Energy-Efficient Virtual Memory System Design for SSD

Energy-Efficient Virtual Memory System Design for SSD

Chia-Lin YangEmbedded Computing Lab

Department of Computer Science and Information EngineeringNational Taiwan University

Embedded Computing Lab

Low-Power 3D graphics processing unit (GPU) design

Power gating and DVS strategies at the architectural level

PPT: Joint Power/Performance/Thermal management of DRAMs in multi-core systems

Orchestrating thread scheduling and page allocationsRARE: Resource-Aware Runtime Environment for thread scheduling in multi-core systemsEnergy-efficient flash memory system

Embedded System Complexity Is Increasing

GPS

Personal information management

Video conference

Operating System for Supporting Multi-tasking & Virtual Memory System

Main Memory

Page Table01101001

Operating System

TaskTaskTaskTaskTaskTaskSecondary storage

Disk as Storage

Traditional virtual memory system assumes disk as the

secondary storage

Hard Disk

Main Memory

Page Table

Disk Buffer

01101001

Flash Memory as Storage

Flash memory has become popular storages in mobile devices

• low power• light weight• shock resistant

Flash memory has very different characteristics from disk

Write onceOut-place updateGarbage collection

Flash Memory as the Secondary Storage ..

Main MemoryMain Memory

Page Table

Flash MemoryFlash Memory

01101001

1. The need to revisit virtual memory system design2. Energy-efficiency is the main design concern

Energy-Aware Flash Memory Management in Virtual Memory System , IEEE transaction on VLSI

An Energy-Efficient Virtual Memory System with Flash Memory as the Secondary Storage, islped’07

Outline

Motivation Background on Flash Memory Interplay between VM and FM Proposed Energy-Efficient VM Design

SubpagingHotCacheDuplication-Aware Garbage Collection

Experimental ResultsConclusions

Flash Storage System Architecture

Physicaldevice

FTL layer

Command translationMTD layer

Logic Block Address

Physical address


(0, 0, 3)1 (0, 1, 2)2 (1, 2, 1)

LBA Physical address(bank, block, page)

……

address translation table

0GarbageCollection

……

Block 0Block 0

Block 1Block 1

Block 2Block 2

Block 3Block 3

Erase one Erase one blockblock

……

Read/Write Read/Write one pageone page

1 Page

Organization of a Typical NAND Flash Memory

Samsung K9F1208R0B 1 Block = 32 pages1 Page = 512B

Flash Memory Characteristics

Write onceWritten page can not be overwritten

Flash Memory

Flash block

A free pageA free page



A

Flash Memory

A live pageA live pageA free pageA free page

Live page Flash block



Out-place update

A’

Flash Memory

A live pageA live pageA dead pageA dead page


Dead page

New data

Flash block

Flash Memory Characteristics (cont’d)

When # of free pages <= GCt (Garbage Collection Threshold)

Trigger Garbage collection to reclaim dead pagesVia erase operations Basic unit of erase operations is a block

Flash Memory

Flash block




Garbage collection to reclaim dead pagesLive data copyingBlock erase

Flash Memory

Flash block




Garbage collection overheadsLive data copyingBlock erase

Flash Memory

Flash block



Writes are Problematic

Writes consume more energy than readsFrequent writes result in dead pages on flash memory

Trigger frequent garbage collections

Operation Latency EnergyRead (page) 47.2 ns 679 nJWrite (page) 533 us 7.66 mJErase (block) 3 ms 43.21 mJ

Key Design Principles for Energy-Efficient Flash Memory

Reduce writes to flash memoryEfficient garbage collection

Block X

Block Y

A live pageA live pageAn invalided pageAn invalided page

11 writes, gain 5 free pages

recycle block x

recycle block y

2 writes, gain 14 free pages

Interplay between VM and FM

A memory page contains n flash pagesAt a page fault, n flash pages of the victim virtual page are written back to back to flash memory

Two important observationsUnnecessary writes from replacing a virtual page Intra-page locality

Flash MemoryMemory Page

1

3

nn Writes

Flash Page Size Swap_out()2

Unnecessary Writes from Replacing a Virtual Page

In conventional virtual memory system, a full victim page is written to the secondary storage.

