1. Power efficiency as an analogue to memory management Sara Alspaugh and Arka Bhattacharya

2. Conflicting goals? To save energy one needs to shut down servers Shutting down servers incurs huge latency penalty on active-set misses. Take Lessons from Memory Management.

3. Memory Management CACHE (<1 ns) MEMORY DRAM (~10 ns) ACTIVE DISK (~10 ms) P miss,cache P miss,memory T avg = T cache + P miss,cache.T memory + P miss,cache.P miss,memory.T disk Penalty : 100 Penalty : 1000 REQUESTS

4. Disk Management CACHE (<1 ns) MEMORY DRAM (~100 ns) ACTIVE DISK (~10 ms) T avg = T cache + P miss,cache.T memory + P miss,cache.P miss,memory.T disk + P miss,cache.P miss,memory.P miss,active-disk.T inactive-disk IN-ACTIVE DISK (~5 sec) IN-ACTIVE DISK (~5 sec) P miss,active-disk P miss,cache P miss,memory Penalty : 100 Penalty : 1000 Penalty : 5000 REQUESTS

5. For interactive jobs, average latency needed < 100 ms

6. Permissible active-disk misses T avg ≈ T active-disk + P miss,active-disk.T non-active-disk

7. Permissible active-disk misses (100ms) ≈ (10ms) + P miss,active-disk.(5 sec) T active-disk ≈ (10ms) T non-active-disk ≈ (5 sec) For interactive jobs : T avg < (100 ms) P miss,active-disk < 2%

8. Permissible active-disk misses T avg ≈ T active-disk + P miss,active-disk.T non-active-disk TO CREATE ENERGY-PROPORTIONAL STORAGE, WE THUS HAVE TO MAINTAIN A NON-ACTIVE STORE AND REDUCE P miss,active

9. Techniques 1.Put disks to sleep after a loiter time, and wake-up on demand 2.Caching 3.Prefetching disk blocks 4.Dynamic object creation/Intelligent layout T avg ≈ T active-disk + P miss,active-disk.T non-active-disk

10. Wikipedia 1-day trace Replicated thrice Partitioned into 5 disks

11. MSR-Cambridge Web/SQL server 7 day trace Not replicated 4 volumes 10 disks each

12. 1. Putting disks to sleep after Loiter

13. ACTIVE DISK Request Stream NON- ACTIVE DISK LOITER TIME ?

14. What loiter time to set ? Energy Improvement Latency Increase Low Loiter TImes High Loiter TImes Desired 100ms

15. Active-set misses vs aggressiveness of loiter

16. Putting it together

17. Active-disk misses

18. Do we really care about average latency ? 95 th percentile Loiter Time = 100 sec MSR

19. 2. Caching

20. File access frequency for top 5000 accessed files

22. Options 1.Caching on an SSD 2.Keep active set on and keep some spinning reserve

23. CACHE (<1 ns) MEMORY DRAM (~10 ns) ACTIVE DISK (~10 ms) SSD cache (~50 μs) REQUESTS

24. What to cache on ? DRAMSSDHard Disk Latency5-10ns60 μs10ms Cost / GB$100$1-$25-10 ¢ Bandwidth1500 MB/s300-500 MB/s50-100MB/s Energynegligible1-3W10W

25. Increase in Average Idle Time with Increase in Cache Size

26. How aggressive are you ? Different loiter times – (1) MSR

27. How aggressive are you ? Different loiter times – (2) Wikipedia

28. CACHE (<1 ns) MEMORY DRAM (~100 ns) FIRST REPLICA (~10 ms) IN-ACTIVE DISK (~5 sec) IN-ACTIVE DISK (~5 sec) SPINNING RESERVE Keeping an active set and spinning reserve REQUESTS

29. Active Set, Spinning Reserve, Normal Wikipedia

30. 3.Prefetching

31. Spatial correlation Average : 0.3 % TIME WINDOW BLOCK NUMBER

32. Graph of different prefetching constants - Microsoft MSR

33. Latency Analysis MSR

34. 4. Intelligent layout of Objects

35. ACTIVE DISK Request Stream

36. ACTIVE DISK Request Stream NON- ACTIVE DISK

37. Wikipedia

38. Summary Not much different from memory management Very simple techniques which could be optimized on the trace. Large energy savings to be had if we are willing to pay slight latency penalties.