系统技术非业余研究

我们知道Erlang的调度器是公平的，当进程的时间片用完了后，会强制切出，但是这个粒度是比较粗的。比如说进程进行了大量的Io操作，这个操作换成时间片是不准确的，会导致某些CPU计算密集型的比较吃亏，IO密集型的合算。

为了避免这个情况，IO密集型的经常可以互动要求短暂放弃执行，最简单的方法就是用消息等待机制。当进程在等消息的时候，就会被切出，我们就达到目的。

我们可以参考mnesia的实现：

%%mnesia_dumper.erl:L1181
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%                                                       
%% Load regulator                                                                                                            
%%                                                                                                                           
%% This is a poor mans substitute for a fair scheduler algorithm                                                             
%% in the Erlang emulator. The mnesia_dumper process performs many                                                           
%% costly BIF invokations and must pay for this. But since the                                                               
%% Emulator does not handle this properly we must compensate for                                                             
%% this with some form of load regulation of ourselves in order to                                                           
%% not steal all computation power in the Erlang Emulator ans make                                                           
%% other processes starve. Hopefully this is a temporary solution.                                                           

start_regulator() ->
    case mnesia_monitor:get_env(dump_log_load_regulation) of
        false ->
            nopid;
        true ->
            N = ?REGULATOR_NAME,
            case mnesia_monitor:start_proc(N, ?MODULE, regulator_init, [self()]) of
                {ok, Pid} ->
                    Pid;
                {error, Reason} ->
                    fatal("Failed to start ~n: ~p~n", [N, Reason])
            end
    end.

regulator_init(Parent) ->
    %% No need for trapping exits.                                                                                           
    %% Using low priority causes the regulation                                                                              
    process_flag(priority, low),
    register(?REGULATOR_NAME, self()),
    proc_lib:init_ack(Parent, {ok, self()}),
    regulator_loop().

regulator_loop() ->
    receive
        {regulate, From} ->
            From ! {regulated, self()},
            regulator_loop();
        {stop, From} ->
            From ! {stopped, self()},
            exit(normal)
    end.

regulate(nopid) ->
    ok;
regulate(RegulatorPid) ->
    RegulatorPid ! {regulate, self()},
    receive
        {regulated, RegulatorPid} -> ok
    end.

祝玩得开心!