bif_info.c b/erts/emulator/beam/erl_bif_info.c
@@ -2937,6 +2937,8 @@ BIF_RETTYPE system_info_1(BIF_ALIST_1)
 	hp = HAlloc(BIF_P, 3);
 	res = TUPLE2(hp, am_min_bin_vheap_size,make_small(BIN_VH_MIN_SIZE));
 	BIF_RET(res);
+    } else if (BIF_ARG_1 == am_bif_timer_count) {
+	BIF_RET(make_small(erts_bif_timer_count()));
     } else if (BIF_ARG_1 == am_process_count) {
 	BIF_RET(make_small(erts_ptab_count(&erts_proc)));
     } else if (BIF_ARG_1 == am_process_limit) {

diff --git a/erts/emulator/beam/erl_hl_timer.c b/erts/emulator/beam/erl_hl_timer.c
@@ -302,6 +302,7 @@ struct ErtsHLTimerService_ {
     ErtsHLTimer *next_timeout;
     ErtsYieldingTimeoutState yield;
     ErtsTWheelTimer service_timer;
+    Uint btm_count;
 };
 
 static ERTS_INLINE int
@@ -635,6 +636,7 @@ erts_create_timer_service(void)
     srv->btm_tree = NULL;
     srv->next_timeout = NULL;
     srv->yield = init_yield;
+    srv->btm_count = 0;
     erts_twheel_init_timer(&srv->service_timer);
 
     init_canceled_queue(&srv->canceled_queue);
@@ -707,6 +709,21 @@ port_timeout_common(Port *port, void *tmr)
     return 0;
 }
 
+static ERTS_INLINE void
+insert_btm(ErtsBifTimer *tmr, ErtsHLTimerService *srv)
+{
+    btm_rbt_insert(&srv->btm_tree, tmr);
+    ++ srv->btm_count;
+}
+
+static ERTS_INLINE void
+remove_btm(ErtsBifTimer *tmr, ErtsHLTimerService *srv)
+{
+    btm_rbt_delete(&srv->btm_tree, tmr);
+    -- srv->btm_count;
+    tmr->btm.tree.parent = ERTS_HLT_PFIELD_NOT_IN_TABLE;
+}
+
 static ERTS_INLINE erts_aint_t
 init_btm_specifics(ErtsSchedulerData *esdp,
                    ErtsBifTimer *tmr, Eterm msg,
@@ -732,7 +749,7 @@ init_btm_specifics(ErtsSchedulerData *esdp,
 
     tmr->btm.proc_tree.parent = ERTS_HLT_PFIELD_NOT_IN_TABLE;
 
-    btm_rbt_insert(&esdp->timer_service->btm_tree, tmr);
+    insert_btm(tmr, esdp->timer_service);
 
     erts_atomic32_init_nob(&tmr->btm.state, ERTS_TMR_STATE_ACTIVE);
     return refc; /* refc from magic binary... */
@@ -1142,8 +1159,7 @@ bif_timer_timeout(ErtsHLTimerService *srv,
     }
 
     if (tmr->btm.tree.parent != ERTS_HLT_PFIELD_NOT_IN_TABLE) {
-	btm_rbt_delete(&srv->btm_tree, tmr);
-	tmr->btm.tree.parent = ERTS_HLT_PFIELD_NOT_IN_TABLE;
+        remove_btm(tmr, srv);
     }
 
 }
@@ -1470,8 +1486,7 @@ cleanup_sched_local_canceled_timer(ErtsSchedulerData *esdp,
     if (roflgs & ERTS_TMR_ROFLG_BIF_TMR) {
         ErtsBifTimer *btm = (ErtsBifTimer *) tmr;
 	if (btm->btm.tree.parent != ERTS_HLT_PFIELD_NOT_IN_TABLE) {
-	    btm_rbt_delete(&esdp->timer_service->btm_tree, btm);
-	    btm->btm.tree.parent = ERTS_HLT_PFIELD_NOT_IN_TABLE;
+            remove_btm(btm, esdp->timer_service);
 	}
     }
 
