Hi abc,
First, your question does not seem very relevant to the LAM/MPI list !
Well, I feel you must think a bit more about the overall design of the code.
Your two loops are very quick, with complexity of order n, and present
no dependencies between different values of the index i. This means
they can be easily parallelized, but as they are so quick, the price of
moving the data across processors may be larger than the price of the
calculation itself. This implies that the data must be distributed and
stay distributed over your mpi processes for the parallelization to be
efficient.
So far so good, but then you have to understand what other dependencies
lie between the distributed data.
In particular I suspect complex dependencies are hidden within the
routine force. Also, I suspect that the real cpu time is used in force.
A good starting point might be to understand where the cpu is used in
the sequential code, using some profiling tool, in order to get some
input for the overall design of the parallelisation scheme.
Hoping this adds more understanding than confusion in your mind.
Pierre V.
abc abc wrote:
> hi everybody,
> can u pls help me parallelize the following code by inserting MPI
> calls. There are 2 DO loops that I am trying to parallelize but I am
> confused a bit. thanks for your time.
> ballo.
> -------------------------------------------------------------------------------------------------------
> subroutine mpi_code_f77
> integer step,deltim,dmptim,iseed
> parameter (n=4096)
> parameter (nlayer=128)
> common /control/ seth,setl,dt,yxrat,yxrat2,rnd,delr,
> & nsp1,nstep,mcstep,ndump,deltim,dmptim,step,iseed
> common /posvel/ isp(n),rx(n),ry(n),rz(n),
> & vx(n),vy(n),vz(n),ax(n),ay(n),az(n)
> common /potential/ s11,s12,s22,
> & rc11,rc11sq,rc12,rc12sq,rc22,rc22sq,
> & ep11,ep12,ep22,xm(2),grav
>
> dt2=dt/2.0d0
>
> do 1000 i=1,n
>
> if (rx(i).gt.0.5) then
> ax(i)=ax(i)-grav
> else
> ax(i)=ax(i)+grav
> endif
>
> vx(i)=vx(i)+ax(i)*dt2
> vy(i)=vy(i)+ay(i)*dt2
> vz(i)=vz(i)+az(i)*dt2
> rx(i)=rx(i)+vx(i)*dt
> if (rx(i).ge.1.0d0) rx(i)=rx(i)-1.0d0
> if (rx(i).lt.0.0d0) rx(i)=rx(i)+1.0d0
> ry(i)=ry(i)+vy(i)*dt
> if (ry(i).ge.yxrat) ry(i)=ry(i)-yxrat
> if (ry(i).lt.0.0d0) ry(i)=ry(i)+yxrat
> rz(i)=rz(i)+vz(i)*dt
> if (rz(i).ge.yxrat) rz(i)=rz(i)-yxrat
> if (rz(i).lt.0.0d0) rz(i)=rz(i)+yxrat
>
> 1000 continue
>
> call force
>
> do 1100 i=1,n
> vx(i)=vx(i)+ax(i)*dt2
> vy(i)=vy(i)+ay(i)*dt2
> vz(i)=vz(i)+az(i)*dt2
> 1100 continue
> return
> end
> ---------------------------------------------------------------------------------------------------------
>
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_/_/_/_/ _/ _/ Dr. Pierre VALIRON
_/ _/ _/ _/ Laboratoire d'Astrophysique
_/ _/ _/ _/ Observatoire de Grenoble / UJF
_/_/_/_/ _/ _/ BP 53 F-38041 Grenoble Cedex 9 (France)
_/ _/ _/ http://www-laog.obs.ujf-grenoble.fr/~valiron/
_/ _/ _/ Mail: Pierre.Valiron_at_[hidden]
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