A Computing Origami: Folding Streams in FPGAs

S. M. FarhadPhD Student

University of Sydney

DAC 2009, California, USA

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Outline

Motivation Stream programming FPGA Problem

Stream Folding Results Conclusion

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Stream Programming Paradigm

Programs expressed as stream graphs Streams: Sequence of data elements Actor: Functions applied to streams

Independent actors with explicit communication

Regular and repeating computation

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Actor/Filter

Streams

Streams

FPGA

FPGAs are widely available as programmable coprocessors

Opportunities to exploit FPGA-based acceleration Multimedia, networking, graphics, and security codes

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Problem

Maximizing throughput subject to Area and latency constraints

Resolving bottleneck actors The replicated filters do not require resynthesis

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Motivating Example

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Motivating Example

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Motivating Example

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Outline

Motivation Stream programming FPGA Problem

Stream Folding Results Conclusion

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Area/Throughput Design Folding

1 foreach Filter f in S do2 workFactor[f] = f.latency.S.runs(f);3 designPointArea + = f.area.workFactor[f];4 scaleLimit = minf.hasState (1/workFactor[f]); 5 scaling = min(AREA/designPointArea, scaleLimit);6 foreach Filter f in S do7 replication[f] = workFactor[f].scaling;8 while area(replication) > AREA do9 replication = reduceThroughput(replication);

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Calculating Throughput

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Calculating Latency

FPGAs that are coupled to host processors Initiation interval (DMA) Replication improves throughput, it often

increases the latency! Major factors for latency variation

Non-periodic data arrival Data-token reordering Local congestion

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Latency constrained design folding

1 latConf= null ; T = ∞;2 while throughput(thrConf) ≤ T do3 if feasibleImprovement(thrConf) then4 candidates = simAnnealing(thrConf, T);5 foreach candidate in candidates do6 if throughput(candidate) < T then7 latConf = candidate;8 T = throughput(latConf);9 thrConf = reduceThroughput(thrConf);10 return latConf

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Results

Benchmark

Minimum area Best throughput Constrained design

LUTs Latency II LUTs Latency II LUTs Latency IIConstraint

Run time

MatrixMult 1498 480 19 7618 185 3 4558 175 7

Latency ≤ 175 1.14s

Serpent 3028 1027 4 3878 773 2 3053 901 4Latency ≤ 910 0.73s

FFT2 37610 1199 3 43370 764 2 39530 868 7AREA ≤ 40000 34.7s

FMRadio 37458 371 39 87564 371 13 62511 371 20AREA ≤ 65000 1.01s

DCT 45752 349 3 137256 349 1 91504 349 2AREA ≤ 120000 0.73s

BitonicSort 43920 1042 3 131760 1042 1 47400 1282 2

AREA ≤ 50000 18.3s

Synthetic 350 309 135 15990 504 2 1490 309 47

AREA ≤ 1500 0.43s

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Questions?