答复: 庰瀻: [Omp] Could you help me about this problem?

[宋刚]

Hi,Ruud,
  Thank you for your kind reply.
  I know what you mean. I will try it later.
  When I get good news, I will tell you!
Yours sinecely,
David Song

-----邮件原件-----
发件人: Ruud.Vanderpas at Sun.COM [mailto:Ruud.Vanderpas at Sun.COM] 
发送时间: 2007年1月4日 lily 15:58
收件人: 宋炚
抄送: omp at openmp.org
主题: Re: ŽðžŽ: [Omp] Could you help me about this problem?

Hi David,

Thanks for the information.

> Just in the subroutine, I get the time like this.
> 
>  ¡­¡­
> 
>  real e,etime,t(2)
> 
>  e = etime(t)
> 
> do .....
> 
> ........
> 
> end do
> 
> e = etime(t)
> 
> print *,¡¯Elapsed time is :¡¯,e,

It seems that you assume "etime" returns the time elapsed
since the previous call to it. Did you check the documentation
to make sure this is the case?

In any case it might be better to use the omp_get_wtime call.
This returns the elapsed time in seconds. It is an absolute
value, so you need to subtract it from the previous call.
Something like this:

    t1 = omp_get_wtime()
    do ...
     ...
    end do
    t2 = omp_get_wtime() - t1

It will be useful to give it a try using this timer instead and
see what you get then.

>  The time I calculate is only in a subroutine.

Thanks.

> The result can be reproduced again. I tested it three more times.

That is interesting.

> Is there something wrong with the time I used? Previously, I also wrote 
> a OpenMP program to compress the data. On a SMP(2 cores) platform, when 
> 4 threads the performance is the best, the speed up sometimes can go to 
> 5.89, I didn¡¯t know the reason. On another platform, a SMP server(4-way 
> Dual core, there are 8 cores), the best performance can be got when 
> there are 12 threads, and the speed up can also be to about to 12. The 
> time  I got is like below:
> 
> time gzip data1
> 
> time ./ompgzip data1

There could be some superlinear behavior, but that is just a
wild guess. You'll probably need to use a profiling tool to
do some more in-depth analysis.

Kind regards,
Ruud
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