Comparison of JVM Phases on Data Cache Performance

Shiwen Hu and Lizy K. John

Laboratory for Computer Architecture

The University of Texas at Austin

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Motivation

Execution of Java programs consists of distinct JVM phases JIT compilation Garbage collection Execution

Efficient execution of Java programs necessitates a comparative study of requirements and characteristics of JVM phases

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Outline

Experimental methodology

Varying cache metrics Cache size, set associativity, block size

Decomposition by miss types

Time varying cache behavior

Conclusion

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Methodology

LaTTe JVM: An open-source, state-of-the-art JVM

Memory management of LaTTe JIT compiler Reusable initial stack – 50KB Allocate dynamic stacks when necessary – recyclable

Heap Management Large object area: indexed by a hash table Small object area: heads indicating object sizes

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Methodology (Cont.)

Experimental workloads: SPECjvm 98 benchmarks Using s10 data set

Cache simulator: Based on Cachesim5 from Sun’s Shade V6 tool suite A JVM phase aware cache simulator

Default configuration: 64KB, 32B blocks, 4-way set associative

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Breakdown of JVM phases

JIT compilation and execution phases dominate In terms of instruction counts, data references, and

data misses

Garbage collector has the highest miss rates Large working set (heap) and pointer-chasings But, rarely affects overall cache performance

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Breakdown of JVM phases (Cont.)

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Varying cache size

Increasing cache size is more effective on JIT compilation than on garbage collection Larger working set of garbage collector Pointer chasing access pattern of garbage collector Stacks of most JIT compilations can be held in

128K cache

Varying effect on execution phase More effective on mpegaudio than on db

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Varying cache size (Cont.)

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Varying set associativity

Increasing set associativity rarely affects JIT compilation and garbage collection Negligible conflict misses due to uniform accesses

to heap or stacks Dominated by capacity misses Short lives of JIT objects

Varying effectiveness on execution phase mtrt: 52% misses eliminated db and javac: 13% misses eliminated

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Varying set associativity (Cont.)

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full 4 way

2 way direct

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Varying block size

Effective on JIT compilation and garbage collection JIT compilation: good spatial locality due to stack

initialization Garbage collection: good spatial locality during

sweep phase

Varying effectiveness on execution phase Larger block: db, jess, mpegaudio, and mtrt Smaller block: compress, jack, javac

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Varying block size (Cont.)

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64B 32B

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Capacity misses dominate Less compulsory misses

Reusable initial stack Overlapped dynamic

stacks

Negligible conflict misses Splitting cache rarely

affects miss type

composition

Decomposition by miss types - JIT

JIT

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cold conf lict capacity

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Fewest compulsory misses in unified cache Cache blocks accessed during execution phase

More compulsory missesin split cache Uniform heap sweeping

Decomposition by miss types - GC

GC

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Decomposition by miss types - EXEC

EXEC

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cold conf lict capacity

Relatively more compulsory misses Heap objects allocation and initialization

Variety reveals program

characteristics Splitting cache rarely

affects miss type

composition

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Time varying behavior

Importance of separating JVM activities from application activities Java programs execute on JVMs, differing with

C/C++ programs Correlating performance results with JVM or

application characteristics is important to design better JVMs

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Time varying behavior (Cont.)

JVM specific operations dominate the startup and end of application execution Class loadings, method compilations

Few garbage collections Corresponding to burst of cache misses

Four passes of JIT compilation correspond to four bursts of cache misses

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Time varying behavior - compress

Less GC and JIT activities Cyclic behavior Two phases during execution

compress

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Time varying behavior - mtrt

More GC and JIT activities No cyclic behavior

mtrt

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Time varying behavior - startup

Identical behavior during startup First 110 million instructions

Sharing of harness classes among SPECjvm 98 benchmarks prolongs the duration

First 150M instructions

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Conclusion

Comparative study of cache performance of distinct JVM phases

Deterministic characteristics of cache behavior JIT compilation: traversing intermediate data

structures Garbage collection: large working set and pointer

chasings

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Conclusion (Cont.)

Near identical cache performance of JIT compilation among applications

Varying cache behavior during execution phase reveal characteristics of applications

Thanks