revisi&ng)lp,nuca)energy) consump&on:)cache)access...

Revisi&ng LP-‐NUCA Energy Consump&on: Cache Access Policies an Adap&ve Block Dropping HIPEAC 2015, Amsterdam

Revisi&ng LP-‐NUCA Energy Consump&on: Cache Access Policies

and Adap&ve Block Dropping D. Suárez-‐Gracia, Alexandra Ferrerón, L. Montesano,

T. Monreal, and V. Viñals

Grupo de Arquitectura de Computadores (gaZ) Departamento de Informá&ca e Ingeniería de Sistemas Ins&tuto de Inves&gación en Ingeniería de Aragón (I3A)

Universidad de Zaragoza

Instituto Universitario de Investigación

de Ingeniería de Aragón


LP-‐NUCA Organiza&on for Embedded Processing

•  First-‐ and second-‐level cache merged into a &led fabric

•  Specialized networks-‐in-‐cache: –  Search –  Transport –  Replacement

•  Single or mul&-‐thread programs

SEARCH: Misses waste energy

DOMINOES REPLACEMENT: Non-‐reused blocks waste energy

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Objec&ves •  Reduce the energy consump&on of LP-‐NUCA without sacrificing performance: –  Is parallel access really useful/needed? Reconsider search policy Energy reduc&ons between 13.2% -‐ 31.7%

–  Dominoes replacement: can we iden&fy harmful blocks without short-‐term reuse? Track temporal locality and drop cache blocks from the RT with no reuse or long-‐term reuse •  How can we track temporal locality? Adap&ve drop ra&o controller (ADR) ADR minimizes replacement and migra&on in low-‐locality phases: energy decreases 22.7% (1SMT) and 29% for (2SMT)

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Agenda

•  Search Policy •  Adap&ve Replacement with ADR •  Methodology •  Evalua&on •  Conclusions

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Agenda


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Search Policy: Parallel vs. Serial Access Parallel Serial

tag data

address address

tag data

Energy Consumptionhitmiss

tag + data 9tag + data 8

hitmiss

Latencytag + data 8max(tag,data) 9

tag + data 9tag 9

High locality

parallel (latency)

Low locality

serial (power)

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Parallel vs. Serial Results

1 SMT 2 SMT

•  Comparable performance •  Parallel policy wastes energy for high-‐RPKI workloads

(RPKI: replacements per kilo-‐instruc&on)

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Agenda

•  Search Policy •  Adap5ve Replacement with ADR •  Methodology •  Evalua&on •  Conclusions

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Non-‐reused Blocks Waste Energy

•  Dominoes replacement in a 3-‐level LP-‐NUCA

•  Non-‐reused blocks: –  Are inserted and evicted

up to 5 &mes –  Can evict other useful

blocks, either from the same or another thread

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Reducing Replacement Energy Waste •  Cache blocks from the RT

with long-‐term reuse or no reuse should be dropped

•  How? Selec&ve block evic&on from the RT taking into account program and phase?

Low locality ! drop (clean blocks discarded; !" " " dirty blocks to next cache level)!

High locality ! evict (usual behavior)!

✔ ! ✔ !✔ !✖ ! ✖ !✖ !

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Temporal Locality Varies across Programs and Time

&me

1M 2M

epoch

1-‐2 Mcycles

< 1 Mcycles

3M

> 2 MCycles

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Temporal Locality Varies across Programs and Time

Short-‐term reuse Near reuse Long-‐term or no reuse

ST: Keep ST: Keep ST: Drop all

MT: Keep MT: careful dropping MT: Drop all

ST: single-‐thread mode MT: mul&-‐thread mode

short-‐term reuse near reuse long-‐term reuse

DROP RATE

no reuse

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473.astar 456.hmmer 470.lbm


Adap&ve Drop Ra&o Controller •  Periodic opera&on (epochs) •  State: drop rate + direc&on (↑, ↓) •  Local search problem: –  Hill climbing – Memory addi&onal rules (avoid minima)

•  Avoid on-‐chip power metering •  How to know we are losing hits? –  Auxiliary tags

epoch control

ADR

RT drop rate

performance & energy metrics

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ADR Opera&on

0.5 ↑

0.5 ↓

1

0.5

0.5

0

0.5

0.5

1 ↑

0.5 ↓

k k + 1epoch k + 2

age jk + 3 k + 4 k + 5

j + 1

trial referencetrial

time

thread 0

trialthread 1

trial

trialreferencetrial

thread 0

thread 1

score 120 100 90 evaluate scores

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Scoring Func&on •  How do we know we are losing hits?

–  Auxiliary tags: insert blocks we drop

•  Scoring func&on: maximize low-‐energy accesses and minimizes high-‐energy accesses

hitsi : low-‐energy accesses (LP-‐NUCA) aux_tag_hitsi :high-‐energy accesses (dropped blocks) k: energy cost ra&o between both (constant)

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Hardware Implementa&on

RT

cache ports ADRcontroller

processorRNGsearch

message

replacement message

...

RT mshr

hits / evictions

state, dir

# th

read

s

epoch id

RT repl.buffer

cmp

drop/insert

auxiliarytags

epochs & age stats.

...

dropped @

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1024-‐entry

32b p. ep

log2(th + 1) log2(states) + 1 p. th


Agenda

•  Search Policy •  Adap&ve Replacement with ADR

• Methodology •  Evalua&on •  Conclusions

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Methodology •  Processor model based on commercial SoCs (IBM/LSI Power PC 476FP)

•  Simula&on –  SMTScalar (SimpleScalar)

•  Energy and Delay –  CACTI 6.5 –  Original LP-‐NUCA layout

•  SPEC CPU2000 and CPU2006 –  Simpoints [Hamerly+06] –  Benchmarks divided into two groups: high-‐ and low-‐RPKI

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Agenda

•  Search Policy •  Adap&ve Replacement with ADR •  Methodology

•  Evalua5on •  Conclusions

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ADR Sensi&vity Analysis

Key Controller Parameters Time/Event-‐based Epoch? Time-‐based, 128KCycles

Op5mal number of drop states? (2-‐9)

Two states (epoch-‐length dependent)

Weight of auxiliary hits? (K) Liple impact Auxiliary Tag Size? 1024 entries Controller Delay? 50-‐100 cycles

no impact on performance

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IPC/Energy Impact: 1 SMT

IPC Energy

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IPC/Energy Impact: 2 SMT

IPC Energy

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ED System Impact: 1 SMT

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ED System Impact: 2 SMT

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Agenda


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Conclusions •  LP-‐NUCA energy reduc&on based on adap&ve mechanisms

•  Search serial policy access reduces energy without hur&ng performance

•  Replacement is op&mized by means of an Adap&ve Drop Ra&o controller –  Hill climbing-‐based –  Detects low locality program phases and useless blocks are silently dropped during them •  Total migra&ons reduced by 81%

–  Straighqorward implementa&on, low area overhead –  Energy reduced by 22.7% (1 SMT) and 29% (2SMT)

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Revisi&ng LP-‐NUCA Energy Consump&on: Cache Access Policies

and Adap&ve Block Dropping D. Suárez-‐Gracia, Alexandra Ferrerón, L. Montesano,

T. Monreal, and V. Viñals

Grupo de Arquitectura de Computadores (gaZ) Departamento de Informá&ca e Ingeniería de Sistemas Ins&tuto de Inves&gación en Ingeniería de Aragón (I3A)

Universidad de Zaragoza

Instituto Universitario de Investigación

de Ingeniería de Aragón

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