Enhancing Lifetime and Security of PCM-Based Main Memory with Start-Gap Wear Leveling

PCM:
- memory technology that can increase main memory capacity in a cost-effective and power-efficient manner
- endure only 10^7 - 10^8 writes, lifetime short. Non-uniformity in writes reduces the available lifetime.
Wear-Leveling: can make writes to PCM cells uniform, but require large storage tables and indirection, thus latency overheads
Propose: Start-Gap, which is a simple, novel, and effective wear-leveling technique that uses only two registers
- Extend lifetime
- Simple extension: more robust

PCM:
- 2x-4x slower than DRAM, can provide up to 4x more density than DRAM
- Limited number of writes to each cell
- Non-uniformity causes the actual lifetime to be 20x lower than lifetime achievable under ideal conditions
Wear leveling
- Makes the writes uniform by remapping heavily written lines to less frequently write lines
- Track write counts associated with each line and an indirection table to perform address mapping to achieve uniform wear-out of the system
- Hardware required scales with RAM that is needed, look up adds latency and increase power consumption, additional design, verification, and testing cost
Goal: simple, effective wear-leveling mechanism that obviates the overheads above and achieve similar lifetime close to perfect wear-leveling
Insight: algebraic mapping between logical and physical address
Idea
- Start-Gap:
  - Gap: keeps track of how many lines have moved
  - When all lines have moved, the Start register is incremented to keep track of the number of times all lines have moved
  - mapping from logical to physical: simple arithmetic operation of Gap and Start registers with the logical address
- Security threat: extend to tolerate such attacks by dividing the memory into few regions and managing each region independently

PCM
- Limited write endurance
- Suffer from non-uniformity
Table-based wear-leveling methods require
- Significant hardware overhead
- Suffer from increased latency
Goal: avoid storage and latency overheads of existing wear-leveling algorithms, and achieve lifetime close to perfect wear-leveling

Key insight: use algebraic mapping between logical and physical addresses, and avoids tracking per-line write counts
Performs wear leveling by periodically moving each line to its neighboring location, regardless of the write traffic to the line
Two registers: Start, Gap
- Gap: number of lines relocated in memory
- Start: how many times all the lines in memory have been relocated
Extra memory line (GapLine): facilitate data movement

Gap is moved by 1 location once every psi writes to memory: copying the content of location gap - 1 to GapLine and decrementing the Gap register (Figure 5(b))
After 8 movements, all the lines from 8-15 get shifted by 1 (Figure 5(c))
Figure 5(e): contents of all lines have shifted by exactly 1 location, and hence the Start register is incremented by 1

The randomizer provides a (pseudo) random mapping of a given Logical Address (LA) to an Intermediate Address (IA)
Due to random assignment of LA to IA, all regions are likely to get a total write traffic very close to the average, and the spatial correlation of heavily written lines among LA is unlikely to be present among IA.

An adversary who knows about the wear leveling technique can design an attack that stresses a few lines in memory and cause the system to reach the endurance limit, and fail
Solution:
- Region Based Start-Gap (RBSG)
  - divides the memory into several regions and manages each region independently using a separate Start and Gap
  - If a region is written heavily it will now undergo Gap Movement faster than other regions, preventing line failure from repeated writes
- Another orthogonal approach to tolerate attacks is to increase the time to write to the same line (Delayed Write Policy with a Delay Write Factor (DWF))

Last updated 2 years ago