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3. Tests
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Summary

Action	ENHANCE-10_acc_fix_traqsr
PI(S)	acc
Digest	Reduce use of 3D allocatable arrays in RGB light penetration schemes
Dependencies	If any
Branch	source:/NEMO/branches/{YEAR}/dev_r{REV}_{ACTION_NAME}
Previewer(s)	Names
Reviewer(s)	Names
Ticket	#XXXX

Description

The current implementation of RGB light penetration in traqsr (either varying or constant chlorophyll) uses 6, domain-sized 3D, temporary arrays which can be reduced to a few 2D arrays. The impact of the current implementation is most evident at lower processor counts where the impact of the extra 3D arrays can cause cache-misses and memory band-width issues. In an extreme case traqsr can switch from consuming 2% of run-time to 68% (comparing ORCA025 running on 200 cores vs 48 cores).

A simple redesign of the algorithm should remove this behaviour.

Implementation

The current code is structured thus:

      CASE( np_RGB , np_RGBc )         !==  R-G-B fluxes  ==!
         !
         ALLOCATE( zekb(jpi,jpj)     , zekg(jpi,jpj)     , zekr  (jpi,jpj)     , &
            &      ze0 (jpi,jpj,jpk) , ze1 (jpi,jpj,jpk) , ze2   (jpi,jpj,jpk) , &
            &      ze3 (jpi,jpj,jpk) , zea (jpi,jpj,jpk) , zchl3d(jpi,jpj,jpk)   )
         !
         ! code to set zchl3d(:,:,1:nskr+1)
         ! (the chlorophyll value projected from the surface values)
         !
         zcoef  = ( 1. - rn_abs ) / 3._wp    !* surface equi-partition in R-G-B
         DO_2D_00_00
            ze0(ji,jj,1) = rn_abs * qsr(ji,jj)
            ze1(ji,jj,1) = zcoef  * qsr(ji,jj)
            ze2(ji,jj,1) = zcoef  * qsr(ji,jj)
            ze3(ji,jj,1) = zcoef  * qsr(ji,jj)
            zea(ji,jj,1) =          qsr(ji,jj)
         END_2D
         !
         DO jk = 2, nksr+1                   !* interior equi-partition in R-G-B depending of vertical profile of Chl
            DO_2D_00_00
               zchl = MIN( 10. , MAX( 0.03, zchl3d(ji,jj,jk) ) )
               irgb = NINT( 41 + 20.*LOG10(zchl) + 1.e-15 )
               zekb(ji,jj) = rkrgb(1,irgb)
               zekg(ji,jj) = rkrgb(2,irgb)
               zekr(ji,jj) = rkrgb(3,irgb)
            END_2D

            DO_2D_00_00
               zc0 = ze0(ji,jj,jk-1) * EXP( - e3t(ji,jj,jk-1,Kmm) * xsi0r       )
               zc1 = ze1(ji,jj,jk-1) * EXP( - e3t(ji,jj,jk-1,Kmm) * zekb(ji,jj) )
               zc2 = ze2(ji,jj,jk-1) * EXP( - e3t(ji,jj,jk-1,Kmm) * zekg(ji,jj) )
               zc3 = ze3(ji,jj,jk-1) * EXP( - e3t(ji,jj,jk-1,Kmm) * zekr(ji,jj) )
               ze0(ji,jj,jk) = zc0
               ze1(ji,jj,jk) = zc1
               ze2(ji,jj,jk) = zc2
               ze3(ji,jj,jk) = zc3
               zea(ji,jj,jk) = ( zc0 + zc1 + zc2 + zc3 ) * wmask(ji,jj,jk)
            END_2D
         END DO
         !
         DO_3D_00_00( 1, nksr )
            qsr_hc(ji,jj,jk) = r1_rho0_rcp * ( zea(ji,jj,jk) - zea(ji,jj,jk+1) )
         END_3D
         !
         DEALLOCATE( zekb , zekg , zekr , ze0 , ze1 , ze2 , ze3 , zea , zchl3d )
         !

Where most of the temporary, full-depth arrays are not necessary because only two vertical levels are required at any one time. In fact even the zea array is unnecessary since the zchl3d array could be repurposed once its value has been used.

Option 1: Minmum memory usage

By rearranging the loop order and placing the vertical loop innermost then the code can be greatly simplified to an equivalent using minimal temporary storage. A couple of other changes seem sensible too, namely:

• rename the zchl3d array to ztmp3d (since it is now used for two purposes)
• only allocate ztmp3d to nksr+1; values below this are not used and nksr + 1 is likely << jpk
• calculate and store the attenuation coefficient look-up table index as soon as the sub-surface chlorophyll value is known. This keeps all LOG operations in one loop and, in the case of constant chlorophyll, removes the LOG from the loop altogether.

