CPU time spent in MPI_Barrier()

• 24 марта 2008 г. 17:05

   

   
  

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  This entry has been transferred from the archived forum site and was originally posted on 3/14/2008 by User ID "Flame"

   

  Hello,
  
  I used microsoft mpi (msmpi) to do message passing for a CFD fortran code. In the code, there are several barriers to synchronize the code. I measured the wall clock time and cpu time spent for calling MPI_Barrier(). I would expect that the cpu time to call MPI_Barrier() is approximately zero and the wall clock time depends on the idle time. However, with msmpi, I observed that the CPU time and the wall clock time to call MPI_Barrier() are always approximately equal. It seems that when a processor is idle the CPU time on the processor still accumulates. Does anyone know what's going on? Do I need to specify some flags to start the mpi job with mpiexec? The test code is in the following:
  
  Thanks,
  Flame
  

  -------------------------------------------------------------------
      program master
  
      use mpi
      implicit none
      integer         :: C1,C2,C3,R,M
      real            :: tm0,tm1,tm2
      integer         :: i,ierr,np,myid,isum,ip,istatus(MPI_STATUS_SIZE)
  
      call SYSTEM_CLOCK (COUNT=C1,COUNT_RATE=R,COUNT_MAX=M)
      call CPU_TIME(tm0)
  
      call MPI_init (ierr)
      call MPI_comm_size (MPI_COMM_WORLD, np, ierr)
      call MPI_comm_rank (MPI_COMM_WORLD, myid, ierr)
  
      if(myid==0) then
  	! do some job only on processor 0
          do i=1,200000000
              C2 = 0
              C2 = sin(0.123)
              C2 = cos(0.0134)
              C2 = exp(dble(C2))
              C2 = log(dble(C2))
          enddo
      endif
  
      call SYSTEM_CLOCK (COUNT=C2,COUNT_RATE=R,COUNT_MAX=M)
      call CPU_TIME(tm1)
  
      write(*,*)
  'Computation+++++++++++++++++++++++++++++++++++++++++++++++++++++++'
      write(*,*) 'Wall Time=', dble(C2-C1)/dble(R)
      write(*,*) 'CPU  Time=', tm1-tm0
  
      call SYSTEM_CLOCK (COUNT=C2,COUNT_RATE=R,COUNT_MAX=M)
      call CPU_TIME(tm1)
  
      call MPI_barrier(MPI_COMM_WORLD,ierr)
  
      call SYSTEM_CLOCK (COUNT=C3,COUNT_RATE=R,COUNT_MAX=M)
      call CPU_TIME(tm2)
      !tm2 = DCLOCK()
  
      write(*,*)
  'Barrier+++++++++++++++++++++++++++++++++++++++++++++++++++++++'
      write(*,*) 'Wall Time=', dble(C3-C2)/dble(R)
      write(*,*) 'CPU  Time=', tm2-tm1
  
      write(*,*) 'Total+++++++++++++++++++++++++++++++++++++++++++++++++++++++'
      write(*,*) 'Wall Time=', dble(C3-C1)/dble(R)
      write(*,*) 'CPU  Time=', tm2-tm0
  
      call MPI_finalize (ierr)
  
      end program master

  -------------------------------------------------------------------
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