On the Relevance of Wire Load Models - PowerPoint PPT Presentation

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On the Relevance of Wire Load Models

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VLSI Power-Law Phenomena (cont.) Vertex degree. Net fanout. P(d) == # vertices ... Power-law vertex degree distribution. New Pref Model. Preferential attachment ... – PowerPoint PPT presentation

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Title: On the Relevance of Wire Load Models


1
On the Relevance of Wire Load Models
  • Kenneth D. Boese, Cadence Design Systems, San
    Jose
  • Andrew B. Kahng, UCSD CSE and ECE Depts., La
    Jolla
  • Stefanus Mantik, UCLA CS Dept., Los Angeles

2
Why WLMs?
  • Synthesis needs placement parasitics
  • Placement needs synthesized netlist
  • Chicken vs. egg
  • WLMs estimate parastics pre-placement
  • Wire cap f(net fanout)
  • But WLMs no longer accurate! (?)

3
Motivating Questions
  • How inaccurate are WLMs?
  • Are WLMs good for optimization?
  • Are WLMs good for estimation?
  • How use WLMs in optimization flow?

4
Previous Works
  • Bad WLMs motivate post-placement synthesis
  • e.g., Lu et al. 7
  • Need for constructive estimators
  • Scheffer and Nequist 8
  • law of small numbers
  • Accuracy vs. fidelity
  • Boese et al. 1,2

5
Kinds of WLMs
  • Custom
  • multiple iterations
  • Statistical
  • from library
  • Structural
  • look at adjacent nets

6
Historical Design Flows
  • No WLM
  • interconnect insignificant
  • Custom WLMs
  • requires iterations
  • Statistical WLMs
  • fewer iterations
  • Add post-placement optimization
  • IPO, PB-Opt
  • Post-placement only (integrated tool)
  • PKS, Physical Compiler, Blast Fusion

7
How Accurate Are WLMs?
  • High Deviations ? WLMs Inaccurate
  • Normalized to Ave. Net Cap

8
Accuracy Skew Measures
  • What if long nets not critical?
  • timing-driven placement
  • buffer insertion
  • So ... Must Test WLMs in Actual Flows

9
Are WLMs Good for Optimization?
  • Sufficient?
  • Necessary?
  • Compare different flows
  • real tool (PKS)
  • real designs

10
Default PKS Flow
RTL
Generic Netlist
Area opt
Technology mapping
Mapped Netlist
Timing opt
Placement
Placed Netlist
Post-placement timing opt
Global routing
Routed Netlist
11
7 Flow Variations
  • 1. Area opt
  • 2. Technology mapping
  • 3. Timing opt
  • 4. Placement
  • 5. Post-pl timing opt
  • 6. Global routing
  • Structural WLM
  • Library WLM
  • Custom WLM
  • No WLM
  • No Pre Pl Opt
  • No Post Pl Logic Opt
  • No Post Pl Opt

12
7 Flow Variations
  • 1. Area opt
  • 2. Technology mapping
  • 3. Timing opt
  • Struct WLM
  • 4. Placement
  • 5. Post-pl timing opt
  • 6. Global routing
  • Structural WLM
  • Library WLM
  • Custom WLM
  • No WLM
  • No Pre Pl Opt
  • No Post Pl Logic Opt
  • No Post Pl Opt

13
7 Flow Variations
  • 1. Area opt
  • 2. Technology mapping
  • 3. Timing opt
  • Lib WLM
  • 4. Placement
  • 5. Post-pl timing opt
  • 6. Global routing
  • Structural WLM
  • Library WLM
  • Custom WLM
  • No WLM
  • No Pre Pl Opt
  • No Post Pl Logic Opt
  • No Post Pl Opt

14
7 Flow Variations
  • 1. Area opt
  • 2. Technology mapping
  • 3. Timing opt
  • Cust WLM
  • 4. Placement
  • 5. Post-pl timing opt
  • 6. Global routing
  • Structural WLM
  • Library WLM
  • Custom WLM
  • No WLM
  • No Pre Pl Opt
  • No Post Pl Logic Opt
  • No Post Pl Opt

15
7 Flow Variations
  • 1. Area opt
  • 2. Technology mapping
  • 3. Timing opt
  • WLM 0
  • 4. Placement
  • 5. Post-pl timing opt
  • 6. Global routing
  • Structural WLM
  • Library WLM
  • Custom WLM
  • No WLM
  • No Pre Pl Opt
  • No Post Pl Logic Opt
  • No Post Pl Opt

16
7 Flow Variations
  • 1. Area opt
  • 2. Technology mapping
  • 3. Timing opt
  • 4. Placement
  • 5. Post-pl timing opt
  • 6. Global routing
  • Structural WLM
  • Library WLM
  • Custom WLM
  • No WLM
  • No Pre Pl Opt
  • No Post Pl Logic Opt
  • No Post Pl Opt

17
7 Flow Variations
  • 1. Area opt
  • 2. Technology mapping
  • 3. Timing opt
  • Lib WLM
  • 4. Placement
  • 5. Post-pl timing opt
  • No logic opt
  • 6. Global routing
  • Structural WLM
  • Library WLM
  • Custom WLM
  • No WLM
  • No Pre Pl Opt
  • No Post Pl Logic Opt
  • No Post Pl Opt

18
7 Flow Variations
  • 1. Area opt
  • 2. Technology mapping
  • 3. Timing opt
  • Lib WLM
  • 4. Placement
  • 5. Post-pl timing opt
  • 6. Global routing
  • Structural WLM
  • Library WLM
  • Custom WLM
  • No WLM
  • No Pre Pl Opt
  • No Post Pl Logic Opt
  • No Post Pl Opt

19
Flow Comparisons
20
Implications
  • WLMs not sufficient
  • post-place opt needed
  • WLMs not necessary for good slacks (?)
  • WLMs reduce running time
  • Some improvement from Post Place Logic Opts

21
Do WLMs Give Good Estimates?
  • WLM slack vs. groute slack
  • Custom WLM

22
How Should We Use WLMs?
  • How much optimism?
  • Which WLM best?
  • Structural (?)
  • no extra input
  • Multi-start?
  • different WLMs / optimism levels
  • is it randomness?

23
Caveats
  • This tool, this version, these designs
  • Global not detailed or extracted routing
  • No clock tree
  • Routability ignored
  • detours, strict
  • Die size fixed
  • Some designs pre-optimized

24
Conclusions / Future Work
  • Strict accuracy not right question
  • WLMs improve run time
  • NOT design quality (?)
  • WLMs may be ballpark estimators
  • Post-place opt indispensible
  • small gains from post-place logic opt
  • Sensitivity analysis of placement and
    post-place opt
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