主题：【求助】：请教信息技术方面的大牛有关多线程的问题 -- newtime

请教为何多线程执行起来花费时间比单线程要多呢？

非常感谢！！！

问题如下：

函数是用MONT CARLO方法计算圆周率。单线程计算函数两次共耗费大约是97s；而两个线程分别计算函数一次耗费的时间是392s.

环境：

CPU: i7920 4核

内存：3GB

操作系统：Windows Vista

编译器:Intel Fortran 90

程序如下：

多线程主程序：

Program TESTPROC0

use ifmt

use threadfunction

implicit none

integer,parameter::n_thread=2,n_c=2

integer(INT_PTR_KIND())::Threads_Handle(n_thread),waitresult,cend,retlog,retint,&

thread_handle_1,thread_handle_2

integer(INT_PTR_KIND()), PARAMETER :: security = 0

integer(INT_PTR_KIND()), PARAMETER :: stack_size = 0

integer(INT_PTR_KIND()) :: thread_id,rc

integer(4)::i,slot

real(kind(1d0))::pi,time_begin,time_end,t0,t1

call cpu_time(time_begin)

slot=0

Thread_Handle_1= CreateThread(security,stack_size,Thread_Proc0,loc(i), 0, thread_id)

Thread_Handle_2=CreateThread(security,stack_size,Thread_Proc0,loc(i), 0, thread_id)

waitresult=waitforsingleobject(thread_handle_1,infinite)

waitresult=waitforsingleobject(thread_handle_2,infinite)

cend=closehandle(thread_handle_1)

cend=closehandle(thread_handle_2)

call cpu_time(time_end)

t1=time_end-time_begin

write(6,*) "multithread computation takes time",t1,t1/n_c

pause

end program TESTPROC0

单线程主程序：

Program TESTPROC0

use threadfunction

implicit none

integer,parameter::n_c=2

integer(INT_PTR_KIND()) :: thread_id,rc

integer(4)::i,slot

real(kind(1d0))::pi,time_begin,time_end,t0,t1

call cpu_time(time_begin)

do i=1,n_c

pi=Thread_Proc0(i)

enddo

call cpu_time(time_end)

t0=time_end-time_begin

write(6,*) "computation takes time",t0,t0/n_c

pause

end program TESTPROC0

函数：

Module threadfunction

contains

real(kind(1d0)) function Thread_Proc0(arg) implicit none

!DEC$ ATTRIBUTES STDCALL, ALIAS:"_thread_proc0" :: Thread_Proc0

integer(4),intent(in):: arg

integer(4),parameter::j=999999999

integer(4)::i

real(kind(1d0))::x,y,u,q

q=0

do i=1,j

call random_number(x)

call random_number(y)

u=x**2.0d0+y**2.0d0

if (u<1.0d0) then

q=q+1.0d0

endif

enddo

Thread_proc0=q/j*4

end function Thread_Proc0

end Module threadfunction