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Flexible Control of Data Transfer between Parallel Programs. Joe Shang-chieh Wu Alan Sussman Department of Computer Science University of Maryland, USA. Particle and Hybrid model. Corona and solar wind. Rice convection model. Global magnetospheric MHD. Thermosphere-ionosphere model.

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Flexible control of data transfer between parallel programs

Flexible Control of Data Transfer between Parallel Programs

Joe Shang-chieh Wu

Alan Sussman

Department of Computer Science

University of Maryland, USA

Flexible control of data transfer between parallel programs

Particle and Hybrid model

Corona and solar wind

Rice convection model

Global magnetospheric MHD

Thermosphere-ionosphere model

Grid 2004

What is the problem
What is the problem?

  • Coupling existing (parallel) programs

    • for physical simulations more accurate answers can be obtained

    • for visualization, flexible transmission of data between simulation and visualization codes

  • Exchange data across shared or overlapped regions in multiple parallel programs

  • Couple multi-scale (space & time) programs

  • Focus on multiple time scale problems (when to exchange data)

Grid 2004


  • Motivation

  • Approximate Matching

  • Matching properties

  • Performance results

  • Conclusions and future work

Grid 2004

Is it important
Is it important?

  • Petroleum reservoir simulations – multi-scale, multi-resolution code

  • Special issue in May/Jun 2004 of IEEE Computing in Science & Engineering

    “It’s then possible to couple several existing calculations together through an interface and obtain accurate answers.”

  • Earth System Modeling Framework

    several US federal agencies and universities. (http://www.esmf.ucar.edu)

Grid 2004

Solving multiple space scales
Solving multiple space scales

  • Appropriate tools

  • Coordinate transformation

  • Domain knowledge

Grid 2004

Matching is outside components
Matching is OUTSIDE components

  • Separate matching (coupling) information from the participating components

    • Maintainability – Components can be developed/upgraded individually

    • Flexibility – Change participants/components easily

    • Functionality – Support variable-sized time interval numerical algorithms or visualizations

  • Matching information is specified separately by application integrator

  • Runtime match via simulation time stamps

Grid 2004

Separate codes from matching







Separate codes from matching

Exporter Ap0

Configuration file

Importer Ap1

Grid 2004

Matching implementation
Matching implementation

  • Library is implemented with POSIX threads

  • Each process in each program uses library threads to exchange control information in the background, while applications are computing in the foreground

  • One process in each parallel program runs an extra representative thread to exchange control information between parallel programs

    • Minimize communication between parallel programs

    • Keep collective correctness in each parallel program

    • Improve overall performance

Grid 2004

Approximate matching
Approximate Matching

  • Exporter Ap0 produces a sequence of data object A at simulation times 1.1, 1.2, 1.5, and 1.9

    • A@1.1, A@1.2, A@1.5, A@1.9

  • Importer Ap1 requests the same data object A at time 1.3

    • A@1.3

  • Is there a match for A@1.3? If Yes, which one and why?

Grid 2004

Supported matching policies
Supported matching policies

<importer request, exporter matched, desired precision> = <x, f(x), p>

  • LUB minimum f(x) with f(x) ≥ x

  • GLB maximum f(x) with f(x) ≤ x

  • REG f(x) minimizes |f(x)-x| with |f(x)-x| ≤ p

  • REGU f(x) minimizes f(x)-x with 0 ≤ f(x)-x ≤ p

  • REGL f(x) minimizes x-f(x) with 0 ≤ x-f(x) ≤ p

  • FASTR any f(x) with |f(x)-x| ≤ p

  • FASTU any f(x) with 0 ≤ f(x)-x ≤ p

  • FASTL any f(x) with 0 ≤ x-f(x) ≤ p

Grid 2004

Acceptable matchable



Acceptable ≠ Matchable

Grid 2004

Region type matches


Region-type matches

Grid 2004

Experimental setup
Experimental setup

Question : How much overhead introduced by runtime matching?

  • 6 PIII-600 processors, connected by channel-bonded Fast Ethernet

  • utt = uxx + uyy + f(t,x,y), solve 2-d diffusion equation by the finite element method.

  • u(t,x,y) : 512x512 array, on 4 processors (Ap1)

  • f(t,x,y) : 32x512 array, on 2 processors (Ap2)

  • All data in Ap2 is sent (exported) to Ap1 using matching criterion <REGL,0.05>

  • Ap1 receives (imports) data with 3 different scenarios. 1001 matches made for each scenario (results averaged over multiple runs)

Grid 2004

Experiment result 1
Experiment result 1

Ap1 execution time (average)

Grid 2004

Experiment result 2
Experiment result 2

Ap1 pseudo code

Ap1 overhead in the slowest process

Grid 2004

Experiment result 3
Experiment result 3

  • Fastest process (P11)

    • - high cost, remote match

  • Slowest process (P13)

    • - low cost, local match

  • High cost match can be hidden

Comparison of matching time

Grid 2004

Conclusions future work
Conclusions & Future work

  • Conclusions

    • Low overhead approach for flexible data exchange between different time scale e-Science components

  • Ongoing & future work

    • Performance experiments in Grid environment

    • Caching strategies to efficiently deal with slow importers

    • Real applications – space weather is the first one

Grid 2004

Main components
Main components

Grid 2004