The Impact of Performance Asymmetry in

Multicore Architectures

SaisanthoshRavi

MichaelKonrad

BalakrishnanRajwarUptonLai

UW-Madison and , Intel Corp.

32nd Annual International Symposium on Computer Architecture

2

Performance asymmetry

- Architectural differences- Micro-architectural parameters- Other

- Heat: Thermal throttling

Why need asymmetry now?

- CMP/ Many cores as commodity systems- Run variety of workloads - Good serial performance and high throughput- Optimal energy consumption

... difference in compute power of processors

Assume an asymmetric multicore system

F FS S

3

Asymmetry & MT workloads

Perf

orm

an

ce

Compute power

Scalable?

N procs. Diff configs.

Need to utilize asymmetry. perform better

Need predictable and robust performance

F FS S

S SS S

Perf

orm

an

ce

Same/Many Runs

Stable?

N procs. Same config.

4

The problems

Algorithm, Correctness, Thread Partitioning

ProgrammersDon’t reason about asymmetry

Characteristics of threadsPartitioning, Synchronization barriers, Interference, Lifetime

Scheduling of threadsOS Kernel, Library, Application, DB/Web

servers,Managed runtime systems (Java, .NET)

8

Configurations

F F

F F S S

S S F F

S S

F F

F S

F S

SS

Symmetric Asymmetric

all fast all slow 1 slow 2 slow 3 slow

F = Full frequency

S = one-eighth of Full frequency (in talk and paper) S = one-fourth of Full frequency (in paper)

9

Studying impact

3 s

low

2 s

low

1 s

low

all s

low

all f

ast

Perf

. M

etr

ic

(Asymm)

Sam

e o

rM

an

y r

un

s

Perf

. M

etr

ic

Scalability Stability

11

SPECjbb

SPECjAppServer

Apache

Zeus

TPC-HSPECOMP

H.264

PMake

Impact of asymmetry

Scalable StableWorkloads

Fix

13

SPECjbbStability (JRockit/Gencon GC) on 2 slow

- Problem: Interference from runtime system (JVM, GC)

4 runs

with kernel fix

- Fix: Kernel scheduler moves jobs from slow to fast if free

Scalable? Stable?

15

Apache

- Problem: light load - threads can be on fast/slow- No issues under heavy load

- Fixes: Kernel scheduler or shorter lifetime of threads

Scalability & Stability (light load)

Stable?Scalable?

16

ZeusScalability & Stability

- Under heavy and light loads: unpredictable- Superior perf. on symmetric configs.

- Problem: Aggressive application-level scheduling

Stable?Scalable?

18

SPECOMP

- OpenMP schedules tasks assuming equal perf. procs.- Problem: Fast processors are held by slow

Scalability

with app. fix

- Fix: Change scheduling of tasks to on-demand- Downside: Overheads

Scalable? Stable?

19

H.264 & PMake

- H.264 slows down significantly with 1 slow proc.

- Speeds up with 1 fast proc.

H.264 PMake

- PMake linearly scalable on all configurations

Scalable? Stable?

21

Conclusions

Asymmetric systems - Good for energy and performance- But can introduce unpredictability

Software to understand asymmetry- Evaluate application’s work partitioning- Scheduling of tasks. Mostly no other changes.- May be, feedback based

Suitable asymmetry- Many slow & few fast processors