Run-Time Power-Down Strategies for Real-Time SDRAM Memory Controllers

Karthik Chandrasekar1, Benny Akesson2, and Kees Goossens2

1TU Delft and 2TU Eindhoven, The Netherlands

Karthik Chandrasekar

TU Delft

Save Energy @ No Performance Impact

Context here: SDRAM Memories

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Problem Statement & Proposed Solutions

SDRAMs contribute significantly to SoC energy profile, even when idle. Powering down impacts performance, due to power-up latencies. Existing SDRAM memory controllers provide :

Either “Low power consumption” or “Real-Time performance” not “Both”. Other existing real-time low-power solutions use compile-time info and

are not suitable for run-time memory controller use. We propose :

Run-time power optimization solutions for real-time SDRAM controllers. We guarantee :

Significant energy savings without impacting bandwidth guarantees. We support :

SDRAM memory controllers using Predictable arbiters such as:

Round-Robin, Time Division Multiplexing, Priority-based arbiters etc.

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Arbiters, Requests & Guarantees Predictable Arbiters such as Round-Robin, TDM, etc. provide:

Maximum Latency Bounds Minimum Bandwidth Guarantee

Such performance guarantees are based on : Request Sizes & Service Cycle Length (SCL)

The smallest SCL (min_SCL) defines Scheduling Interval (SI) and Idle SCL. The longest SCL (max_SCL) defines the guaranteed Net Bandwidth.

Micron 1Gb, DDR3-800 using Closed-Page BC-4, BI-1 for 64B requests.

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Deriving Latency-Rate Arbiter Guarantees A Latency-Rate arbiter guarantees a requester :

Maximum Latency Bounds Minimum Bandwidth Guarantee

Deriving guarantees for R1 when backlogged using Round-Robin arbiter

Maximum Latency Bound(Θ) = tBLOCK + (x+1) * max_SCL + tREFRESH

Net Bandwidth (Net_BW) = num(max_SCL) * Request Size / tREFI

Minimum Guaranteed Bandwidth (β) = ρ* Net_BW

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Proposed Real-Time Power-Down Strategies Conservative Power-Down

Always powers-up within Scheduling Interval (SI) Aggressive Power-Down

Powers-up only when required; with Snooping Point @ SI – tPUP Request misses slot, if it arrives after Snooping point Only latency bounds increase and bandwidth guarantee is not affected. What if the request arrives after Snooping point?

Impact on Θ and β Conservative Power-Down

Θ does not change Max_SCL does not change

Aggressive Power-Down Θ increases by tPUP Max_SCL does not change

Speculative Power-Down Max_SCL increases

Latency Bound(Θ) = tBLOCK + (x+1) * max_SCL + tREFRESH

Net Bandwidth (Net_BW) = num(max_SCL) * request size / tREFI

Bandwidth Guarantee (β) = ρ* Net_BW Θ increases depending on number of interfering requesters (x) Net_BW and β decrease significantly depending on increase in max_SCL

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Impact on Energy & Performance

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Worst-Case Impact: Θ Increase:

Aggressive PD – 2.4% Speculative PD – 12.3%

β Decrease: Aggressive PD – 0.0% Speculative PD – 12.1%

Average Execution Time Penalty: Aggressive PD – 0.25% Speculative PD – 1.32%

Energy Savings: Conservative PD – 42.1% Aggressive PD – 51.3% Theoretical Best PD – 51.4%

4 Requesters/Apps, Round-Robin, Micron 1Gb, DDR3-800, 64B requests

Summary Proposed two real-time power-down strategies:

Conservative Latency-Bandwidth-Neutral and Aggressive Bandwidth-Neutral If memory goes idle, it powers-down (if it is gainful to power-down). @ Run-time, it checks if the memory can go to or continue to be in power-down.

Evaluated their impact on: Latency Bounds (Θ) Bandwidth Guarantee ( )β

Compared them against: Speculative power-down Theoretical best power-down

Showed impact on: Real-time performance guarantees Average-case execution time and energy savings

For more details: Please visit my poster!

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