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

Motivation Use power-gating and turn off unused entries! [Kumar et al., ICCD’07] Allocator 3% Router Power Breakdown Crossbar Switch Input buffer Clock 46% 16% 35% 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. Buffer size Use power-gating and turn off unused entries!

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

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

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

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

Unified mode 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 FLIT 0 ON Primary region ON FLIT 1 FLIT 2 ON OFF Not used ON OFF Secondary region OFF OFF

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. Primary region is empty! Flits are read out from here. Primary region Secondary region Primary region Secondary region FLIT 3 ON Next flit’s place is NOT available. ON FLIT 1 OFF FLIT 2 ON Yet, there are unused entries. FLIT 4 ON Flits are written to here. OFF FLIT 5 ON OFF ON OFF OFF Split queue Unified mode

Summary of Evaluation 13% 39% Simulator : Cycle-accurate OCN simulator - Booksim Power Measurement - Orion 2.0 Parameter Topology 8x8 2D mesh Technology node 32nm # of VCs 4 Clock frequency 1.5GHz VC buffer depth 8 Vdd 1.0 V Performance Power consumption 13% 39%

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

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