Speed up supervisor:count_children/1; simple_one_for_one

Speed up supervisor:count_children/1 for simple_one_for_one
supervisors. This is achieved by avoiding looping through all the
child process and verifying that each one is alive.

For a supervisor with 100,000 'temporary' children the count-time will
drop from approx 25ms to about 0.005ms.

For a supervisor with 100,000 'permanent' or 'transient' children the
count-time will drop from approx 30ms to about 0.005ms.

This avoids having the supervisor block for an extended period while
the count takes place. Under normal circumstances the accuracy of the
result should also improve since the duration is too short for many
processes to die during the count.

1 files changed, 23 insertions, 26 deletions

diff --git a/lib/stdlib/src/supervisor.erl b/lib/stdlib/src/supervisor.erl
index 6a5005b4b6..cecdebd0c8 100644
--- a/lib/stdlib/src/supervisor.erl
+++ b/lib/stdlib/src/supervisor.erl
@@ -116,6 +116,7 @@
 		intensity              :: non_neg_integer(),
 		period                 :: pos_integer(),
 		restarts = [],
+		dynamic_restarts = 0   :: non_neg_integer(),
 	        module,
 	        args}).
 -type state() :: #state{}.
@@ -518,39 +519,26 @@ handle_call(which_children, _From, State) ->
 handle_call(count_children, _From, #state{children = [#child{restart_type = temporary,
 							     child_type = CT}]} = State)
   when ?is_simple(State) ->
-    {Active, Count} =
-	?SETS:fold(fun(Pid, {Alive, Tot}) ->
-			   case is_pid(Pid) andalso is_process_alive(Pid) of
-			       true ->{Alive+1, Tot +1};
-			       false ->
-				   {Alive, Tot + 1}
-			   end
-		   end, {0, 0}, dynamics_db(temporary, State#state.dynamics)),
+    Sz = ?SETS:size(dynamics_db(temporary, State#state.dynamics)),
     Reply = case CT of
-		supervisor -> [{specs, 1}, {active, Active},
-			       {supervisors, Count}, {workers, 0}];
-		worker -> [{specs, 1}, {active, Active},
-			   {supervisors, 0}, {workers, Count}]
+		supervisor -> [{specs, 1}, {active, Sz},
+			       {supervisors, Sz}, {workers, 0}];
+		worker -> [{specs, 1}, {active, Sz},
+			   {supervisors, 0}, {workers, Sz}]
 	    end,
     {reply, Reply, State};
 
-handle_call(count_children, _From,  #state{children = [#child{restart_type = RType,
+handle_call(count_children, _From,  #state{dynamic_restarts = Restarts,
+					   children = [#child{restart_type = RType,
 							      child_type = CT}]} = State)
   when ?is_simple(State) ->
-    {Active, Count} =
-	?DICTS:fold(fun(Pid, _Val, {Alive, Tot}) ->
-			   case is_pid(Pid) andalso is_process_alive(Pid) of
-			       true ->
-				   {Alive+1, Tot +1};
-			       false ->
-				   {Alive, Tot + 1}
-			   end
-		   end, {0, 0}, dynamics_db(RType, State#state.dynamics)),
+    Sz = ?DICTS:size(dynamics_db(RType, State#state.dynamics)),
+    Active = Sz - Restarts,
     Reply = case CT of
 		supervisor -> [{specs, 1}, {active, Active},
-			       {supervisors, Count}, {workers, 0}];
+			       {supervisors, Sz}, {workers, 0}];
 		worker -> [{specs, 1}, {active, Active},
-			   {supervisors, 0}, {workers, Count}]
+			   {supervisors, 0}, {workers, Sz}]
 	    end,
     {reply, Reply, State};
 
@@ -820,8 +808,15 @@ restart(Child, State) ->
 	    {shutdown, remove_child(Child, NState)}
     end.
 
-restart(simple_one_for_one, Child, State) ->
+restart(simple_one_for_one, Child, State0) ->
     #child{pid = OldPid, mfargs = {M, F, A}} = Child,
+    State = case OldPid of
+		?restarting(_) ->
+		    NRes = State0#state.dynamic_restarts - 1,
+		    State0#state{dynamic_restarts = NRes};
+		_ ->
+		    State0
+	    end,
     Dynamics = ?DICTS:erase(OldPid, dynamics_db(Child#child.restart_type,
 					       State#state.dynamics)),
     case do_start_child_i(M, F, A) of
@@ -832,7 +827,9 @@ restart(simple_one_for_one, Child, State) ->
 	    NState = State#state{dynamics = ?DICTS:store(Pid, A, Dynamics)},
 	    {ok, NState};
 	{error, Error} ->
-	    NState = State#state{dynamics = ?DICTS:store(restarting(OldPid), A,
+	    NRestarts = State#state.dynamic_restarts + 1,
+	    NState = State#state{dynamic_restarts = NRestarts,
+				 dynamics = ?DICTS:store(restarting(OldPid), A,
 							Dynamics)},
 	    report_error(start_error, Error, Child, State#state.name),
 	    {try_again, NState}