Flash Memory

Memory Page

Clean Data

Dirty Data

Clean Data

Clean Data

Four Writes

Flash Page Size

Unnecessary Writes from Replacing a Virtual Page

In conventional virtual memory system, a full victim page is written to the secondary storage.

Flash Memory

Memory Page

Clean Data

Dirty Data

Clean Data

Clean Data

Flash Page Size

Four Writes

Dirty Ratio

A victim page often contains a significant amount of unmodified data.

97.62%59.31%72.40%40.90%69.41%Dirty Ratio

gqview+juk

openoffice+juk

kspread+juk

mozilla+juk

kword+juk

Application98.86%66.59%88.61%48.49%89.73%Dirty Ratio

gqviewopenofficekspreadmozillakwordApplication

Dirty ratio =Dirty ratio = the number of dirty 512B block in a dirty memory pagethe number of 512B blocks in a main memory page

DDDDDDDD

CCCCCCCC

BBBBBBBB

What is Intra-page Locality?

Flash pages in one main memory page are written to flash memory back to back

A0 A1 A2 A3

A4 A5 A6 A7

B0 B1 B2 B3

B4 B5 B6 B7

C0 C1 C2 C3

C4 C5 C6 C7

D0 D1 D2 D3

D4 D5 D6 D7

Block X Block YA0

A1

A2

A3

A4

A5

A6

A7

Virtual page

Main memory

A0, A1, A2, A3, A4, A5, A6, A7

Swap_out(A)

Why is Preserving Intra-Page Locality Important?

After page A, B are swapped out

A A A AA A A A

A A A A

A A A A

B B B BB B B B

B B B B

B B B B

C C C CC C C C

C C C CC C C C

C C C CC C C C

C C C CC C C C

D D D DD D D D

D D D DD D D D

D D D DD D D D

D D D DD D D D

Block X Block Y Block X Block Y

Block X Block Y Block X Block Y

It affects the efficiency of garbage collection

After page A, B are swapped out

Garbage Collection Threshold vs. Intra-Page Locality

0.95

1

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

1.45

1.5

1.55

255 257 259 261 263 265 267 269 271 273 275 277 279 281 283 285 287

Garbage collection threshold

Nor

mal

ized

Ene

rgy

Con

sum

ptio

n

GCt : garbage collection thresholdm: # of flash pages in one memory pagen: # of flash pages in one flash block• GCt mod m = 0• GCt mod n ≥ n − m• GCt mod m ≠ 0 and GCt mod n < n − m

Proposed Energy Efficient VM Design

Reduce # of writes to flash memorySubpagingHotCache

Efficient garbage collectionDuplication-aware garbage collection

Subpaging

Divide a virtual memory page into a set of subpages in the granularity of flash page sizeEach subpage is associated with a dirty bit.

Memory Page

Clean Data

Dirty Data

Clean Data

Clean Data

One Write to Flash

Flash Page Size 0

1

0

0Dirty Bit

Flash Memory

HotCache

HotCacheManagement Policy

Caching writes onlyPreserving intra-page localityCapturing hot data

HotCache


Physicaldevice

FTL layer

Command translationMTD layer

(f, 0, 0, 3)1 (f, 0, 1, 2)2 (s, -, 2, -)

LBA Physical address(f/s, bank, block, page)

……

address translation table

0

HotCacheManagement

GarbageCollection

How to Capture Hot Data?

Three management policies Two-level LRU (2L)Time frequency (TF)

Replace the HotCache block with smallesttimestamp * write_counts

Time frequency locality (TFL)TF policy with intra-page locality preserved

………tailhead

1st level list

………

tailhead2nd level list

Two-level LRU

DDDD

1DDDD

1

Duplication-Aware Garbage Collection

Exploit data redundancy between the main memory and flash memory to eliminate unnecessary live page copying during garbage collection

An invalided pageAn invalided page A free pageA free page

Duplication-Aware Garbage

Collection

0 BBBB

AAAA

Main Memory

0

Dirty Bit

Flash Memory Flash MemoryA A A AB B B BC C C C

C C C C

DDDD

1DDDD

11 B

BBB

AAAA

Main Memory

1

Dirty Bit

Duplication-Aware Garbage Collection (cont’d.)

