ECE 506

Reconfigurable Computing

http://www.ece.arizona.edu/~ece506

Lecture 6

Clustering

Ali Akoglu

Before Placement: Clustering° Intra-cluster connections: fast° Inter-cluster connections: slowNeed to pack BLEs ° Goals:

• Reduce stress on routing• Take advantage of local fast

interconnect• Reduce inter-cluster wiring• Minimize critical path (timing-

driven)° How do we do this

• Take advantage of cluster architecture

° Tradeoffs

Basic Clustering (Betz)

° How many distinct inputs should be provided to a cluster of N 4-LUTs?

° How many 4 LUTs should be included in a cluster to create the most area-efficient logic block?

VPACK

Basic Clustering (Betz)

° Flow• Iterate until all BLEs consumed• Start new cluster by selecting a random BLE

- select the currently unclustered BLE with the most used inputs,• Add BLE with most shared inputs with current cluster to

cluster- to minimize the number of inputs that must be routed to

each cluster.• Keep adding until either cluster full or input pins used up• Hill climbing – if some cluster BLEs unused

- Add another BLE even if cluster input count temporarily overflowed

- If input count not eventually reduced select best choice from before hill climbing

Logic Utilization

Number of Inputs per Cluster

• Lots of opportunities for input sharing in large clusters (Betz – CICC’99)

• Reducing inputs reduces the size of the device and makes it faster.

• Most FPGA devices (Xilinx, Lucent) have 4 BLE per cluster with more inputs than actually needed.

TVPACK

Architecture Modeling

Tri-state buffer and pass transistor distributionCluster Size vs. Routing resources (Tile size)Transistor and Buffer Scaling based on segment lengthFlexibility of Switches (Fc=W for large cluster size is a waste?)

Logic Cluster Structure

Timing-Driven Clustering – T-VPACK

° Optimization goals of VPack• Pack each cluster to its capacity

- Minimize number of clusters• Minimize number of inputs per cluster

- Reduce the number of external connections

Timing-Driven Clustering – T-VPACK

° Optimization goal of T-VPack• Minimize number of external connections on critical

path• Why?

- External connections have higher delay and internal connections

- Reducing number of external nets on critical path will reduce delay

Timing-Driven Clustering – T-VPACK

° First stage• Identify connections that are on the critical path

° Second Stage• Pack BLEs sequentially along the critical path • Recompute criticality of remaining BLEs

Slack and Criticality Calculation

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Slack and Criticality Calculation

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Slack and Criticality Calculation

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arrival time/required time

Slack and Criticality Calculation

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Slack = required time - arrival time

Slack and Criticality Calculation

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Slack = required time - arrival time

Slack and Criticality Calculation

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Critical Path

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Timing-Driven Clustering – T-VPACK

° Cost metric now considers both connectivity and timing criticality

° Perform an analysis of criticality at beginning considering all wires to be inter-cluster

° Determine “Base” BLE criticality

Base Criticality

How to break ties?

° Initially, many paths may have the same number of BLEs

° Include “tie-breaking” in performance cost function

Results for T-VPACK versus VPACK

Why does the gap between VPack and T-VPack increase as N increases?

Results for T-VPACK versus VPACK

° T-VPack prefers to cluster a BLE with BLEs that are in its fan-in or fan-out

° VPack favors input sharing° T-VPack completely absorbs many low-fanout nets

• Fewer nets to route!

Results for T-VPACK versus VPACK

Why does area-delay product show an increasing trend beyond cluster size of 10?

Results for T-VPACK versus VPACK

° Increased number of nets that are completely absorbed by T-Vpack

° Area- delay product• Cluster size 7-10 best choice (36-34% better than N=1)

° N=7 vs N=1• 30% less delay, 8% les area

Results for T-VPACK, DELAY !!!

Why do we see a circuit speedup?

Results for T-VPACK, DELAY !!!

18%

40%

° Intra-cluster: Fast, Inter-cluster: Slow !° As N increases

• Number of internal connections on the critical path increase• Number of external connections on the critical path decrease

Why are inter-cluster connections becoming faster?

Reduction in Number of external connections (internal connections are faster)External connections on the critical path are becoming faster

Reduction in routing requirements

Drawback of VPack and T-VPack