Kwangil Choi, Hyunok Oh Hanyang University

1. An Energy Aware Buffer Mapping Technique on Multiple Retention Time STT-MRAMs for Stream Applications Kwangil Choi, Hyunok Oh Hanyang University

2. Outline • Introduction • Non-volatile Memory (NVM) • Synchronous dataflow (SDF) • Problem Definition • Answer Set Programming • Experiment • Conclusion

3. Introduction • Non-Volatile Memory (NVM) • Replace DRAM for main memory

4. STT-MRAM Cell Structure Top Electrode current Top AF Layer Spacing Layer Synthetic Anti ferromagnetic (SyAF) structure Bottom AF Layer free layer MTJ* Buffer Layer tunnel oxide Capping Layer Pinned Layer fixed layer Tunnel Barrier Free Layer Seed Layer Bottom Electrode(substrate) Gate Source Drain (*MTJ : Magnetic Tunnel Junction)

5. Reduction of Retention Time The reduction of the retention time contributes the cell density, leakage power, dynamic power consumption, and performance.

6. STT-MRAMs with Different Retention Times

7. System with Multiple Retention Time Memories 26.5μs STT1 memory 3.24s STT2 memory processor 4.27y STT3 memory

8. Application Specification • Synchronous dataflow (SDF) • represents streaming applications like multimedia that require frequent memory access • Node(Actor) - functional algorithm • Edge - communication between two actors • Producing / Consuming rate • the number of produced and consumed samples • Rate is fixed 3 2 A B

9. Motivational Example } 1ms Tretention= 1 ms Tretention= 4 ms 16 refresh operations no refresh operation

10. Outline • Introduction • Non-volatile Memory (NVM) • Synchronous dataflow (SDF) • Problem Definition • Answer Set Programming • Experiment • Conclusion

11. Problem Definition • Input • Target architecture: A system with multiple relaxed retention time STT-MRAM modules. Note that the memory refreshes memory cells containing valid data. • Characteristics of STT-MRAM : retention time, read/write energy, and refresh energy. • Application : An application is specified in SDF model. A schedule and the execution time of each node are given. • Goal • Minimization the total energy consumption on the memory system for the application. • Output • The mapping of buffers to STT-MRAM modules.

12. Problem Definition Map the buffer to memory to minimize the energy consumption 3 2 A B SDF graph, schedule and execution time are given A system with multiple retention time memories

13. Our Approach 1. Construct a schedule AACBDDFEEEE Energy consumption = 13860 STT- Short STT- Long Energy consumption = 15033 2. Build lifetime chart 3. Determine buffer mapping

14. Buffer Mapping Formulation • Write energy • Refresh energy • Total energy = write energy+refresh energy

15. Outline • Introduction • Non-volatile Memory (NVM) • Synchronous dataflow (SDF) • Problem Definition • Answer Set Programming • Experiment • Conclusion

16. Answer Set Programming (ASP) • Declarative approach for NP problems • Problem - logic predicates “ AND” • Solutions - answer sets • Easy to understand the formulation • Fast ASP solvers have been introduced

17. Problem Solving Flow Answer: 1memory_type(2,1) memory_type(1,1) map(8,2) map(7,2) map(6,2) map(5,2) map(4,2) map(3,2) map(2,2) map(1,2) buffer_energy(8,3123) buffer_energy(7,11196) buffer_energy(6,12438) buffer_energy(5,4850) buffer_energy(4,11196) buffer_energy(3,11196) buffer_energy(2,11196) buffer_energy(1,19269) Optimization: 84464 Answer: 2memory_type(2,2) memory_type(1,1) map(8,2) map(7,2) map(6,2) map(5,2) map(4,2) map(3,2) map(2,2) map(1,2) buffer_energy(8,5004) buffer_energy(7,5994) buffer_energy(6,5994) buffer_energy(5,6660) buffer_energy(4,5994) buffer_energy(3,5994) buffer_energy(2,5994) buffer_energy(1,5994) Optimization: 47628 Answer: 3memory_type(2,2) memory_type(1,1) map(8,2) map(7,2) map(6,2) map(5,1) map(4,2) map(3,2) map(2,2) map(1,2) buffer_energy(8,5004) buffer_energy(7,5994) buffer_energy(6,5994) buffer_energy(5,4850) buffer_energy(4,5994) buffer_energy(3,5994) buffer_energy(2,5994) buffer_energy(1,5994) Optimization: 45818 OPTIMUM FOUND lifetime(E,Inv, Duration) :- fire(A,S), fire(B,F), edge(E,A,B,P,C,I), numFiredBefore(A,S,SN), numFiredBefore(B,F,FN), Inv=FN*C-C+1..FN*C, SN*P-P <Inv, Inv <= SN*P, accRealTime(Start,S), accRealTime(Finish,F), Duration=Finish-Start+ATime, Duration>0, Inv>0, Inv<= R*P,repetition(A,R), S<F, extime(A,ATime). buffer_energy(E,Write*P*Rep+Refresh*Energy) :- Energy = [lifetime(E,T,Duration)=Duration/Retention+1], edge(E,A,_,P,_,I), retention(Type,Retention), memory_type(M,Type), map(E,M), refresh_energy(Type,Refresh), write_energy(Type,Write), repetition(A,Rep). 1 { memory_type(M,T) : retention(T,_) } 1 :- memory(M). 1 { map(E,M) : memory(M) } 1 :- edge(E,_,_,_,_,_).sample(E,S-C,T) :- fire(B,T), edge(E,_,B,P,C,I), sample(E,S,T-1), S>= C, time(T). #minimize [buffer_energy(E,Energy)=Energy : edge(E,_,_,_,_,_)]. 5 2 2 A B 3 3 SDF Graph ASP formulation Result 3 node(1..2). edge(1,1,2,3,3,3).edge(2,2,1,2,2,5). repetition(1,1).repetition(2,1).

18. Outline • Introduction • Non-volatile Memory (NVM) • Synchronous dataflow (SDF) • Problem Definition • Answer Set Programming • Experiments • Conclusion

19. Experiments • Synthetic examples • 7 randomly generated examples • 3 to 7 actors • Real-life applications • Part of CELP • H.263 encoder / decoder • MP3 decoder • Each node n has the execution time • T(n) = k*Ti(n) • where k represents the scale factor, Ti(n) the initial execution time, and T(n) the execution time of node n.

20. Synthetic SDF Graphs

21. Real-life SDF Graphs

22. Assumed STT-MRAM Parameters

23. Normalized Energy Consumption w/ multiple retention memories Scale factor = 1 Scale factor = 10 Scale factor = 50000 Scale factor = 1000

24. Mapped STT-MRAMs ratio scale factor

25. Normalized Energy Consumption w/ an STT-MRAM Scale factor=1 Scale factor=10

26. Conclusion • Buffer mapping algorithm for a system with multiple retention STT-MRAM memories can reduce the energy consumption by 30~70%. • The mapped STT-MRAM memory is dependent on the variable lifetime.

27. Thank you