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REDUCING REFRESH POWER IN MOBILE DEVICES WITH MORPHABLE ECC
Chiachen Chou, Georgia Tech
Prashant Nair, Georgia Tech
Moinuddin K. Qureshi, Georgia Tech
DSN-4506/24/2015
Rio de Janeiro, Brazil
Computer Architecture and Emerging Technologies Lab, Georgia Tech
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GROWING DRAM SIZE IN SMARTPHONES
2
Samsung Galaxy S6 (2015) 3GB DRAM
Samsung Galaxy S2 (2011)1GB DRAM
courtesy: Samsung
Smartphone usability: battery life
30% energy goes to memory system in idle mode
DRAM Refresh accounts for significant energy consumption in idle mode
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LOWER REFRESH RATE FOR ENERGY
Current standard refresh rate: 64ms
3
DRAMCPU
64ms
Energy 1X
1s
0.5X
Refresh Rate
Use ECC to protect DRAM from refresh errors
Bit Error Rate 10-9 10-4
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ECC-6 INCURS LONG LATENCY FOR READ
Decoder latency is on the critical path
4
DRAMCPU
Request
ECCDecoder
Data
ECC-6: 30 cycles
We want energy reduction in idle mode, and maintain performance in active mode
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AGENDA
• Introduction
• Background– DRAM 101– Refresh and Errors– Error Correction Codes (ECC)
• Morphable ECC
• Results
• Summary
5
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6
• Dynamic Random Access Memory (DRAM)
• DRAM stores data as charge on capacitor
Leakage
DRAM Chip
1
DRAM 101
DRAM is a volatile memory charges leak quickly
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7
DRAM maintains data integrity by Refresh operations
DRAM Chip
RefreshRefreshRefreshRefresh
DRAM REFRESH
JEDEC: 64ms 1s
Lowering refresh rate reduces power
64ms 1s0
0.2
0.4
0.6
0.8
1
Refresh Power
Refresh
No
rma
lize
d P
ow
er
64ms 1s0
0.2
0.4
0.6
0.8
1
Idle Power
BackgroundRefresh
No
rma
lize
d P
ow
er
16X50%
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8
DRAM maintains data integrity by Refresh operations
DRAM REFRESH RATE AND ERRORS
Lowering refresh rate increases bit error rate
0.01 0.1 1 10 1001E-10
1E-08
1E-06
1E-04
1E-02
1E+00
Refresh Rate (second)
Cum
ulati
ve E
rror
Rat
e
64ms: 10-9
1s: 10-4.5
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ERROR CORRECTION CODE (ECC)
ECC: tolerate refresh errors
Q: how many errors should the system tolerate?
9
ECC-1
ECC-6
64B cache line
Errors
1 Error
6 Errors
ECC Strength Capability
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ERROR CORRECTION CODE (ECC)
ECC: tolerate refresh errors
Q: how many errors should the system tolerate?
What should be the strength of the ECC?
10
ECC Strength Line Failure System Failure
ECC-1 1.8 X 10-2 1.0
ECC-2 9.8 X 10-7 1.0
ECC-4 1.6 X 10-11 2.7 X 10-4
ECC-5 4.9 X 10-14 8.1 X 10-7
ECC-6 1.2 X 10-16 1.8 X 10-9
Refresh rate of 1s needs ECC-6 for errors
✔
Good✔
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DRAWBACKS OF ECC-6
Single Core, 1MB Cache, 1GB DRAM
11
ECC-6 incurs huge performance degradation
Low Med High ALL0.8
0.9
1
ECC-1 ECC-6
Memory Activity (MPKI)
Nor
mal
ized
IP
C
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WHAT IS THE IDEAL CASE?
12
Goal Strong ECC
Weak ECC
Active Mode Performance(Refresh Power Negligible) Bad Good
Idle Mode Energy(Performance Not Critical) Good Bad
✔
✔ ✘
✘
Ideally, we want ECC-1 in active mode, and ECC-6 in idle mode
?
?
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AGENDA
• Introduction
• Background
• Morphable ECC (MECC)– Overview– Design– ECC Support and Storage
• Results
• Summary
13
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MECC: OVERVIEW
14
MECC uses two ECCs based on modes
Time
REFRESH Normal Slow Normal
ECC Weak Strong Weak
Active Active
Idle
Sleep Wake up
ECC-Upgrade ECC-Downgrade
ECC-Upgrade ECC-Downgrade
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MECC: DESIGN
15
Memory controller uses ECC-mode bits to decode with different ECCs
DRAM Chips
RD Queue
WR Queue
ECCEncoder
Address Data+ECCData+ECC
ECC-1Encoder
ECC-6Encoder
Memory ControllerTo Processor
Data
?
