Programmable processors for wireless basestations

1
Programmable processors for wireless base-stations

• Sridhar Rajagopal
• (sridhar_at_rice.edu)
• December 11, 2003

2
Wireless rates ? clock rates
4 GHz
54-100 Mbps
200 MHz
2-10 Mbps
1 Mbps
9.6 Kbps

• Need to process 100X more bits per clock cycle
  today than in 1996

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Base-stations need horsepower
Sophisticated signal processing for multiple
users Need 100-1000s of arithmetic operations to
process 1 bit Base-stations require gt 100 ALUs
4
Power efficiency and flexibility
implies does not waste power does not imply low
power

• Wireless systems getting harder-to-design
• Evolving standards, compatibility issues
• More base-stations per unit area
• operational and maintenance costs
• Flexibility provides power-efficiency
• Base-stations rarely operate at full capacity
• Varying users, data rates, spreading, modulation,
  coding
• Adapt resources to needs

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Thesis addresses the following problem

• I want to design programmable processors for
  wireless base-stations with 100s of ALUs
• map wireless algorithms on these processors
• power-efficient (adapt resources to needs)
• (c) decide ALUs, clock frequency

how much programmable? as programmable as
possible
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Choice Stream processors

• Single processors wont do
• ILP, subword parallelism not sufficient
• Register file explosion with increasing ALUs
• Multiprocessors
• Data parallelism in wireless systems
• SIMD (vector) processors appropriate
• Stream processors media processing
• Share characteristics with wireless systems
• Shown potential to support 100-1000s of ALUs
• Cycle accurate simulator and compiler tools
  available

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Thesis contributions

• (a)Mapping algorithms on stream processors
• designing data-parallel algorithm versions
• tradeoffs between packing, ALU utilization and
  memory
• reduced inter-cluster communication network
• (b)Improve power efficiency in stream processors
• adapting compute resources to workload variations
  
• varying voltage and frequency to real-time
  requirements
• (c) Design exploration between ALUs and clock
  frequency to minimize power consumption
• fast real-time performance prediction

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Outline

• Background
• Wireless systems
• Stream processors
• Mapping algorithms to stream processors
• Power efficiency
• Design exploration
• Broad impact and future work

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Wireless workloads
Time 1996
2004 ?
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Key kernels studied for wireless

• FFT Media processing
• QRD Media processing
• Outer product updates
• Matrix vector operations
• matrix matrix operations
• Matrix transpose
• Viterbi decoding
• LDPC decoding

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Characteristics of wireless

• Compute-bound
• Finite precision
• Limited temporal data reuse
• Streaming data
• Data parallelism
• Static, deterministic, regular workloads
• Limited control flow

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Parallelism levels in wireless systems

• int i,aN,bN,sumN // 32 bits
• short int cN,dN,diffN // 16 bits packed
• for (i 0 ilt 1024 i)
• sumi ai bi
• diffi ci - di
• Instruction Level Parallelism (ILP) - DSP
• Subword Parallelism (MMX) - DSP
• Data Parallelism (DP) Vector Processor
• DP can decrease by increasing ILP and MMX
• Example loop unrolling

DP
ILP
MMX
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Stream Processors multi-cluster DSPs
Memory Stream Register File (SRF)










ILP MMX















DP
adapt clusters to DP Identical clusters, same
operations. Power-down unused FUs, clusters
VLIW DSP (1 cluster)
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Programming model
Communication
Computation
Your new hardware wont run your old software
Balchs law
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Outline

• Background
• Wireless systems
• Stream processors
• Mapping algorithms to stream processors
• Power efficiency
• Design exploration
• Broad impact and future work

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Viterbi needs odd-even grouping

• Exploiting Viterbi DP in SWAPs
• Use Register exchange (RE) instead of regular
  traceback
• Re-order ACS, RE

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Performance of Viterbi decoding
Ideal C64x (w/o co-proc) needs 200 MHz for
real-time
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Patterns in inter-cluster comm

• Intercluster comm network fully connected
• Structure in access patterns can be exploited
• Broadcasting
• Matrix-vector multiplication, matrix-matrix
  multiplication, outer product updates
• Odd-even grouping
• Transpose, Packing, Viterbi decoding

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Odd-even grouping

• Packing
• overhead when input and output precisions are
  different
• Not always beneficial for performance
• Odd-even grouping required for bringing data to
  right cluster
• Matrix transpose
• Better done in ALUs than in memory
• Shown to have an order-of-magnitude better
  performance
• Done in ALUs as repeated odd-even groupings

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Odd-even grouping
0 1 2 3 4 5 6 7 ? 0 2 4 8 1 3 5 7
Inter-cluster communication
Entire chip length Limits clock frequency Limits
scaling
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A reduced inter-cluster comm network
only nearest neighbor interconnections
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Outline

• Background
• Wireless systems
• Stream processors
• Mapping algorithms to stream processors
• Power efficiency
• Design exploration
• Broad impact and future work

