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Erlang match_spec引擎介绍和应用

October 7th, 2011 2 comments

match_spec是什么呢?

A “match specification” (match_spec) is an Erlang term describing a small “program” that will try to match something (either the parameters to a function as used in the erlang:trace_pattern/2 BIF, or the objects in an ETS table.). The match_spec in many ways works like a small function in Erlang, but is interpreted/compiled by the Erlang runtime system to something much more efficient than calling an Erlang function. The match_spec is also very limited compared to the expressiveness of real Erlang functions.

具体参见这里
说白了它就是个erlang term得过滤器,可以让用户来自己选择需要匹配什么,需要从term里面抽取什么数据。那同学可能就有疑问了,Erlang的函数不是很强大吗,它能做的函数也能做,那为什么要重新费劲做一个呢?
Erlang实现这个match_spec得原因有2个:1. 运行效率 2. 小巧可以在运行期使用。

它的实现思路是: match_spec是个引擎,有自己的语法,先把语句编译成专用的opcode, 然后在在匹配的时候运行opcode,获取结果,可以理解为erlang的DSL。

接下来我带大家先感性的认识下这个DSL:
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Erlang R14B04 released

October 5th, 2011 Comments off

参见官方网站: http://www.erlang.org/news/26, 主要变更如下:

Erlang/OTP R14B04 has been released as planned on October 5:th 2011. It is the fourth R14 service release.

See the release notes in the readme file.

Download the new release from the download page.

This release is mainly a stabilization of the R14B03 release (but as usual there are some new functionality as well).

这次发布主要是bug fix版本,修正了大量的小问题, 谢谢OTP团队!
具体见这里

祝玩得开心!

Categories: Erlang探索 Tags: ,

Erlang Shell实用小技巧

October 5th, 2011 3 comments

Erlang Shell下有很多内置的命令,在平时交互的时候很好用,文档里面都是一行带过,大家可能没什么感觉。
我来重点讲解和演示下:

$ erl
Erlang R14B04 (erts-5.8.5) [source][/source][/source] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.8.5  (abort with ^G)
1> help().
** shell internal commands **
b()        -- display all variable bindings
e(N)       -- repeat the expression in query <N>
f()        -- forget all variable bindings
f(X)       -- forget the binding of variable X
h()        -- history
history(N) -- set how many previous commands to keep
results(N) -- set how many previous command results to keep
catch_exception(B) -- how exceptions are handled
v(N)       -- use the value of query <N>
rd(R,D)    -- define a record
rf()       -- remove all record information
rf(R)      -- remove record information about R
rl()       -- display all record information
rl(R)      -- display record information about R
rp(Term)   -- display Term using the shell's record information
rr(File)   -- read record information from File (wildcards allowed)
rr(F,R)    -- read selected record information from file(s)
rr(F,R,O)  -- read selected record information with options

我最经常用的有以下几个,熟悉了感觉就很爽:
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Categories: Erlang探索 Tags:

Erlang进程简单的主动负载管制实现

October 5th, 2011 Comments off

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!

Categories: Erlang探索 Tags: ,

seq_trace集群消息链跟踪利器

October 3rd, 2011 Comments off

做过网络集群服务器的的同学都知道,集群服务通常由不同的服务器组成,这些不同角色的服务器组合在一起共同完成了特定的服务。一个服务通常需要一个协调者,和不同的工作者。 协调者负责派发任务,接收工作者的完成情况,最终回馈给用户。举个例子来讲,拨打电话:首先需要确认你的号码是在有效的,同时还要看下你的帐号里面有钱不,还要看下你拨打的电话号码是不是由权限,电话打的时候需要扣钱,等等。 这些服务中间的任何一个环节出问题了,服务就不正常了。那么我们在服务出问题的时候,如何定位问题呢?通常的办法是打日志,在所有的参与服务的节点上打开日志记录,之后到所有的节点上收集日志,集中分析日志,相当的麻烦。

这时候seq_trace来救助了,seq_trace的目标就是能够跟踪一条消息经过的所有环节,最终把路径展现给用户。
铺垫材料:
seq_trace工作原理,请参考这里
ttb对seq_trace的支持参考这里
tdbg对seq_trace的支持参考这里
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Categories: Erlang探索 Tags: , , ,

hibernate使用注意事项

October 2nd, 2011 2 comments

hibernate的作用是在进程闲的时候或者内存紧张的时候,通过重新整理进程的堆和栈内存来减少内存的消耗,同时维持进程之前的状态,但是误用会引起些问题,这里我们来展开下。

erlang:hibernate文档参考这里
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Categories: Erlang探索 Tags: ,

Erlang代码反编译以及查看汇编码

October 2nd, 2011 1 comment

Erlang的代码是先翻译成abstract_code,再到目标代码的,如果有符号信息很容易恢复源代码,通常我们部署系统的时候需要把符号信息去掉,reltool就可以干这个事情!

我们演示下:
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