      CASE( np_RGB , np_RGBc )         !==  R-G-B fluxes  ==!
         !
         ALLOCATE( ztmp3d(jpi,jpj,nksr + 1)   )
         !
         ! code to set ztmp3d(:,:,1:nskr+1)

         ! including the following changes after
         IF( nqsr == np_RGBc ) THEN          !*  Variable Chlorophyll
            .
            DO_3D_00_00( 1, nksr + 1 )
            .
            . no change until after
               zCze    = 1.12  * (zchl)**0.803
               !
               ! NB. make sure zchl value is such that: zchl = MIN( 10. , MAX( 0.03, zchl ) )
               zchl = MIN( 10. , MAX( 0.03, zCze * ( zCb + zCmax * EXP( -( (zpsi - zpsimax) / zdelpsi )**2 ) ) ) )
               ! Convert chlorophyll value to attenuation coefficient look-up table index
               ztmp3d(ji,jj,jk) = 41 + 20.*LOG10(zchl) + 1.e-15
            END_3D
         ELSE                                !* constant chrlorophyll
           zchl = 0.05
           ! NB. make sure constant value is such that:
           zchl = MIN( 10. , MAX( 0.03, zchl ) )
           ! Convert chlorophyll value to attenuation coefficient look-up table index
           zlui = 41 + 20.*LOG10(zchl) + 1.e-15
           DO jk = 1, nksr + 1
              ztmp3d(:,:,jk) = zlui
           END DO
         ENDIF
         ! 
         !
         zcoef  = ( 1. - rn_abs ) / 3._wp    !* surface equi-partition in R-G-B
         ! store the surface SW radiation; re-purpose the surface ztmp3d array
         ! since the surface attenuation coefficient is not used
         ztmp3d(:,:,1) = qsr(:,:)
         !
         !* interior equi-partition in R-G-B depending of vertical profile of Chl
         DO_2D_00_00
            zc0 = rn_abs * qsr(ji,jj)
            zc1 = zcoef  * qsr(ji,jj)
            zc2 = zc1
            zc3 = zc1
            zc4 = e3t(ji,jj,1,Kmm)
            DO jk = 2, nksr+1
               irgb = NINT( ztmp3d(ji,jj,jk) )
               zc0 = zc0 * EXP( - zc4 * xsi0r       )
               zc1 = zc1 * EXP( - zc4 * rkrgb(1,irgb) )
               zc2 = zc2 * EXP( - zc4 * rkrgb(2,irgb) )
               zc3 = zc3 * EXP( - zc4 * rkrgb(3,irgb) )
               zc4 = e3t(ji,jj,jk,Kmm)
               ! store the SW radiation penetrating to this location
               ! re-purpose the ztmp3d array since the attenuation coefficient
               ! at this point will not be needed again
               ztmp3d(ji,jj,jk) = ( zc0 + zc1 + zc2 + zc3 ) * wmask(ji,jj,jk)
            END DO
         END_2D
         !
         DO_3D_00_00( 1, nksr )
            qsr_hc(ji,jj,jk) = r1_rho0_rcp * ( ztmp3d(ji,jj,jk) - ztmp3d(ji,jj,jk+1) )
         END_3D
         !
         DEALLOCATE( ztmp3d )

This is code and memory efficient but may perform poorly due to non-contiguous access to the array elements (see performance section below).

Option 2: Low memory use (retain loop order).

A compromise solution, which reduces memory use and should perform better is to remove all unnecessary full-depth arrays but maintain loop order by keeping a few 2D arrays. The same additional changes listed above are also made.

