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Computational Steering. Sathish Vadhiyar Sources/Credits: CUMULVS web sites and papers. Introduction. Parameters of the computation are adjusted on-the-fly to gain understanding of the problem Respond to simulation results as they occur by manipulating the input parameters

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Computational Steering

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Computational Steering

Sathish Vadhiyar

Sources/Credits: CUMULVS web sites and papers


Introduction

  • Parameters of the computation are adjusted on-the-fly to gain understanding of the problem

  • Respond to simulation results as they occur by manipulating the input parameters

  • For carrying out “what if” analysis

  • For experimenting with program characteristics whose effects are not easily understood

  • Allows interactive exploration of simulation in time and/or space


Motivation: Falcon Molecular Dynamics (MD)

  • Simulation of system containing 4800 particles representing an alkane film layered on 2700 particles representing a crystalline base

  • Simulation steps

    • Obtain location information from neighboring particles

    • Calculate intra-molecular forces

    • Calculate inter-molecular forces

    • Apply forces to yield new particle position

    • Publish particle’s new position

  • Other steps

    • Determining system-wide characteristics such as atomic temperature

    • Performing on-line data analysis and visualization

  • Simulation algorithm

    • Domain decomposition

    • Domains assigned to parallel processors

    • Cut-off radius in force calculation


Steering opportunities in MD

  • Changing decomposition geometries

  • Changing cut-off radius – speed/fidelity tradeoffs

  • Changing frequency of global temperature calculations based on temperature stability of the system


Advantages of Steering in MD


Advantages of Steering in MD


CUMULVS

  • Collaborative User Migration, User Library for Visualization and Steering

  • Provides 2 libraries

    • For application program

    • For visualization and steering front end

  • Can dynamically attach multiple viewer front ends to a running parallel application

  • Viewer programs can also steer one or more user-defined parameters

  • Also provides fault tolerance and migration across heterogeneous systems


CUMULVS – Application and Visualization Specification

  • Programmer specifies

    • Data distributions

    • Which parameters are allowed to be steered

  • To reduce network traffic, front-end viewers can specify a region for visualization including the granularity of the desired data


CUMULVS – Visualization and Steering – Coordination and Synchronization

  • Consistent view of the application’s state by the viewer through loose synchronization with the application

    • A viewer brackets the timesteps and ensures that all tasks are on one of the timesteps

    • Tasks A, B, and C can be at timesteps 10,12,11

  • Steering parameter updates are marked with “apply-at” timestep


Execution order


Example


Interface Descriptions


Viewer and steering interface

  • Viewer programs

    • Text-based

    • AVS-compatible viewer

    • Custom TCL/TK viewer

  • Token scheme prevents conflicting adjustments to the same steering parameter by different users

  • All tasks apply the steering changes in unison


Viewer and Steering Interface

  • Interfaces for

    • Initializing communications with the user application

    • Requesting data fields

    • Collecting data frames

    • Allocating data frame storage

    • Dumping data field values

  • User’s data field requests:

    • Set of data fields

    • Specific region of computational domain

    • Frequency of “data frames”

    • Cell size for each axis, i.e., data stride

  • Supports different Data decompositions


Viewer interfaces

  • stv_viewer_init

    • Initializes link with a specific user application

    • Gathers information about the data fields and parameters that are available from that application

    • Returns instance identifying a particular application

  • Another call for selecting data fields and specifying visualization regions

    • Data type and storage order can be specified

    • Set of fields specified in a single field request is called “view field group”

    • Also specifies visualization regions – bounds and cell sizes of data fields


Viewer Interfaces

  • stv_viewer_request_field()

    • Field request

    • Returns VFG instance that represents the group of data fields requested

  • stv_viewer_send_XON

    • Sends “XON” to the application

    • Application waits for XON message, polls for permission to continue with the next iteration


Viewer Interfaces - Miscellanious

  • stv_viewer_set_VisRegion()

    • For modifying visualization region of a VFG on the fly

    • Records new set of visualization bounds and cell sizes and sends update to the application

  • set_viewer_set_VisFrequency()

    • Controlling frequency of data frames


Steering interfaces

  • For iterative control

  • stv_viewer_steering_init

    • Creates loosely synchronized connection with the user application

    • Performs equivalent of special data field request

  • stv_viewer_steering_request

    • Obtains steering token for a particular parameter

    • If token unavailable, i.e. another viewer possesses it

      • When the possessing viewer releases the token, CUMULVS will broadcast a message informing all the requesting viewers


Steering Interfaces

  • stv_viewer_steer_parameter, stv_viewer_steer_vparameter

    • Changes values of parameters

    • The routines copy viewer data to an internal data structure

  • stv_viewer_send_NewParams()

    • Changed parameters are sent

  • stv_viewer_steering_release

    • Will relinquish steering token


Protocols for attaching to running application

  • CUMULVS uses application name supplied in the init call in the application for attaching

  • Application name registered in a database that indicates how to contact instance 0 of the application

  • Different phases – inquiry, request for attachment, data transfer, detachment


Protocols - Inquiry

  • Inquiry

    • Viewer looks up the application name, message context

    • Sends init0 message to task 0.

    • Task0 reply: total number of tasks, data decompositions, steering parameter particulars, current time step

    • Task0 forwards init0 message to other tasks

    • Other tasks contact the viewer and send their data and steering parameters


Protocols – Field Request

  • Field request – 3 phases

    • Viewer sends which fields are required. Tasks return their current time steps, timestep0. tasks compute till timestep0+1.

    • After viewer hears from all tasks, it calculates maximum of timestep0s, timestep1. sends timestep1 to tasks

    • Tasks compute until timestep1 and then send requested data fields to viewers


Example

  • Simulating propagation of an acoustic signal through a heterogeneous media

  • Solved by finite difference equations

  • Equation has pressure fields and sources

  • CUMULVS used to introduce arbitrary “thumps”


Output


Example – CFD code Supercomputing Conference (from CUMULVS web site)

CFD code for simulation of airflow over jet wing

Peak in residual field due to an omitted column in computation


References / Sources / Credits

  • P. M. Papadopoulos, J. A. Kohl, B. D. Semeraro, ``CUMULVS: Extending a Generic Steering and Visualization Middleware for Application Fault-Tolerance,'' Proceedings of the 31st Hawaii International Conference on System Sciences (HICSS-31), Kona, Hawaii, January 1998.

  • G. A. Geist, J. A. Kohl, P. M. Papadopoulos, ``CUMULVS: Providing Fault-Tolerance, Visualization and Steering of Parallel Applications,'' International Journal of High Performance Computing Applications, Volume 11, Number 3, August 1997, pp. 224-236.

  • Falcon: On-line Monitoring and Steering of Parallel Programs, Weiming Gu, Greg Eisenhauer and Karsten Schwan, submitted to the IEEE Transactions on Parallel and Distributed Systems November 1994, revised September 1995.


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