Processor Frequency Setting for Energy Minimization of Streaming Multimedia Application by A. Acquaviva, L. Benini, and B. Riccò, in Proc. 9th Internation Symposium on Hardware/Software Codesign, Apr

Agenda Introduction Power optimization model derivation Power optimization algorithm Experimental results Conclusion

Introduction With technology enhancement, multimedia capabilities are being added to handheld devices. Examples:  Picture taking  MP3 audio playback  Video playback  Audio recording A new problem arises  power management

Introduction Power management options  Shut down devices when in idle mode Problem: Background tasks have to be stopped as well  Better approaches: clock frequency and voltage regulation Lowers system speed in idle states Reduces LCD display brightness

Introduction Real-time media streaming applications  Retrieve stream data from off-CPU interface (e.g. discs, memory cards)  Process data (e.g. decoding, decompression)  Deliver processed data to output interface (e.g. display, speakers)

Power Optimization Model Derivation System settings:  The CPU must communicate with relatively slower I/O interfaces  Clock frequency can be adjusted by software  Frame-based media (e.g. MP3 audio, MPEG video)

Power Optimization Model Derivation Power consumption: Energy per frame: V – supply voltage, C – switched capacitance, f – CPU clock frequency, T f – frame processing time, N f – number of cycles to process frame, t – cycle time

Power Optimization Model Derivation Due to speed difference between the CPU and external hardware: where N idle is a non-decreasing function of f We now have

Power Optimization Model Derivation To satisfy real-time synchronization constraint: T max – maximum allowed time for a frame (e.g. 1/30s in 30fps movie)

Power Optimization Model Derivation In words, target of power optimization is to reduce the term N idle in under the constraint

Power Optimization Model Derivation This technique is more effective if application requires much memory access However, delay and energy spent for frequency adjustment must also be considered

Proposed Algorithm Define three curves FR B (f), FR A (f), FR W (f), the best- case, average-case and worst-case frame rate at f. Compute curves for all bit rate and sampling rate values and obtain FR o B (f), FR o A (f), FR o W (f) Compute FR req by N sample : samples per frame, fixed at 576 for MP1 and MP1 phase 2

Proposed Algorithm Normalize the curves FR o B (f), FR o A (f), FR o W (f) by FR o A (f max ) from a pre-calculated look-up table Intersect FR req with three curves to obtain f min, f av and f max.

Proposed Algorithm CPU frequency can be set to:  f min if we find constantly frames processed faster than the average rate  f av if we want continuous playback, with some buffering storage for decoding rate jitter  f max to guarantee real-time performance on a frame-by-frame basis  Greater than f max if the processor is not dedicated to the application only

Experimental Results Energy consumption per frame

Experimental Results Energy consumption for 16KHz, 16KBit/s audio

Experimental Results Frequency setting

Example Calculation An audio stream of 16KHz, 16KBit/s Without any optimization, E f = mJ FR req = / 576 = fps FR A (f max ) = fps Normalized FR = 27.8 / = fps f min = 85.7MHz, f max = 106.7MHz Choosing f max, E f = 8.9mJ  19% energy reduction

Conclusion Frequency-energy relationship is derived An energy optimization algorithm is proposed Experiment shows dramatic save in power consumption