hibernate your gen_fsm process

关于erlang进程hibernate的好处，去过这次erlang大会的同学应该已经都知道了

简单的理解：
hibernate可以使你闲置的erlang进程马上进行gc,因为进程处于receive状态下是不会gc的
我在性能调优时发现，gc对于消息发送速度的影响还是非常大的

关于erlang gc问题也可以参看这个

引用

Recently, as part of RabbitMQ server development, we ran into an interesting issue regarding Erlang’s per-process garbage collection. If a process is idle — not doing any work at all, simply waiting for an external event — then its garbage-collector will not run until it starts working again. The solution is to hibernate idle processes, which causes a very aggressive garbage-collection run and puts the process into a suspended state, from which it will wake when a message next arrives.

所以查看rabbitMQ源代码可以看到里面的gen_server/fsm等进程统统都会使用hibernate

下面解释下如何让进程hibernate,这里需要注意的是一个进程hiberate后，会清空stack栈，也就是说之前的调用关系全部没有了，也就是说hibernate所在的函数永远不会返回，待有新消息时，进程从hibernate(M,F,A)里指定的M:F处开始运行

gen_server和gen_fsm都提供了hibernate的方式，2种方法

第一种：
在gen_server,或gen_fsm回调接口返回时，指定hibernate

{next_state, NStateName, NStateData, Time1} 

实际这个Time1可以指定为hibernate，则重新回到main loop时会直接hibernate

这种方法可能使你的进程频繁hibernate又被唤醒，效率不是很好

正确的方式应该是，当你预期你的进程一段时间内不会收到消息才hibernate

所以推荐下面的方法，直接看下示例代码：

%% ====================================================================
%% Interface Function
%% ====================================================================
join(UserId)->
	io:format("join UserId=~p~n",[UserId]),
	gen_fsm:sync_send_event(?MODULE, {join,UserId}).

start()->
	gen_fsm:send_event(?MODULE,start).

%% ====================================================================
%% Callback Function
%% ====================================================================
wait_player({join,UserId},_From, State) ->
	print_inner_data(State),
	put(p,get(p)+1),
	ets:insert(State#state.players,{p,UserId}),
	Count=State#state.count+1,
	if
		Count =:= 3 ->
			io:format("jump to next phrase -> start~n"),
			{reply,next_phrase,wait_start,State,5000};
		true->
			NewState=State#state{count=Count},
			{reply,stand_still,wait_player,NewState,5000}
	end.
wait_player(timeout,State)->
	io:format("timeout happened when wait_player,let's hibernate~n"),
	proc_lib:hibernate(gen_fsm, enter_loop, [?MODULE, [], wait_player,State]).

wait_start(start,State)->
	 print_inner_data(State),
	 io:format("recv start event ~n"),
	 {next_state,wait_start,State,5000};

wait_start(timeout,State)->
	io:format("timeout happened when wait_start,let's hibernate~n"),
	proc_lib:hibernate(gen_fsm, enter_loop, [?MODULE, [], wait_start,State]).

start_link()->
	gen_fsm:start_link({local,?MODULE}, ?MODULE, [] ,[]).

init([])->
	put(p,0),
	{ok, 
		wait_player, 
		#state
		{
			count=0,
			players=ets:new(players,[bag])
		},
		5000
	}.

print_inner_data(State)->
	io:format("inner data: dict=~p,ets=~p~n",[get(p),ets:tab2list(State#state.players)]).

运行：

2> {ok,Pid}=fsm:start_link().
{ok,<0.39.0>}

3> process_info(Pid,total_heap_size).
{total_heap_size,233}

4> fsm:join(123).
join UserId=123
inner data: dict=0,ets=[]
stand_still
timeout happened when wait_player,let's hibernate

5> process_info(Pid,total_heap_size).
{total_heap_size,33}

6> fsm:join(456).
join UserId=456
inner data: dict=1,ets=[{p,123}]
stand_still

总体上是让进程在main_loop里先timeout也就是证明一段时间内都没有消息到达，然后gen_fsm进程是发送{'gen_event',timeout}消息处理timeout事件的，所以要在每个Mod:StateName处都加个处理timeout的分支，来进行timeout处理，也就是hibernate掉自己，重新进入的是enter_loop 函数，这个东西很有用呐，大家可以查看源码，我就不多说了

另外，示例中可以看到,hibernate会清空stack，但是不会影响你的进程字典和ets等其它东西，可以放心用了