1. Performance directed energy management using BOS technique
Pratap Ramamurthy
Ramanathan Palaniappan
University of Wisconsin-Madison

2. Outline
Introduction
BOS Mechanisms
Policies
Results
Conclusion

3. Introduction

4. Introduction Energy consumption in mobile devices and laptops
Memory could consume 50% more power than processors
Is there hardware support ?
RAMBUS devices provide capability

5. Problem Statement
How to save power in memory without hurting performance ?
Solution: BOS
Estimating the optimal amount of memory based on paging activity.
Dynamic resizing of memory !

6. What is BOS ? BOS – Ballooning in-OS technique
Tracks current memory requirements
Tracks chip access pattern
Powers down memory chips
Minimizes power consumption

7. BOS Mechanisms Ballooning in-OS technique Chip selection Power Down
Page Migration
Page Reservation
Invisible Buddy
Chip recovery (Power up)
Chip selection
Intercept memory accesses

8. BOS policies Power decision Epoch Chip selection
Disk activity, # chips powered on
Epoch
Time interval between two decisions
Chip selection
Access pattern

9. Inferences
Operating point balances disk accesses and memory power consumption
Cost of thrashing offsets the power saved from a single chip
Most media applications do not need the entire RAM
Least allocated chips are not the least accessed ones

10. BOS Architecture Migrate Chip Power Table (CPT) Pin the page
Wake up
Kpower_d
Wait an Epoch
Get system
parameters
Decide to
Power off/on
Do Nothing ON
Recover PowerPages
Clear flags & unPin
OFF
Select Chip
Loop all pages in chip
Update
Chip Power Table
(CPT)
If occupied
Migrate
If page is free
Pin the page If
Locked or reserved
Wait & Pounce

11. BOS Mechanisms

12. Mechanisms In-OS Ballooning Techniques Chip Selection
Page Migration
Page Reservation
Invisible Buddy
Chip Recovery
Chip Selection
Intercept Memory Accesses
No Actual Power Down

13. So replace page using LRU…
Power down
Selected Chip
Under Memory pressure ???
DISK
So replace page using LRU…
And Migrate!

14. Page Migration When to migrate ? Kinds of pages File pages
Anonymous pages
Swap cache pages
Free Pages

15. Page Migration Mechanism
Data Transfer
Reverse map (thanks to Linux Kernel 2.6)
Remap
Dependant data structures
Page Table
Active LRU and Inactive LRU
Page cache
Swap cache
Buffers
Flags

16. Page Migration Major Data Structures Updated while Migrating
Processes
Buffers
LRU lists
1. Memcopy
2. Remapping using rmap
3. Update buffers
4. Remove from LRU lists

17. Page Reservation Ways to Pin pages Remove from LRU Lists Reserve Page
Lock Page
Page count

18. Free Pages Buddy allocator List of free pages Buddy Order (0-11)
Locality
How to deal with already free pages?

19. Invisibility in Action (Buddy allocator)2 1
Make the ‘power’ pages invisible
Request for a free page of order 0
Request for a page of order 1
(Invisibility in action)

20. Invisible Buddy Power Bit Invisibility – divert page allocation
Other methods?
Why not always remove the page completely from the buddy allocator?
Because the free pages are not necessarily in the buddy

21. Recovery Page in the buddy Pages that were migrated Reverse Recovery
Clear power bit
That’s it !
Pages that were migrated
Clear Power bit
Add to the buddy
Reverse Recovery
Recover the last pinned page and move backwards

22. Partial Recovery Why would you abort a chip? Easy solution
Locked pages
Reserved pages
IO activity
Easy solution
Abort chip
Better solution
Wait and Pounce

23. Policies

24. Policy Epoch Power decision Chip select
Time interval between two decisions
Power decision
Disk activity, # chips powered on
Chip select
Access pattern

25. Chip Selection Mechanism
Chip pattern table
Access history over 32 epochs
Form a number with the bits and Select the minimum !
How to monitor chip access ?
Referenced bit in h/w
Clear bits
Examine every epoch 1
0 0 1
1 0 1
0 0 1
0 1 1
0 0
The least accessed chip is # 6
The least accessed chip is # 7

26. Results

27. Results

28. Chip access pattern

29. Results
# Off
Available Memory (MB)
512 10
352 15
272 20
192

30. Results

31. BOS architecture Migrate Chip Power Table (CPT) Pin the page
Wake up
Kpower_d
Wait an Epoch
Get system
parameters
Decide to
Power off/on
Do Nothing ON
Recover PowerPages
Clear PowerBit & unPin
OFF
Select Chip
Loop all pages in chip
Update
Chip Power Table
(CPT)
If occupied
Migrate
If page is free
Pin the page If
Locked or reserved
Wait & Pounce

32. Power down/up decision
If ( # disk_access < α )
power_down()
elseif ( # disk_access > β )
power_up()
elseif (α < # disk_access < β )
take_no_action()
α, β – thresholds, α < β

33. Results

34. Results Other workloads Application # chips ON Memory used
Xmms Mp3 player 7
112 MB
Totem media player (X-server) 25
400 MB
Mozilla,X-server, Open-office (together) 12
192 MB

35. Conclusion

36. Conclusion BOS technique is feasible
BOS helps us to vigorously control memory size
Chip allocation and access pattern have no correlation
Several applications do not require the full memory
Threshold based policy gives reasonable performance
An incorrect chip select could increase migration overhead

37. Summary BOS philosophy Implementation of BOS Mechanism Policy
Page migration
Invisible buddy
Chip recovery
Chip access pattern
Policy
Study of sample workloads
Threshold based policy

38. Acknowledgements lxr.linux.no
We would like to thank the following people for their valuable time and for the immensely helpful discussions in the course of this project
Remzi H. Arpaci-Dusseau
Muthian Sivathanu
Vijayan Prabhakaran
Lakshmi Bairavasundaram
Amit Jhawar for resources

39. ?

40. References
Huang et.al. “Design and Implementation of Power-Aware Virtual Memory”, USENIX 2003.
Lebeck et.al. “ Power Aware Page Allocation”, ASPLOS 2000.
Li et.al. “Performance directed energy management for main memory and disks”, ASPLOS 2004.
Delaluz et.al. “Scheduler-Based DRAM Energy Management”, DAC 2002.

41. Related Work Power Aware Virtual Memory [1] Per process chip select
Localized per process page allocation
Performance not considered
Execution Time based energy management [ref]
Hardware support for energy management
Various power modes in RAMBUS
Various power modes in disks

42. Tough nuts to crack Absolutely no Documentation for Linux 2.6 memcpy()
Alternate recovery mechanism
Multiple chip select policy
Alternate Allocator mechanism
Removing from LRU