       CASE( np_RGB , np_RGBc )         !==  R-G-B fluxes  ==!
         !
         ALLOCATE( ze0 (jpi,jpj)           , ze1 (jpi,jpj) ,   &
            &      ze2 (jpi,jpj)           , ze3 (jpi,jpj) ,   &
            &      ztmp3d(jpi,jpj,nksr + 1)                     )
         !
         ! code to set ztmp3d(:,:,1:nskr+1)
         ! including the following changes after
         IF( nqsr == np_RGBc ) THEN          !*  Variable Chlorophyll
            .
            DO_3D_00_00( 1, nksr + 1 )
            .
            . no change until after
               zCze    = 1.12  * (zchl)**0.803
               !
               ! NB. make sure zchl value is such that: zchl = MIN( 10. , MAX( 0.03, zchl ) )
               zchl = MIN( 10. , MAX( 0.03, zCze * ( zCb + zCmax * EXP( -( (zpsi - zpsimax) / zdelpsi )**2 ) ) ) )
               ! Convert chlorophyll value to attenuation coefficient look-up table index
               ztmp3d(ji,jj,jk) = 41 + 20.*LOG10(zchl) + 1.e-15
            END_3D
         ELSE                                !* constant chlorophyll
            zchl = 0.05
            ! NB. make sure constant value is such that:
            zchl = MIN( 10. , MAX( 0.03, zchl ) )
            ! Convert chlorophyll value to attenuation coefficient look-up table index
            zlui = 41 + 20.*LOG10(zchl) + 1.e-15
            DO jk = 1, nksr + 1
               ztmp3d(:,:,jk) = zlui
            END DO
         ENDIF
         !
         zcoef  = ( 1. - rn_abs ) / 3._wp    !* surface equi-partition in R-G-B
         DO_2D_00_00
            ze0(ji,jj) = rn_abs * qsr(ji,jj)
            ze1(ji,jj) = zcoef  * qsr(ji,jj)
            ze2(ji,jj) = zcoef  * qsr(ji,jj)
            ze3(ji,jj) = zcoef  * qsr(ji,jj)
            ! store the surface SW radiation; re-use the surface ztmp3d array
            ! since the surface attenuation coefficient is not used
            ztmp3d(ji,jj,1) =       qsr(ji,jj)
         END_2D
         !
          !* interior equi-partition in R-G-B depending of vertical profile of Chl
         DO_3D_00_00( 2, nksr+1 )
            irgb = NINT( ztmp3d(ji,jj,jk) )
            ze3t = e3t(ji,jj,jk-1,Kmm)
            ze0(ji,jj) = ze0(ji,jj) * EXP( - ze3t * xsi0r )
            ze1(ji,jj) = ze1(ji,jj) * EXP( - ze3t * rkrgb(1,irgb) )
            ze2(ji,jj) = ze2(ji,jj) * EXP( - ze3t * rkrgb(2,irgb) )
            ze3(ji,jj) = ze3(ji,jj) * EXP( - ze3t * rkrgb(3,irgb) )
            ! store the SW radiation penetrating to this location
            ! re-use the ztmp3d array since the attenuation coefficient
            ! at this point will not be needed again
            ztmp3d(ji,jj,jk) = ( ze0(ji,jj) + ze1(ji,jj) + ze2(ji,jj) + ze3(ji,jj) ) * wmask(ji,jj,jk)
         END_3D
         !
         DO_3D_00_00( 1, nksr )
            qsr_hc(ji,jj,jk) = r1_rho0_rcp * ( ztmp3d(ji,jj,jk) - ztmp3d(ji,jj,jk+1) )
         END_3D
         !
         DEALLOCATE( ze0 , ze1 , ze2 , ze3 , ztmp3d )

Performance and evaluation

Both these options produce identical results to the original code (based on an ORCA2_ICE_PISCES test using SETTE (which includes variable surface chlorophyll inputs). ln_timing was activated and the CPU time (averaged across all processors) spent in tra_qsr used as a simple measure of performance. Unfortunately, variations in runtime between successive tests (even with the same code) on the NOC cluster were almost as great as any difference arising from algorithmic differences. Each test was repeated 6 times with the following results:

code option	CPU seconds spent in tra_qsr						Average
original code	0.34	0.34	0.35	0.35	0.34	0.34	0.3433
minimum memory option	0.36	0.36	0.37	0.36	0.36	0.37	0.3633
low memory option	0.35	0.35	0.35	0.36	0.36	0.35	0.3533

from which the tentative conclusion is that the minimum memory option does perform consistently worst but the low memory option appears to be a suitable replacement to the original code. More stringent tests are require to confirm this.

These initial tests were performed using the standard 32 processor SETTE test for ORCA2_ICE_PISCES. To search for a better distinction between the options further tests were made by varying the number of processors. Tests with 2, 8, 32 and 60 processors were performed (3 for each option at each core count). The following table shows the percentage of CPU time spent in tra_qsr and the rank of the tra_qsr routine in the CPU time-sorted list of routines (a higher rank means tra_qsr is taking proportionally less of the overall CPU time). In each case the average of the 3 samples is given.:

% CPU spent in tra_qsr
#CPUs	original	min-mem	low-mem
2	1.76	1.82	1.83
8	1.38	1.48	1.46
32	0.48	0.49	0.5
60	0.24	0.26	0.26

Rank in sorted list of routines by CPU usage
#CPUs	original	min-mem	low-mem
2	14	12.67	12
8	16.33	15.67	15
32	22.33	21.33	23.33
60	26	25	25

Unfortunately the message is still mixed and, although the new codes reduce the size and number of temporary arrays; they have a slight detrimental effect on performance and do not alter the tendency for tra_qsr to consume higher percentages of the CPU time as domain size increases.

The low-memory option does, at least, lend itself to tiling.

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