1. Zombie Memory: Extending Memory Lifetime by Reviving Dead Blocks
John D. Davis2,
Rodolfo Azevedo1, John D. Davis2, Karin Strauss2,
Parikshit Gopalan2, Mark Manasse2, Sergey Yekhanin2
University of Campinas1 & Microsoft Research2

2. The “End” of the Road for DRAM
DRAM scaling wall
Fabrication limitations
Variability
Increasing error correction overhead (more transient errors)
Increasing active/standby/refresh power
Industry looking for byte-addressable alternatives
…but, main gating factor is memory lifetime
(see DRAM session on Tuesday)

3. Coming on the Horizon: NEW *RAM!
Phase Change Memory (PCM), CBRAM, Memristors, etc.
Fabrication friendly
Value stability
“Zero” standby power
Shorter lifetime (108) vs. DRAM (1015)
Mismatch in memory cell failure mechanisms
Zombie Page
Dead Page
4 KB Page

4. Cell Failure Remediation Mismatch
I am NOT Dead Yet!

5. Why Should You Care About Zombies?
Not all dead things are bad for you!
Lots of good cells in “dead” pages
Single-level cell (SLC) & multi-level cell (MLC) mechanisms
The first resistance drift + cell failure mechanism for MLC PCM
Adaptive error correction mechanisms
Maximizes memory capacity over the lifetime

6. Zombies in the Paper SLC MLC Error sources Wearout Wearout + drift
Mechanisms
ZombieECP
ZombieERC
ZombieXOR
ZombieMLC
Lifetime improvement
58%-92%
11x-17x
Service lifetime
~2.2 years  years
~5 months ~5 years
Performance impact
0-25%

7. Outline Zombie ECP Zombie ERC Zombie XOR Zombie ECP Zombie MLC
Block Pairing
Zombie Memory
Zombie ECP
Zombie ERC
Zombie XOR
Zombie MLC
How Long do Zombies Live? (Evaluation)
Conclusions
Zombie ECP
Single-Level Cell
Zombie ERC
Multi-Level Cell

8. The Basics Reintegrating Zombies back into the memory system
Primary Page
Zombie Page
Reintegrating Zombies back into the memory system
Phase Change Memory + 6 Error Correcting Pointers (ECP)
Other error correction schemes can be used
512 bit blocks + 64 bits error correction, 64 blocks/ 4 KB page
Differential writes
Simulation details in the paper, SPEC CPU2006
Pairing can be adaptive to maximize memory capacity.

9. Error Correcting Pointers Review
Use pointer + replacement bit for cell failure
9 bits pointer + 1 bit
Additional metadata
ISCA ‘10
12% EC Overhead
512-bit
block
Good Block
Failed Cell
Worn Block
ECP Entry

10. Adaptive Block Pairing
Pairing with different sized spare blocks
EC bits in the primary point to the spare
Reuse intrinsic error correction in the spare block
Re-pairing at the sub-block and block levels
Re-pair with different spare blocks
Gives Zombie a second chance
Zombie block pools
On a re-pair, the “new” spare can be a fresh or old zombie
Good Block
Primary
Primary
Worn Block
Spare Block
Spare
Spare
Spare

11. Zombie XOR
Pairs primary and spare blocks using XOR aligned bits to produce data
Bias wear to spare block to maximize primary lifetime
Reuse spare error correction bits to correct aligned cell failures in the primary and spare
Re-pair with “new” spare
Good Block
Failed Cell
Mention second spare could be new or old.
Worn Block
ECP Entry
Spare Block
Pairing Pointer
Primary
Spare
Spare

