automating shift-register-lut based run-time reconfiguration karel heyse, brahim al farisi, karel...
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Automating Shift-Register-LUT Based Run-Time Reconfiguration
Karel Heyse, Brahim Al Farisi, Karel Bruneel, Dirk Stroobandt
ARC 2012 2
Run-Time Reconfiguration (RTR)
• Changing (part of) circuit at run-time• To save area, power, time … money
zz z
zz z
i0-3
f(i0-3) g(i0-3)h(i0-3)
ARC 2012 3
Run-Time Reconfiguration – cont.
• Reconfiguration time– Time during which (part of) circuit is disabled– Can nullify gains of RTR• Defines when RTR is feasible
– Faster reconfiguration is important
SRL reconfiguration
ARC 2012 4
SRL
Reconfiguration methods
• ICAPHow: Similar to configuration interface, frame based
• SRL reconfigurationHow: Shift-register functionality of LUT’s truth table
inputsshift-in
shift-enableshift-clock
shift-out
output
SRL SRL
ARC 2012 5
Reconfiguration methods – cont.
• SRL reconfiguration+ Very fine grained+ Lower overhead+ Bandwidth adjustableFAST− Only LUTs
TLUTMAP
• ICAP− Coarse grained− Higher overhead− Fixed bandwidth
+ Full reconfiguration
ARC 2012 6
TLUTMAP - Technology mapper
• Takes an HDL design with some slow inputs• Creates configuration:– Dynamically specialisable for the slow inputs– By reconfiguring only part of the LUTs
• Smaller & faster specialised design– FIR filter: -39% LUTs, +38% max clock freq.– TCAM: -66% LUTs, +30% max clock
freq.• Fast RTR
ARC 2012 7
You are here (★)
• Run-time reconfiguration• Reconfiguration methods• TLUTMAP – Technology mapper• Generating reconfiguration chains• Modelling as mTSP• Solution method• Results
ARC 2012 8
Generating reconfiguration chains
• SRLs have to be chained, connected to configuration manager
Configuration manager
ARC 2012 9
Optimising reconfiguration chains
• Influence on the design– Shares routing resources: routability, clock speed
Minimise combined length of reconfiguration chains
• Influence on the reconfiguration time– Clock cycles
Minimise #SRLs in longest chain
– Clock speed reconfigurationMinimise longest connection
ARC 2012 10
Modelling as mTSP
• We chose: Generating chains after placement– Position of SRLs fixed & known
• Model: multiple Travelling Salesman Problem– Minimise influence on design
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Constrained mTSP
• Extra constraints to optimise reconfiguration time– Minimise # cities per salesman– Minimise longest link
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Solution method:Simulated Annealing
• Summary– Iterative heuristic:• Repetitive small, random alterations to a solution
•
– Temperature (T): • Starts high: exploration• Ends low: converge to minimum
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Solution method:Simulated Annealing – cont.
• Solution space– Common starting point for salesmen– No fixed end point for salesmen– Every salesman visits same number of cities (±1)
Minimise # cities per salesman
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Solution method:Simulated Annealing – cont.
• Random alterations
1)
2)
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Solution method:Simulated Annealing – cont.
• Cost function
0 0.2 0.4 0.6 0.8 1 1.20
0.20.40.60.8
11.21.41.6
l/lmax
C(l)
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Experimental results
• Designs– TCAM: 60% reconfigurable LUTs– FIR filter: 37% reconfigurable LUTs
• Evaluated using Xilinx Tools– Flow:• Place• Insert reconfiguration chains• Place & route
ARC 2012 17
Results: Clock of the design
Relative to design without reconfiguration chainsAveraged over experiments with 1, 4, 16 and 32 chains
128 elem.
256 elem.
512 elem.
1024 elem.
-20%
-10%
0%
10%
20%
30%
40%
50%
TCAM
SRL only
Random
mTSP
Manual
32 taps 64 taps 128 taps 256 taps-20%
-10%
0%
10%
20%
30%
40%
50%
FIR
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Results
• Second placement step– VPR: -4% to -28% clock speed– Xilinx: +130% longer reconfiguration chain
• Number of reconfiguration chains– No influence on clock design– Small influence on clock reconfiguration
• Max clock speed reconfiguration– 1x to 2x clock speed design
ARC 2012 19
Conclusion
• Automated method to generate reconfiguration chains
• Takes into account routability of design and reconfiguration speed
• Better than random, almost as good as manual• Could be improved by avoiding second placement
step
Automating Shift-Register-LUT Based Run-Time Reconfiguration
Karel Heyse, Brahim Al Farisi, Karel Bruneel, Dirk Stroobandt