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Flexibility needed in workloads
3G Workload variation from 1 GOPs for 4 users,
constraint 7 viterbi to 23 GOPs for 32 users,
constraint 9 viterbi
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Flexibility affects DP
U - Users, K - constraint length, N - spreading
gain, R - decoding rate
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When DP changes

• 4 ? 2 clusters
• Data not in the right SRF banks
• Overhead in bringing data to the right banks
• Via memory
• Via inter-cluster communication network

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Adapting clusters to Data Parallelism
SRF
Turned off using voltage gating to eliminate
static and dynamic power dissipation
Adaptive Multiplexer Network
Clusters
C
C
C
C
No reconfiguration
4 2 reconfiguration
41 reconfiguration
All clusters off
C
C
C
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Cluster utilization variation
Cluster Index
Cluster utilization variation on a 32-cluster
processor (32, 9) 32 users, constraint length
9 Viterbi
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Frequency variation on 32 clusters
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Operation

• Dynamic Voltage-Frequency scaling when system
  changes significantly
• Users, data rates
• Coarse time scale (every few seconds)
• Turn off clusters
• when parallelism changes significantly
• Memory operations
• Exceed real-time requirements
• Finer time scales (100s of microseconds)

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Power Voltage Gating Scaling
Power can change from 12.38 W to 300 mW (40x
savings) depending on workload changes
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Outline

• Background
• Wireless systems
• Stream processors
• Mapping algorithms to stream processors
• Power efficiency
• Design exploration
• Broad impact and future work

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Deciding ALUs vs. clock frequency

• No independent variables
• Clusters, ALUs, frequency, voltage (c,a,m,f)
• Trade-offs exist
• How to find the right combination for lowest
  power!

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Static design exploration
also helps in quickly predicting real-time
performance
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Sensitivity analysis important

• We have a capacitance model Khailany2003
• All equations not exact
• Need to see how variations affect solutions

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Design exploration methodology

• 3 types of parallelism ILP, MMX, DP
• For best performance (power)
• Maximize the use of all
• Maximize ILP and MMX at expense of DP
• Loop unrolling, packing
• Schedule on sufficient number of
  adders/multipliers
• If DP remains, set clusters DP
• No other way to exploit that parallelism

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Setting clusters, adders, multipliers

• If sufficient DP, linear decrease in frequency
  with clusters
• Set clusters depending on DP and execution time
  estimate
• To find adders and multipliers,
• Let compiler schedule algorithm workloads across
  different numbers of adders and multipliers and
  let it find execution time
• Put all numbers in power equation
• Compare increase in capacitance due to added ALUs
  and clusters with benefits in execution time
• Choose the solution that minimizes the power

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Design exploration for clusters (c)
DP
ILP

• For sufficiently large
• adders, multipliers per cluster
• Explore Algorithm 1 32 clusters
• Explore Algorithm 2 64 clusters
• Explore Algorithm 3 64 clusters
• Explore Algorithm 4 16 clusters

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Clusters frequency and power
32 clusters at frequency 836.692 MHz (p 1) 64
clusters at frequency 543.444 MHz (p 2) 64
clusters at frequency 543.444 MHz (p 3)
3G workload
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ALU utilization with frequency
3G workload
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Choice of adders and multipliers
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Exploration results

• Final Design Conclusion
• Clusters 64
• Multipliers/cluster 1
• Multiplier Utilization 62
• Adders/cluster 3
• Adder Utilization 55
• Real-time frequency 568.68 MHz for 128
  Kbps/user
• Exploration done in seconds.

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Outline

• Background
• Wireless systems
• Stream processors
• Mapping algorithms to stream processors
• Power efficiency
• Design exploration
• Broad impact and future work

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Broader impact

• Results not specific to base-stations
• High performance, low power system designs
• Concepts can be extended to handsets
• Mux network applicable to all SIMD processors
• Power efficiency in scientific computing
• Results 2, 3 applicable to all stream
  applications
• Design and power efficiency
• Multimedia, MPEG,

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Future work

• Dont believe the model is the reality
• (Proof is in the pudding)
• Fabrication needed to verify concepts
• Cycle accurate simulator
• Extrapolating models for power
• LDPC decoding (in progress)
• Sparse matrix requires permutations over large
  data
• Indexed SRF may help
• 3G requires 1 GHz at 128 Kbps/user
• 4G equalization at 1 Mbps breaks down (expected)

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Need for new architectures, definitions and
benchmarks

• Road ends - conventional architecturesAgarwal2000
  
• Wide range of architectures DSP, ASSP, ASIP,
  reconfigurable,stream, ASIC, programmable
• Difficult to compare and contrast
• Need new definitions that allow comparisons
• Wireless workloads
• Typically ASIC designs
• SPEC benchmark needed for programmable designs

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Conclusions

• Utilizing 100-1000s ALUs/clock cycle and mapping
  algorithms not easy in programmable architectures
• Data parallel algorithms need to be designed and
  mapped
• Power efficiency needs to be provided
• Design exploration needed to decide ALUs to meet
  real-time constraints
• My thesis lays the initial foundations