[Omp] OpenMP Parallel Do Loops

[Kay Diederichs]

If I use "schedule(static,n/(2*np))" or just "schedule(guided)" in the
!$OMP do
statement I get a (slightly better than) 2-fold speedup on a four-processor 
Opteron for a 1000x1000 matrix using the ifort 9 compiler and the Intel MKL in 
32bit mode.

Kay

ta.cbra at maths.strath.ac.uk wrote:
> Neil,
> 
> Thank you for your reply. I have attached a more detailed version of my 
> code that actually applies the Given Rotations (this uses BLAS routines 
> drotg and drot).
> 
> I have tried to implement your suggestions in this new code but I am still 
> unable to get any kind of speed up when I increase the number of 
> processors.
> 
> Any obvious reason why to anyone? Any help is greatly appreciated!
> 
>       program CDGR
>       include 'omp_lib.h'
> 
> c     Declare variable types
>       integer :: i, j, x, m, n
>       double precision, dimension(:,:), allocatable :: W
>       double precision :: cc,ss,time
>       integer np, me
> 
> c     Get the size of matrix to use
>       write(*,*) 'What size of matrix do you wish to use?'
>       write(*,*) 'Number of rows (m) ='
>       read(*,*) m
>       write(*,*) 'Number of columns (n) ='
>       read(*,*) n
>       write(*,*)'m= ',m,' and n = ',n,' thank you.'
> 
>       allocate(W(m,n))
> 
>       do i=1,m
>          do j=1,n
>             W(i,j)=1000*rand(i+j)
>          end do
>       end do
> 
> !      do i=1,m
> !         write(*,*)(W(i,j),j=1,n)
> !      end do
> 
>       time=dtime(timearray)
> 
> c     Show time step (i) that each element would be annihilated during 
> c     to leave the matrix W upper triangular
> 
> !$OMP parallel private(x,cc,ss,me,i) shared(W,np,m,n)
>       np = omp_get_num_threads()
>       me = omp_get_thread_num()
> 
>       do i=1,m+n-2
> c     Every node uses the same value of i but the j values
> c     are shared out and can be preformed at the same time 
> 
> !$OMP do schedule(dynamic,1)
>          do j=1,n
>             x=m+2*j-i-1
> c     make sure element W(x,j) is with-in the matrix W
>             if (j .lt. x) then 
>                if (x .le. m) then
>                    call drotg(W(x-1,j), W(x,j), cc, ss)
>                    W(x,j)=0d0
>                    call drot(n-j,W(x-1,j+1:n),1,W(x,j+1:n),1,cc,ss)
> !                   W(x,j)=i
>                endif
>             endif
>          enddo
> !$OMP end do
>       enddo
> !$OMP end parallel
> 
>       time=dtime(timearray)
>       write(*,*)'CDGR with ',np
>       write(*,*)'m=',m,'n=',n,'time=',time,
> 
> c  Print W if you want to see how it was annihilated
> !      do i=1,m
> !         write(*,*)(W(i,j),j=1,n)
> !      end do
> 
>       deallocate(W)
> 
>       stop
>       end
> 
> 
> On Mon, 14 Nov 2005, Neil Summers wrote:
> 
> 
>>2 things i have noticed on a quick scan of your code.
>>
>>1) you should define the parallel region outside
>>the i loop, creating a parallel region within a do loop
>>causes excessive overhead, as the program fork/joins excessively.
>>You should define the parallel region outside the i loop
>>to reduce overhead then use omp do to split work up
>>between threads. ie
>>
>>!$OMP parallel private(x,me,np)
>> do i=1,m+n-2
>>!$OMP do
>>  do j=1,n
>>   ...
>>  enddo
>> enddo
>>!$OMP end parallel
>>
>>2) i'm supprised you get the right results,
>>by defining firstprivate(m,n), these are then undefined
>>on exiting the parallel region, so i would guess the second
>>iteration of i would not happen correctly.
>>you don't need these private, so i'd leave them shared
>>
>>Neil
> 
> 


-- 
Kay Diederichs   http://strucbio.biologie.uni-konstanz.de/~kay
email: Kay.Diederichs at uni-konstanz.de  Tel +49 7531 88 4049 Fax 3183
Fachbereich Biologie, Universität Konstanz, Box M647, D-78457 Konstanz
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