SEESAW: Set Enhanced Superpage are caching¡SEESAW provides low-associative access to superpages,...

SEESAW:Set Enhanced Superpage

Aware cachingMayank Parasar∑, Abhishek

BhattacharjeeΩ, Tushar Krishna∑

∑School of Electrical and Computer EngineeringGeorgia Institute of Technology

ΩDepartment of Computer Science Rutgers University

[email protected]

Set

Associativityhttp://synergy.ece.gatech.edu/

mailto:[email protected]

Outline¡Motivation

¡SEESAW: Concept

¡SEESAW: Micro-architecture

¡Evaluation Methodology

¡Results

¡Conclusion

Mayank Parasar, School of Electrical and Computer Engineering, Georgia Tech

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L1 Cache Characteristics

Fast lookup

High hit-rate

EnergyEfficiency

Virtually Indexed Physically Tagged [VIPT] Cache


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TLB

PPN Page Offset

tag Data blockv

VPN Page Offset

Setindex

block offset

Cache HIT/MISS

VA

PA

Way-1Way-2Way-3Way-4


set-1

set-NWay-1Way-2Way-3Way-4


=

Virtually Indexed Physically Tagged [VIPT] Cache


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TLB

PPN Page Offset

tag Data blockv

VPN Page Offset

Setindex

block offset

Cache HIT/MISS

VA

PA



set-1

set-NWay-1Way-2Way-3Way-4


=

VIPT Caches necessitate:

(set-index + block-offset) <= Page-offset

Impact of Associativity on Access Latency and Energy of cache


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Cache Access Latency Cache Access Energy

Effect of associativity on MPKI of cache


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High Associativity hurts latency and energy without commensurately improving hit rate


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Revisiting L1 Cache Characteristics for VIPT Cache

Fast lookup

High hit-rate

EnergyEfficiency

Virtual

memory!

Virtual

memory!


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Opportunity: SuperpageIs it possible to relax constrains of

Traditional VIPT cache? Yes

How ?

4-KB2-MB

1-GB

More page-offset bitsfor superpage!

HW and OS Support for Superpagesin modern processors

Baseline Page

Super Page

Offset-bits:12

Offset-bits:21

Offset-bits:30

Prevalence of superpages in modern OSes under memory fragmentation


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Ran on 32-core; Sandybridge; 32 GB RAMMemhog causes memory fragmentation; higher

%age indicates higher fragmentation

Outline¡Motivation

¡SEESAW: Concept



¡Results

¡Conclusion


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SEESAW: Concept


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Less-setsMore-associativity

More-setsLess-associativity

super-page

Base-page

tag Data blockv

Way-1 Way-1

Way-1

Way-1

Way-1

Way-1Way-2 Way-2

Way-2

Way-2

Way-2

Way-2

Way-3 Way-3

Way-3Way-3

Way-3 Way-3

Set:1

Set:2

Set:3

tag Data blockv

Way-1 Way-1

Way-1

Way-1

Way-1

Way-1Way-1 Way-1

Way-1

Way-1

Way-1

Way-1

Way-1 Way-1

Way-1Way-1

Way-1 Way-1

Set:1

Set:3Set:2

Set:4Set:5Set:6

Set:7Set:8Set:9

Faster

Energy-Efficient

Outline¡Motivation

¡SEESAW: Concept



¡Results

¡Conclusion


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SEESAW: Micro-architecture


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VPNSet

indexblock offset

Cache

VA

TLB

PPN Basepage OffsetPA

set-N

set-1

tag Data blockvWay-3Way-4

Way-3Way-4

Way-3Way-4

Way-3Way-4

Basepage Offset


Way-1

Way-2

Way-1Way-2

Way-1Way-2

set-1

set-N

Partition bit

Translation Filter Table

(TFT)

Partitiondecoder

Predicts whether page is superpage

Partition-0 Partition-1

Superpage offsetDecodes

partition index from partition bit

SEESAW: Micro-architecture


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VPNSet

indexblock offset

VA

TLB


set-N

set-1


Way-3Way-4

Way-3Way-4

Way-3Way-4

Basepage Offset


Way-1

Way-2

Way-1Way-2

Way-1Way-2

set-1

set-N

Partition bit


(TFT)

