steve blackburn department of computer science australian national university perry cheng tj watson...

Steve BlackburnDepartment of Computer Science

Australian National University

Perry ChengTJ Watson Research Center

IBM Research

Kathryn McKinleyDepartment of Computer Sciences

University of Texas at Austin

IBM Research

Myths & RealitiesThe Performance Impact of Garbage

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Tuesday, April 18, 2023Myths & Realities: The performance impact of garbage

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Background• No prior apples-to-apples comparisons

• MMTk• Canonical policies implemented (SS, MS, RC, genX,

etc)

– Shared mechanisms– Good performance (match/beat old Watson GCs)

– Ideal platform for apples-to-apples comparisons


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Some Questions• Architecture

– How well do modern OO languages play to modern architectures?

• Collection– Is generational GC “a waste of time”?– Are write barriers expensive?

• Allocation– Free list or bump pointer?

• “Locality is everything”– Really???– Is it different for young & old? Why?

• Locality and architecture– What is the impact, what is the trend?


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Methodology• Jikes RVM & MMTk• Platforms• 1.6GHz G5 (PowerPC 970) • 1.9GHz AMD Athlon 2600+• 2.6GHz Intel P4• Linux 2.6.0 with perfctr patch & libraries– Separate accounting of GC & Mutator perf counts

• SPECjvm98 & pseudojbb


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Architecture


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Relative PerformanceAthlon

2600+ 1.9GHz

P42.6GHz

G51.6GHz

compress 0.93 1.00 1.18

jess 0.88 1.00 1.20

raytrace 0.71 1.00 0.73

db 0.97 1.00 1.68

javac 0.67 1.00 1.37

mtrt 0.69 1.00 0.75

jack 0.62 1.00 1.11

pseudojbb 0.77 1.00 1.24


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Architecture - Q & A

How big is the mismatch between modern arch & modern

languages???


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Allocation


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Allocation Choices• Bump pointer– ~70 bytes IA32 instructions, 726MB/s

• Free list– ~140 bytes IA32 instructions, 654MB/s

• Bump pointer 11% faster in tight loop– < 1% in practical setting– No significant difference (?)

• Second order effects?– Locality??– Collection mechanism??


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Implications for Locality• Compare SS & MS mutator– Mutator time = total – GC time– Mutator memory performance: L1, L2 & TLB


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jess

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 61

1.05

1.1

1.15

1.2

1.25

1.3

1.35

1.4

jess mutator time

MarkSweepSemiSpace

Normalized Heap Size

Nor

mal

ized

mu

tato

r ti

me


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jess

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 61

1.2

1.4

1.6

1.8

2

jess L1 misses

MarkSweepSemiSpace


Nor

mal

ized

L1

mis

ses


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jess

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 61

2

3

4

5

6

7

8

9

jess L2 misses

MarkSweepSemiSpace


Nor

mal

ized

L2

mis

ses


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jess

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 61

1.5

2

2.5

3

jess TLB misses

MarkSweepSemiSpace


Nor

mal

ized

TLB

mis

ses


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javac

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 6

1

1.05

1.1

1.15

1.2

javac mutator time

MarkSweep

SemiSpace


No

rma

lize

d m

uta

tor

tim

e

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 6

1

1.1

1.2

1.3

1.4

1.5

javac L1 misses

MarkSweep

SemiSpace


No

rma

lize

d L

1 m

isse

s

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 6

1

1.2

1.4

1.6

1.8

javac L2 misses

MarkSweep

SemiSpace


No

rma

lize

d L

2 m

isse

s

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 6

1

1.2

1.4

1.6

1.8

javac TLB misses

MarkSweep

SemiSpace


No

rma

lize

d T

LB m

isse

s


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pseudojbb

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 6

1

1.05

1.1

1.15

1.2

1.25

jbb mutator time

MarkSweep

SemiSpace


No

rma

lize

d m

uta

tor

tim

e

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 6

1

1.1

1.2

1.3

1.4

jbb L1 misses

MarkSweep

SemiSpace


No

rma

lize

d L

1 m

isse

s

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 6

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

jbb L2 misses

MarkSweep

SemiSpace


No

rma

lize

d L

2 m

isse

s

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 6

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

jbb TLB misses

MarkSweep

SemiSpace


No

rma

lize

d T

LB m

isse

s


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db

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 6

1

1.02

1.04

1.06

1.08

1.1

1.12

db L1 misses

MarkSweep

SemiSpace


No

rma

lize

d L

1 m

isse

s

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 6

1

1.025

1.05

1.075

1.1

1.125

1.15

db mutator time

MarkSweep

SemiSpace


No

rma

lize

d m

uta

tor

tim

e

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 6

1

1.01

1.02

1.03

1.04

1.05

1.06

1.07

db L2 misses

MarkSweep

SemiSpace


No

rma

lize

d L

2 m

isse

s

1 1.21 1.44 1.93 2.47 3.07 3.72 4.43 5.19 6

1

1.05

1.1

1.15

1.2

1.25

db TLB misses

MarkSweep

SemiSpace


No

rma

lize

d T

LB m

isse

s


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Locality


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Bump Pointer & Free List

• Is the locality differential age-dependant?• Re-run experiment with GenCopy &

GenMS– Generational variants of MarkSweep &

SemiSpace– Young objects treated identically– Mature objects either SemiSpace or

MarkSweep


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Whole Gen Whole Gen Whole Gen Whole Genjess 1.26 1.02 1.73 0.87 2.27 0.53 1.91 1.07

