For more QCon posts, see my live blog table of contents. This presentation is about the compiler and also the runtime.
- Execution engine
- Heap management/garbage collection
- JIT compilation
- VM Class loading
- Byte code verification,etc
Runtime goal – convert from bytecode to native code and do optimizations along the way
Compilation Techniques and Notes
- Pre-compiled/ahead of time
- profile guided – based on critical hotspots
- Adaptive optimization (Java uses Profile guided and Adaptive optimization)
- Identify root of compilation
- replace method or on stack – depends on number of times through loop
- Server compiler has a higher threshold than client compiler for the threshold at which you need optimizations
- Tiered compilation – tier 1 is client compiler with no profiling info, tier 2 and 3 are client compiler with profiling info. Then comes server compiler
- CodeCache order of magnitude larger when tiered compilation is enabled. If need more can use -XX:ReservedCodeCacheSize
- Inlining – many different parameters when figuring out when to inline
- Vectorization – SIMD (SIngle Instruction Multiple Data). Can generate stubs and benefit from caching size chunks. For SuperWord Level Parallelism, you need to unroll the loop, do analysis/pre-optimization, etc. Still in infancy with Hotspot.
- Escape analysis – Want to see if object only is used in a compile method. Need entire graph to confirm not in a static field/returned from method/passed as parameter/etc. If really local, can optimize by storing in registers.
- Objects are 8 byte aligned by default. Fields are aligned by type.
- OOP (ordinary object pointer) is a managed pointer. The size can be changed to optimize.
- Compressed Class Pointers – part of the Metaspace. Class data is outside of heap.
- dependency issues
- class unloading/redefinition
- uncommon path
- profiled info isn’t useful for path [like with databases when db assumes something different than you want]
If curious about details
To get information about what compiler thinks/did:
- PrintCompilation – ex: what level instructions were compiled at
- PrintInlining – use -XX:+UnlockDiagonsticVMOptions