M0
ECC-ModeECC-Mode
Data + ECC
ECCDecoder
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MECC: ECC STORAGE
16
64B Data Block 8B ECC
SECDEC SECDEC SECDEC SECDEC
Byte 0-7 Byte 8-15 Byte 48-55 Byte 56-63
Conventional
MECC ECC-1
MECC ECC-6
0000
1111
ECC-Mode
ECC-1 for 64B Data
ECC-6 for 64B Data
60 bits
11 bits
Unused
MECC uses existing space to store 2 ECCs
11 bits
60 bits
+ = 60 bits
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AGENDA
• Introduction
• Background
• Morphable ECC
• Results
• Summary
17
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Core Chips:• 1 cores 1.6 GHz• 2-wide In-Order• 1MB cache
METHODOLOGY
18
Off-chip DRAMCPU
LPDDR2
Capacity 1GB
Bus DDR 200MGHz
Organization1 channels,
4 banks
• Baseline: No Error Correction Code
• SPEC2006 (exclude mcf): low, medium, high MPKI workloads
• USIMM for DRAM model and power
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POWER AND ENERGY CONSUMPTION
19
Parameters Values DescriptionVDD 1.7 V Operating VoltageIDD0 95 mA 1 bank active precharge current
IDD2P 0.6 mA Precharge power-down standby currentIDD3P 3 mA Active power-down standby currentIDD4 135 mA Burst read/write: 1 bank activeIDD5 100 mA Auto refreshIDD8 1.3 mA Self refresh
Power in Idle Mode = (Prefresh original * Toriginal / TMECC ) + Pother
courtesy: Micron LPDDR2 Specs
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PERFORMANCE IN ACTIVE MODE
20
povraywrf
hmmer
h264ref
gromacs
astar
dealII
cactu
sgcc
sphinx
xalanc
libq
lbm0.7
0.75
0.8
0.85
0.9
0.95
1
PerformanceECC-6 MECC
Nor
mal
ized
IPC
-2%
MECC limits the degradation within 2%
-1%
Low MPKI
-2%
Med MPKI
-3%
High MPKI
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POWER SAVING IN IDLE MODE
21
Baseline MECC0
0.2
0.4
0.6
0.8
1
Refresh Power
Refresh
No
rma
lize
d P
ow
er
Baseline MECC0
0.2
0.4
0.6
0.8
1
Idle Power
Background Refresh
No
rma
lize
d P
ow
er
16X
50%
MECC saves idle power by 50%
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TOTAL ENERGY SAVINGS
22
Baseline MECC0
0.2
0.4
0.6
0.8
1
Total System Energy
Acitve IdleN
orm
aliz
ed E
nerg
y
15%
MECC saves total energy by 15%
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ECC-UPGRADE
Active
IdleECC-Upgrade
DRAM Chips
ECCEncoder
Data + ECC-1Data + ECC-6
ECCDecoder
Lines
Can we enhance the ECC-Upgrade?
Entire Memory
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MEMORY DOWNGRADE TRACKING (MDT)
24
1
0
Address
Index
Need ECC-Upgrade
Don’t Need ECC-Upgrade
povr
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hmm
er
h264
ref
gobm
kpe
rl
bzip
2
sopl
ex
calcu
lix
zeus
mp
sphi
nx
xala
nc libq
lbm
ALL10.24
102.4
1024
Memory Downgrade Tracking
Acc
ess
ed
Me
mo
ry
Re
gio
n (
MB
)
MDT avoids unnecessary ECC-Upgrades
128MB
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FREQUENT TRANSITION OF ECC STATES
25
Active
Idle
ECC-Downgrade
Can we enhance the ECC-Downgrade?
courtesy: Samsung, Bluetooth, Facebook, Twitter
Frequent Transition
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SELECTIVE MEMORY DOWNGRADE (SMD)
26
Memory Activity(MPKC)
> THLD?Enable
ECC-Downgrade
DisableECC-Downgrade
Yes
No
0
40
80
Selective Memory Downgrade
ECC-Downgrade Disabled ECC-Downgrade Enabled
Exe
cutio
n T
ime (
%)
in
each
Sate
SMD avoids frequent transition of ECCs
Low MPKI
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EXECUTIVE SUMMARY
• Energy consumption determines the usability of emerging mobile computing devices
• DRAM refresh operations accounts for significant fraction of memory system’s energy
• Results: -50% idle power, -15% overall energy, with only 2% performance degradation
27
Strong ECC Weak ECCActive Mode
(refresh power negligible)Bad
PerformanceGood
PerformanceIdle Mode
(performance not critical)Huge Energy
SavingsNo Energy
Saving
Morphable ECC
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REDUCING REFRESH POWER IN MOBILE DEVICES WITH MORPHABLE ECC
Chiachen Chou, Georgia Tech
Prashant Nair, Georgia Tech
Moinuddin K. Qureshi, Georgia Tech
DSN-4506/24/2015
Rio de Janeiro, Brazil
Computer Architecture and Emerging Technologies Lab, Georgia Tech