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Of course a JIT is capable of producing code as good as an AOT and perhaps even better since it can capture more profiling data. The problem with a JIT is the s

Of course a JIT is capable of producing code as good as an AOT and perhaps even better since it can capture more profiling data. The problem with a JIT is the startup time and this has not gone unnoticed by Oracle as even they've started working on AOT.

pron 11y ago
Not startup but warmup (i.e. the time until the application is fully optimized). There are other problems with a JIT on small devices, such as increased memory and energy consumption (each may be significant or not, depending on how the JIT works). I am a big fan of JITs, but as with everything in software, it is a tradeoff.
- strcat 11y ago
  Increased memory consumption and reduced page sharing between processes using the same code means lower performance. Memory bandwidth and cache capacity are a much bigger bottleneck than CPU throughput for most use cases. A JIT compiler is not competitive in performance with an AOT compiler if both are using the same code generation implementation. And profiling is limited in what it can do, especially when AOT compilation with PGO as used by projects like Firefox is considered.
  - pron 11y ago
    > A JIT compiler is not competitive in performance with an AOT compiler if both are using the same code generation implementation. If a JIT uses the same code generation as an AOT, it will work as an AOT, namely, only when the application starts, without additional burden during its runtime. Most good JITs use profiling information, which is collected at runtime -- this has some memory overhead, but results in better code. > And profiling is limited in what it can do, especially when AOT compilation with PGO as used by projects like Firefox is considered. Modern JITs are like _very_ sophisticated PGO AOTs, but they have true profiles exposed to them. Besides, the overhead for even a good JIT is not that large, it's just that on mobile devices even a bit more RAM is more than you're willing to afford.

pron 11y ago

Not startup but warmup (i.e. the time until the application is fully optimized). There are other problems with a JIT on small devices, such as increased memory and energy consumption (each may be significant or not, depending on how the JIT works). I am a big fan of JITs, but as with everything in software, it is a tradeoff.

strcat 11y ago
Increased memory consumption and reduced page sharing between processes using the same code means lower performance. Memory bandwidth and cache capacity are a much bigger bottleneck than CPU throughput for most use cases. A JIT compiler is not competitive in performance with an AOT compiler if both are using the same code generation implementation. And profiling is limited in what it can do, especially when AOT compilation with PGO as used by projects like Firefox is considered.
- pron 11y ago
  > A JIT compiler is not competitive in performance with an AOT compiler if both are using the same code generation implementation. If a JIT uses the same code generation as an AOT, it will work as an AOT, namely, only when the application starts, without additional burden during its runtime. Most good JITs use profiling information, which is collected at runtime -- this has some memory overhead, but results in better code. > And profiling is limited in what it can do, especially when AOT compilation with PGO as used by projects like Firefox is considered. Modern JITs are like _very_ sophisticated PGO AOTs, but they have true profiles exposed to them. Besides, the overhead for even a good JIT is not that large, it's just that on mobile devices even a bit more RAM is more than you're willing to afford.