@@ -1779,8 +1794,7 @@ setup_bif_timer(Process *c_p, int twheel, ErtsMonotonicTime timeout_pos,
 					  ERTS_P2P_FLG_INC_REFC);
 	if (!proc) {
             if (tmr->btm.tree.parent != ERTS_HLT_PFIELD_NOT_IN_TABLE) {
-                btm_rbt_delete(&esdp->timer_service->btm_tree, tmr);
-                tmr->btm.tree.parent = ERTS_HLT_PFIELD_NOT_IN_TABLE;
+                remove_btm(tmr, esdp->timer_service);
             }
 	    if (tmr->btm.bp)
 		free_message_buffer(tmr->btm.bp);
@@ -1884,8 +1898,7 @@ access_btm(ErtsBifTimer *tmr, Uint32 sid, ErtsSchedulerData *esdp, int cancel)
     }
     else {
         if (tmr->btm.tree.parent != ERTS_HLT_PFIELD_NOT_IN_TABLE) {
-	    btm_rbt_delete(&esdp->timer_service->btm_tree, tmr);
-	    tmr->btm.tree.parent = ERTS_HLT_PFIELD_NOT_IN_TABLE;
+	    remove_btm(tmr, esdp->timer_service);
 	}
         if (is_hlt) {
             if (cncl_res > 0)
@@ -2361,8 +2374,7 @@ exit_cancel_bif_timer(ErtsBifTimer *tmr, void *vesdp, Sint reds)
         }
 
         if (tmr->btm.tree.parent != ERTS_HLT_PFIELD_NOT_IN_TABLE) {
-	    btm_rbt_delete(&esdp->timer_service->btm_tree, tmr);
-	    tmr->btm.tree.parent = ERTS_HLT_PFIELD_NOT_IN_TABLE;
+            remove_btm(tmr, esdp->timer_service);
 	}
         if (is_hlt)
             hlt_delete_timer(esdp, &tmr->type.hlt);
@@ -2898,6 +2910,12 @@ erts_read_port_timer(Port *c_prt)
     return get_time_left(NULL, timeout_pos);
 }
 
+Uint
+erts_bif_timer_count_in_timer_service(ErtsHLTimerService *service)
+{
+    return service->btm_count;
+}
+
 /*
  * Debug stuff...
  */

diff --git a/erts/emulator/beam/erl_hl_timer.h b/erts/emulator/beam/erl_hl_timer.h
@@ -87,4 +87,6 @@ erts_debug_callback_timer_foreach(void (*tclbk)(void *),
 					       ErtsMonotonicTime,
 					       void *),
 				  void *arg);
+
+Uint erts_bif_timer_count_in_timer_service(ErtsHLTimerService *service);
 #endif /* ERL_HL_TIMER_H__ */
diff --git a/erts/emulator/beam/erl_process.c b/erts/emulator/beam/erl_process.c
@@ -15248,6 +15248,16 @@ void erts_halt(int code, ErtsMonotonicTime htmo)
     }
 }
 
+Uint erts_bif_timer_count()
+{
+    Uint result = 0;
+    ERTS_FOREACH_RUNQ(rq,
+        ErtsHLTimerService *srv = rq->scheduler->timer_service;
+        result += erts_bif_timer_count_in_timer_service(srv);
+    );
+    return result;
+}
+
 #if defined(ERTS_ENABLE_LOCK_CHECK)
 int
 erts_dbg_check_halloc_lock(Process *p)

diff --git a/erts/emulator/beam/erl_process.h b/erts/emulator/beam/erl_process.h
@@ -759,6 +759,7 @@ extern ErtsAlignedSchedulerData * ERTS_WRITE_UNLIKELY(erts_aligned_scheduler_dat
 extern ErtsAlignedSchedulerData * ERTS_WRITE_UNLIKELY(erts_aligned_dirty_cpu_scheduler_data);
 extern ErtsAlignedSchedulerData * ERTS_WRITE_UNLIKELY(erts_aligned_dirty_io_scheduler_data);
 
+Uint erts_bif_timer_count(void);
 
 #if defined(ERTS_ENABLE_LOCK_CHECK)
 int erts_lc_runq_is_locked(ErtsRunQueue *);

diff --git a/erts/emulator/test/system_info_SUITE.erl b/erts/emulator/test/system_info_SUITE.erl
@@ -39,7 +39,7 @@
 
 -export([process_count/1, system_version/1, misc_smoke_tests/1,
          heap_size/1, wordsize/1, memory/1, ets_limit/1, atom_limit/1,
-         procs_bug/1,
+         procs_bug/1, bif_timer_count/1,
          ets_count/1, atom_count/1, system_logger/1]).
 