SWAP system

GarbageCollection

FTL

Read(LBA) Write(LBA,PID,VPN)

(0, 0, 1)(0, 0, 2)(0, 0, 3)

LBAPhysical address(bank, block, page)

………

Block Allocation Map (BAM)

InvalidValid

FreeState

98

0PID

87

311

0VPN

10

1In_memory

Swap_clean(LBA) Swap_free(PID,VPN)

(0, 0, 1)(0, 1, 2)(1, 2, 5)

LBA Physical address(bank, block, page)

……

Address Translation Table (ATT)

12

0

Experimental SetupTrace-driven simulation

Valgrind: captures the memory access trace while application executing in real-time.

Applicationskword, kspread, mozilla, openoffice, gqviewMulti-programming workloads: kword+juk, kspread+juk, mozilla+juk, openoffice+juk, gqview+juk

Configuration Main memory

16MB, 4K page size.Flash memory

16K block, 512B page128K block, 2KB page. MP3 jukebox

programjuk

image viewergqview

popular office suite similar to Microsoft office

openoffice

web browsermozilla

spreadsheet application

kspread

word processorkword

DescriptionApplication

Subpaging

Smaller flash page size leads to more energy reduction 512B page size: 20% energy reduction on average2KB page size: 8% energy reduction on average

Save more energy for multiprogramming workload Single-program workload: openoffice (14% energy reduction)Multi-program workload: openoffice+juk (31% energy reduction)

00.10.20.30.40.50.60.70.80.9

1

kword mozilla kspread openoffice gqview kword+juk

mozilla+juk

kspread+juk

openoffice+juk

gqview+juk

Nor

mal

ized

Ene

rgy

Con

sum

ptio

n

Flash page size = 2KB Flash page size = 512B

HotCache:Hit Rates & Energy Savings

9.98%10%8.62%3.91%3.34%Average hit rate

TFL TF 2L LRU

1MBCache size

FIFO Replacement Policy

5%5.02%4.72%0.49%0.41%Average hit rate

TFL TF 2L LRU FIFO Replacement Policy

512KBCache size

00.10.20.30.40.50.60.70.80.9

1

2L TFTFL 2L TFTFL 2L TFTFL 2L TFTFL 2L TFTFL 2L TFTFL 2L TFTFL 2L TFTFL 2L TFTFL 2L TFTFL

kword mozilla kspread openoffice gqview kword+juk mozilla +juk kspread+juk

openoffice+juk

gqview+juk

Nor

mal

ized

Ene

rgy

Con

sum

ptio

n

SRAM read write energy garbage collection

HotCache: Energy Breakdown

TF causes higher overhead per GC due tobreaking intra-page locality

Duplication-Aware Garbage Collection

Up to 50% of energy reductionAverage energy reduction rate is 24%

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

kword mozilla kspread openoffice gqview kword +juk mozilla+juk

kspread+juk

openoffice+juk

gqview+juk

average

Norm

aliz

ed E

nerg

y Co

nsum

ptio

n

HotCache + Subpaging + DA-GC

Energy reduction of HotCache + Subpaging + DA-GCRanging from 9.3% to 75%

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

kword mozilla kspread openoffice gqview kword +juk mozilla+juk

kspread+juk

openoffice+juk

gqview+juk

Nor

mal

ized

Ene

rgy

Con

sum

ptio

n

1MB HotCache & 512KB flash pages

Conclusion

We revisit virtual memory system design with flash memory as the secondary storage Three energy-efficient VM design

SubpagingHotCache managementDuplication-aware garbage collection

Joint use of Subpaging & TFL policy & DA-GCReduce up to 75% of flash memory energy

Flash Memory ControllerCPU core SRAM

HostInterface

FlashInterface

Flash memory bus

Flash memory chips

Host Interface

On-Going Works

SSD in server platformHigh-throughput multi-bank flash system Data placement in SLC/MLCReliability issue

ATA

or S

ATA

micro controller

SRAMMLC

MLC

MLC

MLC

MLC

MLC

MLCMLC

SLC

Documents

Energy-Efficient Virtual Memory System Design for SSD