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Adaptive Backpressure: Efficient Buffer Management for On-Chip Networks Daniel U. Becker, Nan Jiang, George Michelogiannakis, William J. Dally Stanford.

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Presentation on theme: "Adaptive Backpressure: Efficient Buffer Management for On-Chip Networks Daniel U. Becker, Nan Jiang, George Michelogiannakis, William J. Dally Stanford."— Presentation transcript:

1 Adaptive Backpressure: Efficient Buffer Management for On-Chip Networks Daniel U. Becker, Nan Jiang, George Michelogiannakis, William J. Dally Stanford University Concurrent VLSI Architecture Group ICCD 2012, 9/30/12–10/3/12, Montreal, Canada

2 Overview Input buffer sharing is attractive in NoCs Improves area and power efficiency But facilitates spread of congestion Adaptive Backpressure mitigates performance degradation by avoiding unproductive use of buffer space in the presence of congestion Avoid downsides of buffer sharing while maintaining benefits in benign case Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure 210/3/12

3 Dynamic Buffer Management Buffer space is expensive resource in NoCs – 30-35% network power (MIT RAW, UT TRIPS) Dynamic management increases utilization by sharing buffer space among multiple VCs – Optimize use of expensive buffer resources – Decrease incremental cost of VCs Improved area and power efficiency 25% more throughput or 34% less power [Nicopoulos06] Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure 310/3/12

4 Buffer Monopolization Blocked flits from congested VC accumulate in buffer Effective buffer size reduced for other VCs Performance degradation (latency / throughput) Congestion spreads across VCs (flows / apps / VMs / …) Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure 410/3/12 VC 0 VC 1

5 Adaptive Backpressure Goal: Avoid unproductive use of buffer space But allow sharing when beneficial Approach: Match arrival and departure rate for each VC by regulating credit availability (backpressure) Derive quota from credit round trip times 10/3/12 Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure 5

6 Quota Motivation (1) Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure T crt,0 Without congestion, full throughput requires T crt,0 credits Router 0Router 1Router 0Router 1 610/3/12 Credit stall Insufficient credit supply causes idle cycle downstream Idle cycle time

7 Quota Motivation (2) Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure Congestion stall Credit stall Matching stalls avoids unproductive buffer occupancy Router 0Router 1Router 0Router 1 Excess drained 710/3/12 Queuing stall T crt,0 +T stall Congestion stall Queuing stall Excess flits Congestion stall causes unproductive buffer occupancy Excess flits time

8 Quota Heuristic Track credit RTT for each output VC RTT=RTT min set quota to RTT min – No downstream congestion Allow one flit in each cycle of RTT interval RTT>RTT min subtract difference from RTT min – Each congestion and queuing stall adds to RTT Allow one credit stall per downstream stall Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure 810/3/12

9 Implementation Network design determines RTT min for each link Track RTT for single in-flight credit per VC Update quota value upon return Switch allocator masks all VCs that exceed quota Simple extension to existing flow control logic No additional signaling required < 5% overhead for 16x64b buffer with 4 VCs 10/3/12 Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure 9

10 Evaluation Methodology BookSim 2.0 8x8 2D mesh, 64-bit channels, DOR 16-slot input buffers, 4 VCs Combined VC and switch allocation Synthetic traffic and application benchmarks Compare ABP to unrestricted sharing 10/3/12 Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure 10

11 Network Stability (1) For adversarial traffic, throughput in Mesh is unstable at high load – Traffic merging causes starvation – Tree saturation causes widespread congestion ABP improves stability – Throttles sources that inject at very high rate – Efficient buffer use reduces tree saturation Faster recovery from transient congestion 10/3/12 Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure 11

12 Network Stability (2) Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure [tornado traffic] 6.3x 1210/3/12

13 Network Stability (3) Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure [foreground traffic at 50% injection rate] 3.3x -13% saturation rate 1310/3/12

14 Performance Isolation (1) Inject two classes of traffic into network – Shared buffer space, separate VCs Sharing causes interference between classes ABP reduces interference – Contains effects of congestion within a class Better isolation between workloads, VMs, … 10/3/12 Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure 14

15 Performance Isolation (2) Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure [uniform random foreground traffic] [hotspot background traffic][uniform random background traffic] -33% -38% 1510/3/12

16 Performance Isolation (3) Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure [50% uniform random background traffic] -31% w/o background 1610/3/12

17 Application Performance (1) 10/3/12 Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure 17 Array of stream processors Streaming data to memory Modeled as hotspot traffic In-order general purpose core Running at 4x network frequency Executing PARSEC benchmarks Modeled using Netrace [Hestness11] Common network Disjoint VC ranges Shared buffer space 8 interleaved memory controllers Heterogeneous network nodes

18 Application Performance (2) Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure [12.5% injection rate for streaming traffic] -31% w/o background 1810/3/12

19 Conclusions Sharing improves buffer utilization, but can lead to undesired interference effects Adaptive Backpressure regulates credit flow to avoid unproductive use of shared buffer space Mitigates performance degradation in presence of adversarial traffic But maintains key benefits of buffer sharing under benign conditions Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure 1910/3/12

20 THE END Thank you for your attention! Becker, Jiang, Michelogiannakis, Dally: Adaptive Backpressure 2010/3/12


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