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Page 1: Distributed computing of the GEOS-Chem model

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Distributed computing of the GEOS-Chem model

Kevin Bowman

Lei Pan, Qinbin Li, and Paul von Allmen

California Institute of Technology

Jet Propulsion Laboratory

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Objectives

• The objective of this activity is to develop a scalable, parallel version ofthe GEOS-Chem code based on a distributed computing architecturethat is suitable for the JPL 1024 processing element (PEs) institutionalcluster

• The goal is to improve the GEOS-Chem wall-clock performance by atleast one order of magnitude over the current capability

• The current capability:the speedup of GEOS-Chem with the number ofCPUs currently plateaus at 4 processors on a shared memory platformsuch as SGI O2K. Best wall-clock performance is completion of a 1-month model simulation on a 200 x 250 km grid within 1 day.

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Approach

• The primary calculations in GEOS-Chem are:

– Chemistry (60%)

– Transport, deposition, emissions (40%)

• The chemistry component is inherently parallel and therefore the mostlogical starting point.

• The initial stage is to use a master/slave architecture for theparallelization of the chemistry

• The second stage is to migrate towards a domain decompositiondesign that will handle both transport and chemistry.

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Timeseries diagnostics (diag49)

Initialization

Start 6-h loop

Start dynamic time stepArchive diagnostics (diag3)Met fields (a3 & a6): unzip, read

Transport

Turbulent Mixing

Convection

End 6-h loop

Dry Deposition

Emissions

Chemistry

Wet Deposition

End dynamic time step

Seasonal, monthly, daily dataInterpolate met fieldsCompute air mass quantitiesUnit conversion: kg -> v/v

Compute air mass quantities

Upper boundary flux conditions

Unit conversion: kg -> v/v

DO_CHEMISTRYchemistry_mod.f

CHEM chem.f

PHYSPROC physproc.f

CALCRATE calcrate.fSMVGEAR smvgear.f

DO_WETDEPwetscav_mod.f

WETDEP wetscav_mod.f

DO_EMISSIONSemissions_mod.f

EMISSDR emissdr.f

DO_DRYDEPdrydep_mod.f

DEPVEL drydep_mod.f

DO_CONVECTIONconvection_mod.f

FVDAS_CONVECT fvdas_convect_mod.f

NFCLDMX convection_mod.for

TURBDAYturbday.f

DO_TRANSPORTtransport_mod.f

TPCORE_FVDAS tpcore_fvdas_mod.f90

TPCOREtpcore_mod.for

15 min

15 min

60 min

60 min

60 min

15 min

15 min

GEOS-Chem computational flow

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Master/slave architecture

Transport, turbulent mixing,convectionDry deposition, emissions

GEOS-Chem master node

Chemistry Chemistry Chemistry Chemistry

GEOS-Chem master node

Wet deposition

Logical sequence/one tim

e-step

Slave node

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Chemistry

Physproc

FOR ii = 1, 2300 DO

CALCRATE

SMVGEAR

ENDDO

Physproc

FOR ii = 1, 2300/N DO

CALCRATE

SMVGEAR

MPI-SEND

MPI-RECEIVE

CALCRATE

SMVGEAR

CALCRATE

SMVGEAR

ENDDO

PE 1

PE 2

PE N

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Amdahl’s Law

Amdahl’s law describes the speed-up from parallelizationas a function of processor number, non-parallelizable component,processor communication and contention

Speedup =Tseq

Tnp + Tcom (P)+Tcont (P)+Tseq − Tnp

P

Tseq : Sequential timeTnp : Non-parallelizable component timeTcom: Communication time between processorsTcont: Contention time between processorsP : Number of processors

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Performance

•Test run on 4x5 deg, full chemistry•1024 processor (dual CPU/node) Dell cluster

• Xeon Processors, ~3 Tflops theoretical peak, ~2 Tbyte RAM•Pentium 3.2 Ghz and 2 GB RAM

•Communication and contention cost removed for analysis

•Tseq: 649.83 sec•Chemistry (seq) : 432.21 sec (66.5%)•SMVGEAR+CALCRATE: 0.0076 sec/node•Reach optimal trade-off in speedup-processorperformance with 32 processors

However,•Total time with master/slave architecture is 2230 sec•Contention time: 1825.95 sec or 82% of wall-clocktime.•Communication time: ~0.0063*2300 sec

Master/slave architecture not a viable option forChemistry or transport.

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Domain Decomposition

Grid cell:GhostBoundaries:

All computations (transport, chemistry) for a grid cell are performed on one processorFor transport, ghost boundaries must be used

PE 1,1

PE 1,2

PE 1,3

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Ghost Boundaries

PE1

PE2

t+2dtt t+dt

MES

SAG

E PA

SSIN

G

t+3dtProcess Time t: current values of fields on allgrid points are accessible by PE1 andPE2. Time t+dt and t+2dt: current valuesof fields are accessible by both PE1and PE2 on a reduced set of gridpoints. Message passing: current values offields are made accessible to both PE1and PE2 on all grid points. Time t+3dt: situation identical totime t.

Salient Features Information is exchanged betweenPE1 and PE2 every 3 time steps. Fields on all the grid points in theghost boundary are exchanged. Fields on some grids points arecomputed redundantly by both PE1 andPE2.

Optimization of ghost boundary size

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Future Directions and Conclusions

• We have a preliminary design for the domain decomposition

• We expect to achieve ~P1/2 speed-up with this design.

• The I/O bottleneck (lots of data written to files) will be resolved by usinga Parallel Virtual File System (PVFS) and MPI ROM/IO in order tomaintain the scaling for a larger number of processors.

• We see this approach will enable GEOS-Chem user’s to address abroad range of questions that are currently inhibited by computationalconstraints.

• These techniques will be beneficial not only to large systems, such asthe JPL institutional cluster, but also to more modest cluster systems.

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Distributed Computation

Data on full grid

Distribute Data (MP)

Distributed Computation

InjectBoundary Data (MP)

Gather Data (MP)

P1 PNP2

Chemistry

Transportt→t+dt

Chemistry

Transportt→t+dt

P1 P2 PN

Data on full grid


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