javac 1.13 1.05 1.32 1.02 1.38 1.25 1.53 1.20pseudojbb 1.15 1.08 1.25 1.14 1.44 1.22 1.45 1.26

db 1.10 1.10 1.07 1.09 1.01 1.05 1.17 1.17

Mutator L2MS/SS

Mutator TLBMS/SSMS/SS

Mutator Time Mutator L1MS/SS


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javac 1.13 1.05 1.32 1.02 1.38 1.25 1.53 1.20pseudojbb 1.15 1.08 1.25 1.14 1.44 1.22 1.45 1.26

db 1.10 1.10 1.07 1.09 1.01 1.05 1.17 1.17

Mutator L2MS/SS



• Why? Mature space locality?

• Nursery absorbs most allocs – lower frag• Relatively frequent copying in SS

Contigious allocation in nursery?


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javac 1.13 1.05 1.32 1.02 1.38 1.25 1.53 1.20pseudojbb 1.15 1.08 1.25 1.14 1.44 1.22 1.45 1.26

db 1.10 1.10 1.07 1.09 1.01 1.05 1.17 1.17

Mutator L2MS/SS



• Why?• Mature space locality?


• Contigious allocation in nursery?


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javac 1.13 1.05 1.32 1.02 1.38 1.25 1.53 1.20pseudojbb 1.15 1.08 1.25 1.14 1.44 1.22 1.45 1.26

db 1.10 1.10 1.07 1.09 1.01 1.05 1.17 1.17

Mutator L2MS/SS



• Why? Mature space locality


Contigious allocation in nursery


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Run SS & MS in “infinite” heap


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MarkSweep SemiSpace1.5X 1.5X

jess 3.37 3.44 1.02 2.63 3.00 1.14javac 8.51 8.34 0.98 7.38 7.60 1.03

pseudojbb 10.82 11.04 1.02 9.58 9.68 1.01db 14.12 14.40 1.02 13.06 11.88 0.91

geomean 1.01 1.02

1.5/ 1.5/


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MarkSweep SemiSpace1.5X 1.5X

jess 3.37 3.44 1.02 2.63 3.00 1.14javac 8.51 8.34 0.98 7.38 7.60 1.03

pseudojbb 10.82 11.04 1.02 9.58 9.68 1.01db 14.12 14.40 1.02 13.06 11.88 0.91

geomean 1.01 1.02

1.5/ 1.5/

• Infinite heap does not degrade locality (!?)– Exceptions: jess (degrades), db (improves)

why?– Is spatial locality unimportant in mature

space???


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BP & FL Locality Implications

• Is spatial locality unimportant in mature space??– No [Huang et al OOPSLA 2004]– But perhaps temporal locality is more significant

• Seems clear contiguous allocation is good– Vast majority of objects < cache line– h/w prefetcher may be significant

• Hard to improve over alloc order, easy to mess up?– Unlikely to be true: MarkSweep < Compacting <

SemiSpace


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Locality &Architecture


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MS/SS Crossover: 1.6GHz PPC

1

1.5

2

2.5

3

1 2 3 4 5 6

Heap Size Relative to Minimum

Normalized Total Time

1.6GHz PPC SemiSpace

1.6GHz PPC MarkSweep


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MS/SS Crossover: 1.9GHz AMD

1

1.5

2

2.5

3

1 2 3 4 5 6





1.9GHz AMD SemiSpace

1.9GHz AMD MarkSweep


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MS/SS Crossover: 2.6GHz P4

1

1.5

2

2.5

3

1 2 3 4 5 6







2.6GHz P4 SemiSpace

2.6GHz P4 MarkSweep


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MS/SS Crossover: 3.2GHz P4

1

1.5

2

2.5

3

1 2 3 4 5 6







2.6GHz P4 SemiSpace

2.6GHz P4 MarkSweep

3.2GHz P4 SemiSpace

3.2GHz P4 MarkSweep


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1

1.5

2

2.5

3

1 2 3 4 5 6







2.6GHz P4 SemiSpace

2.6GHz P4 MarkSweep

3.2GHz P4 SemiSpace

3.2GHz P4 MarkSweep

MS/SS Crossover

2.6GHz2.6GHz

1.9GHz1.9GHz

1.6GHz1.6GHz

locality space

3.2GHz3.2GHz


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Conclusions• Need for (re) evaluation of GC

performance– Key GC insights > 20yrs old– Technology has changed– Absence of apples-to-apples comparisons– Highly architecturally sensitive

• MMTk + perf counters– High performance infrastructure– Multiple GCs, shared mechanisms

• Some myths exposed & new realities

steve blackburn department of computer science australian national university perry cheng tj watson...

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