Phase Change Memory What to wear out today?

Chris Craik, Aapo Kyrola, Yoshihisa Abe

Memory Technologies

• Concerns– Density– Latency– Energy

• Off Chip Technologies– DRAM• Moderately dense, but not very fast

– Flash• Fairly dense, but near-disk slowness

Evaluation of Technologies

DRAM NAND Flash NOR Flash

Density 1 4 0.25

Read Latency 60ns 25,000ns 300ns

Write Speed 1000MB/s 2.4MB/s 0.5MB/s

Endurance Eff. Infinite 10^4 10^4

Retention? Refresh 10 Years 10 Years

Phase Change Memory

• Bit recorded in ‘Phase Change Material’– SET to 1 by heating to crystallization point– RESET to 0 by heating to melting point– Resistance indicates state

Phase Change Memory

• Density– 4x increase over DRAM

• Latency– 4x increase over DRAM

• Energy– No leakage– Reads are worse(2x), writes much worse (40x)

• Wear out– Limited number of writes (but better than Flash)

• Non-volatile– data persists in memory

Evaluation of Technologies

DRAM NAND Flash NOR Flash PCM

Density 1 4 0.25 2-4

Read Latency 60ns 25,000ns 300ns 200-300ns

Write Speed 1000MB/s 2.4MB/s 0.5MB/s 100MB/s

Endurance Eff. Infinite 10^4 10^4 10^6 to 10^8

Retention? Refresh 10 Years 10 Years 10 Years

Solutions to wearing & energy• Partial writes = write only bits that

have changeda) Caches keep track of written

bytes/words per cacheline (Lee et. al)• storage overhead vs.

accuracyb) When writing a row to memory,

first read old row and compare => write only modified bits (Zhou et al.)

Writes cause thermal expansion / contraction that wears the material and requires strong current. But contrary to DRAM, PCM does not leak energy.

Most written bits redundant!

Solutions to wearing & energy (cont.)

• Buffer organisation (Lee et al.)– DRAM uses one row buffer (2048B)– propose using up to 32 * 64B

narrow buffers, each with own association• capture coalescing writes:

temporal locality more important than spatial locality

• find 4*512B most effective• area-neutral• also helps decrease latency

• Small DRAM buffer for PCM (Qureshi et al.)– combine low latency of DRAM with

high capacity of PCM– similarly use Flash cache for Disk

Solutions to wearing & energy• Wear leveling (Zhou et al.)

– row shifting: even out writes among cells in a row• needs extra hardware

– segment swapping: even out between pages• implemented in memory controller

Spatial locality is now a problem!

PCM as On-chip Cache• Hybrid on-chip cache architecture consisting of multiple memory

technologies• PCM, SRAM, embedded DRAM (eDRAM), and Magnetic RAM (MRAM)

• PCM is slow compared to SRAM etc.– But high density, non-volatility etc. help

• Use as complement to faster memory technologies• As “slow” L2 cache, as L3 cache etc.

PCM

Cache Structure Example• Use PCM as huge L3 cache• SRAM and eDRAM both as L2

– Faster and smaller SRAM region– Slower and larger eDRAM region

L3 PCM (32MB)

L2 eDRAM (Slow: <4MB) L2 SRAM (Fast: 256KB)

Core w/ L1L3 SRAM

1MB

L2 SRAM256KB

Corew/ L1

Same Footprint

• Compared to 3-level SRAM cache model:• 18% improvement in instructions per cycle• Comparable power consumption

• Despite additional layer of PCM and its large capacity

• Various design possibilities• PCM as “third” L2 cache etc.

Summary

• PCM can be viable approach towards next-generation memory architecture– High density, non-volatility– Various techniques to overcome shortcomings

• Short endurance, high-energy writes, latencies– Could be used as main memory or in on-chip cache hierarchy

Questions

• How well do results obtained on benchmark apps translate to real usage?

• Variance of endurance of memory cells?– may some cells wear out very quickly?

• Possibilities of PCM non-volatilityinstant wake-up from hibernation etc.