FlexiBuffer : Reducing Leakage Power in On-Chip Network Routers

1. FlexiBuffer: Reducing Leakage Power in On-Chip Network Routers SungjooYoo Dept. of Electronic and Electrical Engineering Pohang University of Science and Technology Gwangsun Kim, John Kim Dept. of Computer Science Korea Advance Institute of Science and Technology

2. Motivation Router Power Breakdown Buffer size • On-chip network is becoming more critical. • Buffer size has a huge impact on performance. • Buffers take a large portion of router power. • However, not all of the buffers are fully utilized even at a high load. Allocator 3% Clock buffer Crossbar Switch 16% 35% Input buffer 46% [Kumar et al., ICCD’07] Use power-gating and turn off unused entries!

3. Our Approach • Dynamically adjust the active window size. • Active window: set of ON (or active) entries of a buffer. Active window F F At a low traffic load ON OFF F F F F F F F At a high traffic load

4. Issue 1: Flow Control Case 1: Increase the active window size using early credit • When? • There is an incoming flit. • There is an OFF buffer entry. • There is congestion in both upstream and local router. • Need to communicate the availability of buffers Router 0 Router 1 Router 2 flit flit F ON OFF credit credit CR 2 1

5. Issue 1: Flow Control (cont’d) Case 2: decrease the active window size by withholding credit. • When? • There is an outgoing flit. • There is more than the minimum # of ON entries. Router 0 Router 1 Router 2 flit flit F F credit credit 2 CR

6. Issue2: Circular Queue Problem • When utilization is low, each incoming flit turns on an entry. →Each activation of an entry incurs power overhead! • Ideal buffer management • The same entry is reused. • Problematic circular buffer • Each flit activates an entry. ON OFF OFF ON FLIT 0 FLIT 0 OFF FLIT 2 FLIT 4 FLIT 1 FLIT 3 FLIT 1 OFF ON OFF FLIT 2 OFF ON OFF ON OFF FLIT 3 OFF ON OFF OFF FLIT 4 Large power overhead No power overhead

7. Split Queue • A buffer is separated into two regions. • Use the primary region only (as long as possible). • Adjust the active window size dynamically. Operate like a circular queue Unified mode ON FLIT 0 Primary region FLIT 1 ON Not used ON FLIT 2 OFF ON OFF Secondary region OFF OFF

8. Split Queue (cont’d) • Cannot stay in the unified mode indefinitely. • Switch to split mode. • When the primary region is empty, • Switch back to unified mode. FLIT 3 ON Primary region FLIT 1 ON Next flit’s place is NOT available. FLIT 2 ON OFF Primary region OFF FLIT 4 ON Flits are written to here. Flits are read out from here. Secondary region OFF ON FLIT 5 Primary region is empty! OFF ON OFF Yet, there areunused entries. Secondary region Unified mode Split queue

9. Summary of Evaluation Simulator : Cycle-accurate OCN simulator - Booksim Power Measurement - Orion 2.0 Performance Power consumption 13% 39%

10. Conclusions • There’s a huge opportunity of power-saving with fine-grained power gating when buffers are large. • Proposed modified credit-based flow control. • Split queue is proposed to minimize activation power overhead. • Our simulation results show that, with minimal performance loss, FlexiBuffer + SQ can save • 39% of router power at low traffic load • 13% of router power at high traffic load

11. Thank you! Questions? For more discussion, please come to my poster!