Partitiondecoder

Partition-0 Partition-1Cache

Superpage offset

SEESAW: Superpage access


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VPNSet

indexblock offset

VA

TLB


set-N

set-1


Way-3Way-4

Way-3Way-4

Way-3Way-4

Basepage Offset


Way-1

Way-2

Way-1Way-2

Way-1Way-2

set-1

set-N

Partition bit


(TFT)

Partitiondecoder


Super Page

Superpage offset

= HIT/MISS

SEESAW: Basepage access


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VPNSet

indexblock offset

VA

TLB


set-N

set-1


Way-3Way-4

Way-3Way-4

Way-3Way-4

Basepage Offset


Way-1

Way-2

Way-1Way-2

Way-1Way-2

set-1

set-N

Partition index


(TFT)

Partitiondecoder


Not a Super Page

= HIT/MISS

SEESAW: TFT and Partition Decoder


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Superpage?Tag: VA[63:21]

Partitiondecoder


(TFT)

Translation Filter TableØ TFT Lookup

Ø Direct mappedØ False negative due to size

Ø TFT UpdateØ VA mispredictionØ 2MB L1-TLB fillØ 2MB L1-TLB Invalidation

Partition DecoderØ For 32kB CacheØ For 64kB Cache

SEESAW: Cache line insertion policy


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VPNSet

indexblock offset

VA

TLB

PPN Baseline Page OffsetPA

set-N

set-1


Way-3Way-4

Way-3Way-4

Way-3Way-4

Baseline Page Offset


Way-1

Way-2

Way-1Way-2

Way-1Way-2

set-1

set-N

Partition bit


(TFT)

Partitiondecoder


Which partition should cache-

line be inserted?

SEESAW: Cache line insertion policy¡4way-8way¡Superpage miss: victim within the partition¡Basepage miss: victim within the set

¡4way¡Uses LRU within the associated partition¡Avoid installing the same line twice¡Saves energy


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SEESAW: System Level Optimization¡Cache coherence ¡Cache coherence lookups use physical address ¡Snoopy provide higher energy benefits over Directory based

coherence

¡Page table modifications¡Superpage splintered into multiple basepages¡Multiple basepages promoted to superpages


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Outline¡Motivation

¡SEESAW: Concept



¡Results

¡Conclusion


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SEESAW: Simulated system


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SEESAW: Workloads

¡Spec¡Parsec¡Cloudsuite¡Tunkrank

¡Biobench¡Mummer¡Tiger

¡MongoDB


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¡Server Workload¡graph500¡Nutch Hadoop

¡Social-event web service¡Olia

¡Key value store¡Redis

Outline¡Motivation

¡SEESAW: Concept



¡Results

¡Conclusion


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SEESAW: Performance improvement


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SEESAW observes 3-10% better runtime over baseline

SEESAW: Performance improvement


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Out-of-orderCPU

in-orderCPU

~10% performance improvement for 64kB cache in OoO CPUs

SEESAW: Energy savings


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10-20% more energy savings over CPUs using baseline VIPT caches!

Approx. one-third of energy savings from coherence

SEESAW: TFT analysis and Way-Prediction


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TFT Analysis SEESAW + Way-prediction

16-entry TFT drives miss-rate under 10%SEESAW+WP shows symbiotic behavior

Outline¡Motivation

¡SEESAW: Concept



¡Results

¡Conclusion


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Revisiting L1 Cache Characteristic

Fast lookup

High hit-rate

EnergyEfficiency

SEESAW: Conclusion


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SetAssociativity

¡ L1 caches are optimized for latency¡ VIPT imposes indirect restriction on number of

sets in a L1 cache, increasing associativity¡ There is non-linear relation between associativity

and access latency/energy of the L1 cache

¡ Superpages are often used in modern OSes¡ SEESAW provides low-associative access to

superpages, providing both latency and energy benefits

¡ Up to 10 % performance improvement and 20 % energy reduction in modern workloads

¡ SEESAW has extremely low-overhead and is readily implementable

SEESAW: Set Enhanced Superpage are caching¡SEESAW provides low-associative access to superpages,...

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