Fwd: [Beowulf] OpenMP on AMD dual core processors

Nathan Moore ntmoore at gmail.com
Fri Nov 21 07:35:15 PST 2008


---------- Forwarded message ----------
From: Nathan Moore <ntmoore at gmail.com>
Date: Fri, Nov 21, 2008 at 9:35 AM
Subject: Re: [Beowulf] OpenMP on AMD dual core processors
To: Bill Broadley <bill at cse.ucdavis.edu>


You're right about the recursive definition,

    v(i,j) = 0.25*(v(i-1,j)+v(i+1,j)+v(i,j+1)+v(i,j-1))

It is an old serial programming trick that makes the computation go faster
with little convergence penalty. I was thinking that two arrays would have a
memory latency (reading in and out simultaneously), but I see what you mean
about forcing the computation to be serial.



On Thu, Nov 20, 2008 at 11:47 PM, Bill Broadley <bill at cse.ucdavis.edu>wrote:

> OpenMP only works on loops that are independent.  So something like:
> do j=1,Ny
>   v(j) = v(j) + 1
>
> So 100 CPUs could each run with a different value for J and not conflict.
>
> Your code however:
> do i=1,Nx
>  do j=1,Ny
>    if(boundary(i,j).eq.0) then
>     old_v = v(i,j)
>     v(i,j) = 0.25*(v(i-1,j)+v(i+1,j)+v(i,j+1)+v(i,j-1))
>
> Neither the i loop nor the j loop can be parallelized because the value if
> i-1
> and j-1 have been referenced.  Does that code even work?  Is it intentional
> that the v(i-1) value is from the current iteration, but v(i+1) value is
> from
> the previous iteration?
>
> Seems like a much better idea to have a new array that is built entirely
> from
> the previous timestep.  That would allow it to converge faster, coverge is
> more cases, and also parallelize.
>
> Make sense?
>



-- 
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Nathan Moore
Assistant Professor, Physics
Winona State University
AIM: nmoorewsu
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-- 
- - - - - - -   - - - - - - -   - - - - - - -
Nathan Moore
Assistant Professor, Physics
Winona State University
AIM: nmoorewsu
- - - - - - -   - - - - - - -   - - - - - - -
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