diff --git a/src/wpool.erl b/src/wpool.erl
index bbebfa3..126a849 100644
--- a/src/wpool.erl
+++ b/src/wpool.erl
@@ -18,14 +18,6 @@
 
 -behaviour(application).
 
--define(DEFAULTS,
-        [{overrun_warning, infinity},
-         {max_overrun_warnings, infinity},
-         {overrun_handler, {error_logger, warning_report}},
-         {workers, 100},
-         {worker_opt, []},
-         {queue_type, fifo}]).
-
 %% Copied from gen.erl
 -type debug_flag() :: trace | log | statistics | debug | {logfile, string()}.
 -type gen_option() ::
@@ -115,7 +107,7 @@ start_pool(Name) ->
 -spec start_pool(name(), [option()]) ->
                     {ok, pid()} | {error, {already_started, pid()} | term()}.
 start_pool(Name, Options) ->
-    wpool_pool:start_link(Name, all_opts(Options)).
+    wpool_pool:start_link(Name, wpool_utils:add_defaults(Options)).
 
 %% @doc Stops the pool
 -spec stop_pool(name()) -> true.
@@ -136,7 +128,7 @@ start_sup_pool(Name) ->
 -spec start_sup_pool(name(), [option()]) ->
                         {ok, pid()} | {error, {already_started, pid()} | term()}.
 start_sup_pool(Name, Options) ->
-    wpool_sup:start_pool(Name, all_opts(Options)).
+    wpool_sup:start_pool(Name, wpool_utils:add_defaults(Options)).
 
 %% @doc Stops the pool
 -spec stop_sup_pool(name()) -> ok.
@@ -243,9 +235,3 @@ get_workers(Sup) ->
 -spec broadcast(wpool:name(), term()) -> ok.
 broadcast(Sup, Cast) ->
     wpool_pool:broadcast(Sup, Cast).
-
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-%% PRIVATE
-%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
-all_opts(Options) ->
-    Options ++ ?DEFAULTS.
diff --git a/src/wpool_process.erl b/src/wpool_process.erl
index 15153d5..7e8a8bf 100644
--- a/src/wpool_process.erl
+++ b/src/wpool_process.erl
@@ -22,11 +22,16 @@
         {name :: atom(),
          mod :: atom(),
          state :: term(),
-         options :: [{time_checker | queue_manager, atom()} | wpool:option()]}).
+         options ::
+             #{time_checker := atom(),
+               queue_manager := atom(),
+               overrun_warning := timeout(),
+               _ => _}}).
 
 -type state() :: #state{}.
 -type from() :: {pid(), reference()}.
 -type next_step() :: timeout() | hibernate | {continue, term()}.
+-type options() :: [{time_checker | queue_manager, atom()} | wpool:option()].
 
 %% api
 -export([start_link/4, call/3, cast/2, send_request/2]).
@@ -38,13 +43,14 @@
 %%% API
 %%%===================================================================
 %% @doc Starts a named process
--spec start_link(wpool:name(), module(), term(), [wpool:option()]) ->
+-spec start_link(wpool:name(), module(), term(), options()) ->
                     {ok, pid()} | ignore | {error, {already_started, pid()} | term()}.
 start_link(Name, Module, InitArgs, Options) ->
-    WorkerOpt = proplists:get_value(worker_opt, Options, []),
+    FullOpts = wpool_utils:add_defaults(Options),
+    WorkerOpt = proplists:get_value(worker_opt, FullOpts, []),
     gen_server:start_link({local, Name},
                           ?MODULE,
-                          {Name, Module, InitArgs, Options},
+                          {Name, Module, InitArgs, FullOpts},
                           WorkerOpt).
 