 -export([init/1, handle_event/2, handle_call/2]).
@@ -53,7 +53,7 @@ suite() ->
 all() -> 
     [process_count, system_version, misc_smoke_tests,
      ets_count, heap_size, wordsize, memory, ets_limit, atom_limit, atom_count,
-     procs_bug,
+     procs_bug, bif_timer_count,
      system_logger].
 
 
@@ -519,6 +519,44 @@ ets_count_do(Opts) ->
     ets:delete(T),
     Before = erlang:system_info(ets_count).
 
+bif_timer_count(Config) when is_list(Config) ->
+    NumTmrs = 1000, ShortTimeout = 1_000, LongTimeout = 1_000_000_000,
+    bif_timer_count_single_threaded(NumTmrs, ShortTimeout),
+    bif_timer_count_single_threaded(NumTmrs, LongTimeout),
+    bif_timer_count_multi_threaded(NumTmrs, ShortTimeout),
+    bif_timer_count_multi_threaded(NumTmrs, LongTimeout),
+    ok.
+
+bif_timer_count_do(NumTmrs, Fun) ->
+    Before = erlang:system_info(bif_timer_count),
+    Tmrs = Fun(),
+    After = erlang:system_info(bif_timer_count),
+    MaxJitter = 10,
+    ActualJitter = abs(After - Before - NumTmrs),
+    true = ActualJitter < MaxJitter,
+    Tmrs.
+
+bif_timer_count_single_threaded(NumTmrs, Timeout) ->
+    Tmrs = bif_timer_count_do(NumTmrs, fun () ->
+        [erlang:start_timer(Timeout, self(), bif_timer_count_test) || _ <- lists:seq(1, NumTmrs)] end),
+    lists:foreach(fun (Tmr) -> erlang:cancel_timer(Tmr) end, Tmrs).
+
+bif_timer_count_multi_threaded(NumTmrs, Timeout) ->
+    SpawnerPid = self(),
+    _ = bif_timer_count_do(NumTmrs, fun () ->
+        [erlang:spawn_link(fun () ->
+            Tmr = erlang:start_timer(Timeout, SpawnerPid, bif_timer_count_test),
+            SpawnerPid ! {timer_initialized, Tmr},
+            % We're cancelling the timers in the spawned processes, because cancelling timers across schedulers is done lazily,
+            % and we want to make sure that the timers are cancelled before we start the next test (otherwise the second multithreaded test fails).
+            % 2s should be enough for the counting to be long finished (it takes about 1s to count 1M timers on my machine, so 2s for 1k timers is very conservative)
+            receive after 2_000 -> ok end,
+            erlang:cancel_timer(Tmr),
+            SpawnerPid ! timer_cancelled
+        end) || _ <- lists:seq(1, NumTmrs)],
+        [receive {timer_initialized, Tmr} -> Tmr end || _ <- lists:seq(1, NumTmrs)]
+        end),
+    [receive timer_cancelled -> ok end || _ <- lists:seq(1, NumTmrs)].
 
 %% Verify system_info(ets_limit) reflects max ETS table settings.
 ets_limit(Config0) when is_list(Config0) ->

diff --git a/erts/preloaded/src/erlang.erl b/erts/preloaded/src/erlang.erl
@@ -10251,6 +10251,7 @@ the `CpuTopology` type to change.
       Alloc :: atom();
          (atom_count) -> pos_integer();
          (atom_limit) -> pos_integer();
+         (bif_timer_count) -> non_neg_integer();
          (build_type) -> opt | debug |
                          gcov | valgrind | gprof | lcnt | frmptr;
          (c_compiler_used) -> {atom(), term()};

diff --git a/lib/dialyzer/src/erl_bif_types.erl b/lib/dialyzer/src/erl_bif_types.erl
@@ -957,6 +957,8 @@ type(erlang, system_info, 1, Xs) ->
 		     t_non_neg_fixnum();
 		   ['trace_control_word'] ->
 		     t_integer();
+		   ['bif_timer_count'] ->
+		     t_non_neg_fixnum();
 		   ['version'] ->
 		     t_string();
 		   ['wordsize'] ->