我是Erlang的新手,我正在阅读Joe Armstrong的书“并发编程”一章。我正在尝试运行一个进程列表来计算一个数字是否是一个素数(天真的方法)。但我的代码运行就像没有进程一样。两种方法具有相同的持续时间。我哪里错了?
shell.erl:
c(prime).
%a list of primes
NUMS=[102950143,102950143,102950143,102950143,102950143].
%time start
NOW1=now().
io:fwrite("Monothread~n").
%for each number test if it is a prime
lists:foreach( fun(N)->
RESULT=prime:is_prime(N),
io:fwrite("Result N=~p ~n",[RESULT])
end, NUMS).
%display the duration
io:fwrite("Duration N=~p ~n",[timer:now_diff(now(),NOW1)/1.0e6]).
%time start
NOW2=now().
io:fwrite("Multithread~n").
%for each number, spawn a new process and test if it is a prime
lists:foreach( fun(N)->ProcId = prime:start(),
io:fwrite("Calculating : procId=~p N=~p ~n",[ProcId,N]),
RESULT=prime:is_prime(ProcId,N),
io:fwrite("Result N=~p ~n",[RESULT])
end, NUMS).
%display the duration
io:fwrite("Duration N=~p ~n",[timer:now_diff(now(),NOW2)/1.0e6]).
halt().
file prime.erl:
-module(prime).
-export([start/0,is_prime/1,is_prime/2]).
%run the forever_function
start()->spawn(fun forever_function/0).
%catch the queries
forever_function()->
receive
{ From,Number} -> From! is_prime(self(),2,Number),
forever_function()
end.
%monothreaded function
is_prime(Number)->is_prime(self(),2,Number).
%multithreaded function
is_prime(ProcessID,Number)->
ProcessID ! {self(),Number},
receive
RESULT->RESULT
end.
%recursive function scanning all the numbers from 2 to Number
is_prime(ProcessID,Div,Number)->
if
Div =:= Number -> {{number,Number},{prime,true}};
Number rem Div =:= 0 -> {{number,Number},{prime,false}};
true-> is_prime(ProcessID,Div+1,Number)
end.
谢谢,
皮尔
答案 0 :(得分:12)
在您的列表中:在第二个区块中调用foreach,您正在调用RESULT=prime:is_prime(ProcId,N)
,即为结果执行receive
。因此,您正在生成一个进程,然后等待它完成,直到您生成下一个进程。这就是为什么它需要与单线程方法相同的时间:两种方式,你一次只做一次。
相反,您需要首先生成所有进程(使用lists:map
之类的内容来跟踪所有PID),然后在单独的步骤中等待结果。请注意,这意味着您必须将ProcessID ! {self(),Number}
部分从receive
部分拆分,以便在第一步中完成,否则您只会产生一堆空闲进程。
所以,比如:
Pids = lists:map( fun(N)-> ProcId = prime:start(N),
io:fwrite("Calculating : procId=~p N=~p ~n",[ProcId,N]),
ProcId end, NUMS).
lists:foreach( fun(ProcId) -> {N,RESULT}=prime:is_prime(ProcId),
io:fwrite("Result procId=~p N=~p Result=~p ~n", [ProcId,N,RESULT]) end, Pids).
start(N)->spawn(?MODULE, forever_function, [N]).
forever_function(Number)->
Result = is_prime(self(),2,Number),
receive
{ From, get_result } -> From! {Number,Result},
% unnecessary since we never call this Pid again, but do it anyway :)
forever_function()
% could also add more cases here to set the number
% or even do a one-shot function like before
end.
%multithreaded function
is_prime(ProcessID)->
ProcessID ! {self(),get_result},
receive
RESULT->RESULT
end.
注意:这是未经测试的,因此可能需要进行一些调整。