Mellow Writes Extending Lifetime in Resistive Memories through - - PowerPoint PPT Presentation

Mellow Writes

Extending Lifetime in Resistive Memories through Selective Slow Write Backs

Zhang, Lunkai; et al. Presented by: Hiwot Kassa and Andrew McCrabb 27 Sept 2017

Motivation

• PRAM and ReRAM have short write-endurance
• Current solutions: Reduce the number of writes
• Switching speed is inversely

proportional to write endurance.

• Memory banks often have long idle time.

Endurance P e r f

• r

m a n c e Switching Speed

Prior Work: Slow Writes*

• Fast writes need high power
• High power damages memory cells
• Slow writes: allow write operation to take longer to reduce damage

*Information from this slide comes from Dr. Strukov’s paper See References

Research Objective

• Optimize when to make writes “Slow”
• Improve memory lifetime to 8 years
• Minimize Area/Energy Overhead

E n d u r a n c e Performance Switching Speed

Solution - 1. Bank-Aware Mellow Writes

• Execute SLOW Writes when no other read/write requests are waiting

○ Slow Write Latency = 3x Normal

• Slow/Normal Decisions are

bank specific

• Only Bank 1 may execute

a slow write

Zhang [1]

Solution - 2. Eager Mellow Writes

• Include additional queue for data you don’t need again
• Execute when no other read/write requests are queued
• Utilizes bank idle time

without performance loss

Zhang [1]

Solution - 3. Wear Quota

• Limit number of normal writes

in a sampling period

• After “X” normal writes, use
• nly slow writes
• Guarantee lifetime with
• ccasional performance loss

Total Write Damage Time Guaranteed Lifetime Failure Threshold FAILURE

Solution - continued

Bank aware mellow write

• mechanism to detect bank conflict in the read and write queues

Eager mellow write

• allows the cache to eagerly write back some dirty data
• the LLC needs to identify what items to use as candidates for Eager Mellow Writes

Wear quota

• divide the execution period into multiple sample periods and the memory

controller calculates .

Putting it all together

Eager mellow write Bank aware mellow write Request on the same bank? Exceeds wear quota?

Slow write Normal write

yes No No yes

Experimental Evaluation

• OoO Core, Alpha ISA , 12 memory-intensive

workloads

From conference presentation on this paper

Results

• 2.58x lifetime and 1.06x performance
• 8 years life time with minimal performance degradation
• High bank utilization // Less memory idle time
• 39% more energy consumption in main memory system
• Lifetime increases with more memory banks (figure below)

• Reducing high-speed write operations can help guarantee a

minimum lifetime.

• We can maintain performance by optimizing slow-write placements.
• Utilizing memory idle time costs minimal extra energy and area.

Technical Insights

Conclusion

• for non-volatile memories, there exists an endurance advantage to

performing writes slowly using a smaller write dissipated power

• a combination of two Mellow Writes schemes extends lifetime with

minimal performance impact

• Wear Quota can guarantee the minimal expected lifetime with small

performance loss.

Discussion

• Is there a benefit to having multiple latencies? (e.g. 1x, 3x, and 20x

write latencies)

• Would this help or hurt write-intensive applications?
• Should “Wear Quota” be included in other platforms (mobile, server,

etc)?

References

• D. B. Strukov, “Endurance-write-speed tradeoffs in non-volatile memories,” Applied Physics A, vol.

122, no. 4, pp.1–4, 2016. Zhang, Lunkai et al., “Mellow Writes: Extending Lifetime in Resistive Memories through Selective Slow Write Backs,” ISCA, Seoul, South Korea, June 2016