12. Zombie MLC Number String Codeword Must handle drift and cell failures
Rank modulation* to handle drift 11 10 01 00
Relative cell
values
Number
Fixed guard bands
String
Codeword
Resistance drift over time normalized to the initial resistance.
The inset graph shows no evidence of electric field acceleration of drift.
RM: -> transform number into strings
Cell values are relative, not absolute and groups of small cells
See tech report.
Shuffling equations done over a finite field.
Show pictorially stuck-at with RM (add Cells and pull one down.)_
*N. Papandreou et al. IMW, 2011
Reprint of D. Ielmini et al., IEDM2007

13. y=𝑎𝑥+𝑏* *over a finite field
Zombie MLC
Must handle drift and cell failures
Rank modulation* to handle drift
Anchor symbols are added to handle cell failures
Known anchor location and/or known values
Optimal encoding: # replacement cells = # failed cells
Resistance drift over time normalized to the initial resistance.
The inset graph shows no evidence of electric field acceleration of drift.
RM: -> transform number into strings
Cell values are relative, not absolute and groups of small cells
See tech report.
Shuffling equations done over a finite field.
Show pictorially stuck-at with RM (add Cells and pull one down.)_
2 Cells Stuck-at 0
1 Cell Stuck-at 0
See the paper for 3 stuck-at cells mechanism.
Anchors
Anchor 1 2 3 1 2 1 2 3 2 3 1 1 2 3 3
Codeword
Original string
Original non-uniform string
Codeword
y=𝑎𝑥+𝑏* *over a finite field
Coordinate shuffle equation
Bit positions
*N. Papandreou et al. IMW, 2011

14. Zombie ECP & ERC Pairing + existing error correction mechanisms
Adaptive: 1/4, 1/2, and full block pairing
ECP [ISCA ‘10]: Use spare block to add more Error Correcting Pointers to the primary block
ERC [PIT ‘74, HPCA ‘13] : Change the model to an erasure model
Instead of correcting (d-1)/2 errors (error model), can correct d-1 errors
Bias wear to spare block to maximize primary lifetime
Maximize memory capacity
Also see Jacobvitz et al. Coset coding paper in HPCA 2013

15. How Long do Zombies Live?

16. Zombie SLC Write Capacity

17. Zombie SLC Write Capacity
58% longer life

18. Zombie SLC Write Capacity
58% longer life

19. Zombie SLC Write Capacity
92% longer life

20. Zombie SLC Performance
< 6% slowdown on SPEC workloads
< 0.5% slowdown on SPEC workloads

21. I’m NOT Dead YET!

22. I’m Still NOT Dead YET!

23. I’m STILL NOT Dead YET!

24. Squeezed Blood From a Turnip!

25. Zombie MLC Write Capacity

26. Zombie MLC Write Capacity
17X longer life

27. Zombie MLC Write Capacity
11X longer life

28. Zombie MLC Performance
< 4% slowdown on SPEC workloads

29. Zombies Can Be Rehabilitated!
Zombie framework
Using dead blocks to extend memory lifetime
Versatile and adaptive
Low implementation overhead
MLC: First drift + cell failure solution
Using fixed positions and/or fixed values for anchors
Lifetime improvement 11X – 17X
SLC: Multiple mechanisms
Maximize lifetime or capacity
Lifetime improvement of 58-92%
Allowing different trade offs
ZOXR, ZERC, ZECP

30. Zombie Memory: Extending Memory Lifetime by Reviving Dead Blocks
Questions?
For more details: Read the paper, read the tech report, and/or talk to us
&
{john.d, kstrauss, parik, manasse,

31. More About Zombie…

32. Zombie SLC Performance

33. Zombie MLC Performance

34. Mitigating Drift-Induced Soft Errors
Previous Assumptions:
Fixed guard band for cell value
Uniform distribution of resistance values.
~2 second data lifetime….
Relaxing the drift-induced soft error constraint
Rank modulation (no fixed guard band)
Non-uniform distribution of resistance values
Cluster the low levels and spread apart the high levels
~5 Days of data lifetime (worst-case wear is 5 seconds)
More knobs:
Tighten resistance distribution
Use different drift coefficients