Reducing Compilation Overhead in J9/tr marius Pirvu, Derek Inglis, Vijay Sundaresan

Reducing Compilation Overhead in J9/TR

• Marius Pirvu, Derek Inglis, Vijay Sundaresan

Agenda

• Adaptive compilation in the TR JIT

• Class Load Phase

• Asynchronous compilation

• Limiting the negative effect of very long compilations

• Ongoing work

• Improvements to async compilation
• AOT

Adaptive Compilation in TR JIT

• Methods start out being interpreted

• After N invocations methods get compiled at ‘warm’ level

• Sampling thread used to identify hot methods

• Methods may get recompiled at ‘hot’ or ‘scorching’ levels

• Transition to ‘scorching’ goes through a temporary profiling step

Performance

Performance

How About Applications Without Hotspots

• WebSphere AppServer startup

• Very flat profile

Class-Load-Phase

• Intuition: Methods compiled during startup phase may not be important during application run phase

• Detect phases when class loading is intense

• Reduce optimization level to “cold” during such phases

Asynchronous Compilation

• Synchronous compilation

• Application thread places compilation request and blocks waiting for the compilation to finish

• Asynchronous compilation

• Application thread does not wait for the compilation result

• JIT compilations performed on a separate compilation thread

Asynchronous Compilation

• Implementation

• Synchronous compilation still needed in some cases (e.g. pre-existence)
• Synchronous and asynchronous compilation must coexist
• Queue of compilation requests

• Advantages

• Takes advantage of available processors on SMP machines by increased parallelism
• Allows performance improvements in uniprocessors by changing compilation thread priority
• Allows reordering of compilation requests

Performance Results – Short Running Apps.

• Asynchronous compilation on SMP reduces execution time of short benchmarks

Limiting Negative Effect of Long Compilations

• Compilations may impede GC operation

• GC requires exclusive VM access
• Cannot allow class unloading while compilation in progress  Compilations require VM access

• Solution

• Compilation thread periodically releases and reacquires VM access allowing GC to cut-in
• Upon re-acquiring VM access, check if GC unloaded any classes
• If classes were unloaded, abort current compilation and retry

Ongoing Improvements to Asynchronous Compilation

• Idea

• Use thread priorities to smooth out the effects of compilation - effectively interleave compilation with execution

• Implementation

• Don’t use more than X% CPU for compilation
• Use the queue of methods as a buffer
• accumulate work during periods of heavy utilization
• solve the backlog when CPU is lightly used or idle (due to IO for instance)
• Prioritize compilation requests in the queue

Performance - Uniprocessors

Ahead Of Time (AOT) Compilation

• Using the JIT as a static compiler

• Fully compliant code

• Used by J2ME customers to decrease footprint by eliminating the JIT

• Will be used by the Real Time offering to eliminate the possibility of nondeterministic behaviour introduced by the JIT

• Experimenting with combining AOT and JIT compilation to improve startup times

Eclipse 3.0.1 Startup Times

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