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An Improved Block-Based Thermal Model in HotSpot 4.0 with Granularity Considerations

An Improved Block-Based Thermal Model in HotSpot 4.0 with Granularity Considerations. Wei Huang 1 , Karthik Sankaranarayanan 1 , Robert Ribando 3 , Mircea Stan 2 and Kevin Skadron 1. Departments of 1 Computer Science, 2 Electrical and Computer Engineering and

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An Improved Block-Based Thermal Model in HotSpot 4.0 with Granularity Considerations

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  1. An Improved Block-Based Thermal Model in HotSpot 4.0 with Granularity Considerations Wei Huang1, Karthik Sankaranarayanan1, Robert Ribando3, Mircea Stan2 and Kevin Skadron1 Departments of 1Computer Science, 2Electrical and Computer Engineering and 3Mechanical and Aerospace Engineering, University of Virginia

  2. Hi! I’m HotSpot • Temperature is a primary design constraint today • HotSpot – an efficient, easy-to-use, microarchitectural thermal model • Validated against measurements from • Two finite-element solvers [ISCA03, WDDD07] • A test chip with a regular grid of power dissipators [DAC04] • A Field-Programmable Gate Array [ICCD05] • Freely downloadable from http://lava.cs.virginia.edu/HotSpot

  3. A little bit of History • Version 1.0 – a block-based model • Version 2.0 – TIM added, better heat spreader modeling • Version 3.0 – grid-based model added • Version 4.0 coming soon!

  4. Why this work? • Michaud et. al. [WDDD06] raised certain accuracy concerns • A few of those had already been addressed pro-actively with the grid-based model • This work tries to address the remaining and does more • Improves HotSpot to Version 4.0 – downloadable soon!

  5. Outline • Background • Overview of HotSpot • Accuracy Concerns • Modifications to HotSpot • Results • Analysis of granularity • Conclusion

  6. Outline • Background • Overview of HotSpot • Accuracy Concerns • Modifications to HotSpot • Results • Analysis of granularity • Conclusion

  7. Overview of HotSpot Analogy between thermal and electrical conduction • Similarity between thermal and electrical physical equations • HotSpot discretizes and lumps ‘electrical analogues’ (thermal R’s for steady-state and C’s for transient) • Lumping done at two levels of granularity • Functional unit-based ‘block-model’ • Regular mesh-based ‘grid-model’ • Thermal circuits formed based on floorplan • Temperature computation by standard circuit solving

  8. Structure of the `block-model’ Sample thermal circuit for a silicon die with 3 blocks, TIM, heat spreader and heat sink (heat sources at the silicon layer are not shown for clarity)

  9. Outline • Background • Overview of HotSpot • Accuracy Concerns • Modifications to HotSpot • Results • Analysis of granularity • Conclusion

  10. Accuracy concerns from [WDDD06] • Spatial discretization – partly addressed with the `grid-model’ since version 3.0 • For the same power map, temperature varies with floorplan • Floorplans with larger no. of blocks better • Floorplans with high-aspect-ratio blocks inaccurate • Transient response • Slope underestimated for small times • Amplitude underestimated

  11. Other issues and limitations • Forced isotherm at the surface of the heat sink • Temperature dependence of material properties – not part of this work

  12. Outline • Background • Overview of HotSpot • Accuracy Concerns • Modifications to HotSpot • Results • Analysis of granularity • Conclusion

  13. Block sub-division Version 3.1 – a block is represented by a single node Version 4.0 – sub-blocks with aspect ratio close to 1

  14. Heat sink boundary condition Version 3.1 – single convection resistance, isothermal surface Version 4.0 – parallel convection resistances, center modeled at the same level of detail as silicon

  15. Other modifications • Spreading R and C approximation formulas replaced with simple expressions (R = 1/k x t/A, C = 1/k x t x A) • Distributed vs. lumped capacitance scaling factor – 0.5 • ‘grid-model’ enhancements – apart from the above: • First-order solver upgraded to fourth-order Runge-Kutta • Performance optimization of the steady-state solver

  16. Outline • Background • Overview of HotSpot • Accuracy Concerns • Modifications to HotSpot • Results • Analysis of granularity • Conclusion

  17. Experiment 1 – EV6-like floorplan

  18. Results with good TIM (kTIM= 7.5W/(m-K))

  19. Results with worse TIM (kTIM= 1.33W/(m-K))

  20. Transient response – bpred Transient response for different power pulse widths applied to the branch predictor. Power density is 2W/mm2(kTIM= 7.5W/(m-K)). Other blocks have zero power dissipation.

  21. Experiment 2 – 1 mm2 square heat source Version 3.1 Version 4.0

  22. Results Center temperature for different heat source sizes with a power density of 1.66W/mm2 – (a) with good TIM (kTIM= 7.5W/(m-K)) (b) with worse TIM (kTIM= 1.33W/(m-K))

  23. Transient response: high power density, worse TIM Transient temperature response for 1mm x 1mm source with 10Watts with worse TIM material (kTIM= 1.33W/(m-K)).

  24. Outline • Background • Overview of HotSpot • Accuracy Concerns • Modifications to HotSpot • Results • Analysis of granularity • Conclusion

  25. Spatial filtering • The Norton equivalent first-order thermal spatial RC circuit • Low-pass filter in the spatial domain • Blocks with high power density need not be hot spots (when small enough)

  26. Spatial filtering – continued... Comparison of 3-ladder thermal spatial RC model and ANSYS simulation for different heat source sizes. • Thermal RC is distributed • First-order approximation not sufficient • 3-ladder RC (similar to HotSpot) approximates well

  27. Outline • Background • Overview of HotSpot • Accuracy Concerns • Modifications to HotSpot • Results • Analysis of granularity • Conclusion

  28. Summary, limitations and caveats • This work acknowledges and addresses the concerns in [WDDD06] • `grid-model’ [DAC04] had addressed part of the discretization aspect earlier • HotSpot 4.0 addresses remaining and does more • Careful use of vertical layers necessary, material properties’ dependence on T not modeled • Soon to be available at http://lava.cs.virginia.edu/HotSpot

  29. Questions?

  30. Backup – ATMI [MoBS07] • Analytical model, has good accuracy • A diversity in modeling is good for the community • Vis-a-vis HotSpot – advantages • Immune to spatial discretization • Disadvantages • Less flexibility (esp. in vertical layers) • Computationally intensive (esp. when looking for temperature with a particular property)

  31. Backup – Transient response: high power density, good TIM Transient temperature response for 1mm x 1mm source with 10Watts power and a good TIM (kTIM= 7.5W/(m-K)).

  32. Backup – Transient response: low power density, good TIM Transient temperature response for a 7mm x 7mm source with 10Watts power and a good TIM (kTIM= 7.5W/(m-K)).

  33. Backup – Granularity (1) • A first-order electrical RC circuit

  34. Backup – Granularity (2) • The Thevenin equivalent first-order thermal spatial “RC” circuit.

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