 %% @equiv gen_server:call(Process, Call, Timeout)
@@ -66,9 +72,10 @@ send_request(Name, Request) ->
 %%% init, terminate, code_change, info callbacks
 %%%===================================================================
 %% @private
--spec init({atom(), atom(), term(), [wpool:option()]}) ->
+-spec init({atom(), atom(), term(), options()}) ->
               {ok, state()} | {ok, state(), next_step()} | {stop, can_not_ignore} | {stop, term()}.
-init({Name, Mod, InitArgs, Options}) ->
+init({Name, Mod, InitArgs, LOptions}) ->
+    Options = maps:from_list(LOptions),
     wpool_process_callbacks:notify(handle_init_start, Options, [Name]),
     case Mod:init(InitArgs) of
         {ok, ModState} ->
@@ -180,15 +187,9 @@ format_status(Opt, [PDict, State]) ->
 %% @private
 -spec handle_cast(term(), state()) ->
                      {noreply, state()} | {noreply, state(), next_step()} | {stop, term(), state()}.
-handle_cast(Cast, State) ->
-    Task =
-        wpool_utils:task_init({cast, Cast},
-                              proplists:get_value(time_checker, State#state.options, undefined),
-                              proplists:get_value(overrun_warning, State#state.options, infinity),
-                              proplists:get_value(max_overrun_warnings,
-                                                  State#state.options,
-                                                  infinity)),
-    ok = notify_queue_manager(worker_busy, State#state.name, State#state.options),
+handle_cast(Cast, #state{options = Options} = State) ->
+    Task = wpool_utils:task_init({cast, Cast}, Options),
+    ok = notify_queue_manager(worker_busy, State#state.name, Options),
     Reply =
         try (State#state.mod):handle_cast(Cast, State#state.state) of
             {noreply, NewState} ->
@@ -206,7 +207,7 @@ handle_cast(Cast, State) ->
                 {stop, Reason, State#state{state = NewState}}
         end,
     wpool_utils:task_end(Task),
-    ok = notify_queue_manager(worker_ready, State#state.name, State#state.options),
+    ok = notify_queue_manager(worker_ready, State#state.name, Options),
     Reply.
 
 %% @private
@@ -217,15 +218,9 @@ handle_cast(Cast, State) ->
                      {noreply, state(), next_step()} |
                      {stop, term(), term(), state()} |
                      {stop, term(), state()}.
-handle_call(Call, From, State) ->
-    Task =
-        wpool_utils:task_init({call, Call},
-                              proplists:get_value(time_checker, State#state.options, undefined),
-                              proplists:get_value(overrun_warning, State#state.options, infinity),
-                              proplists:get_value(max_overrun_warnings,
-                                                  State#state.options,
-                                                  infinity)),
-    ok = notify_queue_manager(worker_busy, State#state.name, State#state.options),
+handle_call(Call, From, #state{options = Options} = State) ->
+    Task = wpool_utils:task_init({call, Call}, Options),
+    ok = notify_queue_manager(worker_busy, State#state.name, Options),
     Reply =
         try (State#state.mod):handle_call(Call, From, State#state.state) of
             {noreply, NewState} ->
@@ -255,13 +250,10 @@ handle_call(Call, From, State) ->
                 {stop, Reason, Response, State#state{state = NewState}}
         end,
     wpool_utils:task_end(Task),
-    ok = notify_queue_manager(worker_ready, State#state.name, State#state.options),
+    ok = notify_queue_manager(worker_ready, State#state.name, Options),
     Reply.
 
-notify_queue_manager(Function, Name, Options) ->
-    case proplists:get_value(queue_manager, Options) of
-        undefined ->
-            ok;
-        QueueManager ->
-            wpool_queue_manager:Function(QueueManager, Name)
-    end.
+notify_queue_manager(Function, Name, #{queue_manager := QueueManager}) ->
+    wpool_queue_manager:Function(QueueManager, Name);
+notify_queue_manager(_, _, _) ->
+    ok.
diff --git a/src/wpool_process_callbacks.erl b/src/wpool_process_callbacks.erl
index 5ed7623..6f59897 100644
--- a/src/wpool_process_callbacks.erl
+++ b/src/wpool_process_callbacks.erl
@@ -35,14 +35,11 @@ handle_call(Msg, State) ->
     {ok, {error, {unexpected_call, Msg}}, State}.
 
 %% @doc Sends a notification to all registered callback modules.
--spec notify(event(), [wpool:option()], [any()]) -> ok.
-notify(Event, Options, Args) ->
-    case lists:keyfind(event_manager, 1, Options) of
-        {event_manager, EventMgr} ->
-            gen_event:notify(EventMgr, {Event, Args});
-        _ ->
-            ok
-    end.
+-spec notify(event(), #{event_manager := any(), _ => _}, [any()]) -> ok.
+notify(Event, #{event_manager := EventMgr}, Args) ->
+    gen_event:notify(EventMgr, {Event, Args});
+notify(_, _, _) ->
+    ok.
 
 %% @doc Adds a callback module.
 -spec add_callback_module(wpool:name(), module()) -> ok | {error, any()}.
diff --git a/src/wpool_queue_manager.erl b/src/wpool_queue_manager.erl
index 1afba3f..add2e12 100644
--- a/src/wpool_queue_manager.erl
+++ b/src/wpool_queue_manager.erl
@@ -27,7 +27,7 @@
         {wpool :: wpool:name(),
          clients :: queue:queue({cast | {pid(), _}, term()}),
          workers :: gb_sets:set(atom()),
-         monitors :: gb_trees:tree(atom(), monitored_from()),
+         monitors :: #{atom() := monitored_from()},
          queue_type :: queue_type()}).
 
 -type state() :: #state{}.
@@ -128,7 +128,7 @@ init(Args) ->
      #state{wpool = WPool,
             clients = queue:new(),
             workers = gb_sets:new(),
-            monitors = gb_trees:empty(),
+            monitors = #{},
             queue_type = QueueType}}.
 
 -type worker_event() :: new_worker | worker_dead | worker_busy | worker_ready.
@@ -149,12 +149,11 @@ handle_cast({worker_ready, Worker}, State0) ->
            queue_type = QueueType} =
         State0,
     State =
-        case gb_trees:is_defined(Worker, Mons) of
-            true ->
-                {Ref, _Client} = gb_trees:get(Worker, Mons),
+        case Mons of
+            #{Worker := {Ref, _Client}} ->
                 demonitor(Ref, [flush]),
-                State0#state{monitors = gb_trees:delete(Worker, Mons)};
-            false ->
+                State0#state{monitors = maps:remove(Worker, Mons)};
+            _ ->
                 State0
         end,
     case queue_out(Clients, QueueType) of
@@ -218,12 +217,11 @@ handle_call(pending_task_count, _From, State) ->
 handle_info({'DOWN', Ref, Type, {Worker, _Node}, Exit}, State) ->
     handle_info({'DOWN', Ref, Type, Worker, Exit}, State);
 handle_info({'DOWN', _, _, Worker, Exit}, State = #state{monitors = Mons}) ->
-    case gb_trees:is_defined(Worker, Mons) of
-        true ->
-            {_Ref, Client} = gb_trees:get(Worker, Mons),
+    case Mons of
+        #{Worker := {_Ref, Client}} ->
             gen_server:reply(Client, {'EXIT', Worker, Exit}),
-            {noreply, State#state{monitors = gb_trees:delete(Worker, Mons)}};
-        false ->
+            {noreply, State#state{monitors = maps:remove(Worker, Mons)}};
+        _ ->
             {noreply, State}
     end;
 handle_info(_Info, State) ->
@@ -286,7 +284,7 @@ now_in_milliseconds() ->
 
 monitor_worker(Worker, Client, State = #state{monitors = Mons}) ->
     Ref = monitor(process, Worker),
-    State#state{monitors = gb_trees:enter(Worker, {Ref, Client}, Mons)}.
+    State#state{monitors = maps:put(Worker, {Ref, Client}, Mons)}.
 
 queue_out(Clients, fifo) ->
     queue:out(Clients);
diff --git a/src/wpool_utils.erl b/src/wpool_utils.erl
index c9bb9c0..dbb4c78 100644
--- a/src/wpool_utils.erl
+++ b/src/wpool_utils.erl
@@ -18,26 +18,25 @@
 -author('ferigis@gmail.com').
 
 %% API
--export([task_init/4, task_end/1]).
+-export([task_init/2, task_end/1, add_defaults/1]).
 
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 %% Api
 %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
 
 %% @doc Marks Task as started in this worker
--spec task_init(term(), atom(), infinity | pos_integer(), infinity | pos_integer()) ->
+-spec task_init(term(), #{overrun_warning := timeout(), _ => _}) ->
                    undefined | reference().
-task_init(Task, _TimeChecker, infinity, _MaxWarnings) ->
-    Time =
-        calendar:datetime_to_gregorian_seconds(
-            calendar:universal_time()),
+task_init(Task, #{overrun_warning := infinity}) ->
+    Time = erlang:system_time(second),
     erlang:put(wpool_task, {undefined, Time, Task}),
     undefined;
-task_init(Task, TimeChecker, OverrunTime, MaxWarnings) ->
+task_init(Task,
+          #{overrun_warning := OverrunTime,
+            time_checker := TimeChecker,
+            max_overrun_warnings := MaxWarnings}) ->
     TaskId = erlang:make_ref(),
-    Time =
-        calendar:datetime_to_gregorian_seconds(
-            calendar:universal_time()),
+    Time = erlang:system_time(second),
     erlang:put(wpool_task, {TaskId, Time, Task}),
     erlang:send_after(OverrunTime,
                       TimeChecker,
@@ -50,3 +49,16 @@ task_end(undefined) ->
 task_end(TimerRef) ->
     _ = erlang:cancel_timer(TimerRef),
     erlang:erase(wpool_task).
+
+-spec add_defaults([wpool:option()]) -> [wpool:option()].
+add_defaults(Opts) ->
+    lists:ukeymerge(1, lists:sort(Opts), defaults()).
+
+-spec defaults() -> [wpool:option()].
+defaults() ->
+    [{max_overrun_warnings, infinity},
+     {overrun_handler, {error_logger, warning_report}},
+     {overrun_warning, infinity},
+     {queue_type, fifo},
+     {worker_opt, []},
+     {workers, 100}].