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Maybe my knowledge isn't up to date, but I always understood (since Hotspot anyway) that not all bytecode is necessarily JIT'd to native code. From the Hotspot

Maybe my knowledge isn't up to date, but I always understood (since Hotspot anyway) that not all bytecode is necessarily JIT'd to native code. From the Hotspot Wikipedia page: "Both VMs compile only often-run methods, using a configurable invocation-count threshold to decide which methods to compile."[1] Also see very old discussions at StackOverflow[2][3]. Then of course there are compilers (eg. gcj) which compile to native up-front. [1] https://en.wikipedia.org/wiki/HotSpot#Features https://en.wikipedia.org/wiki/HotSpot#Features [2] https://stackoverflow.com/questions/7100365/why-doesnt-javas-jit-compiler-translate-everything-to-native-code https://stackoverflow.com/questions/7100365/why-doesnt-javas... [3] https://stackoverflow.com/questions/16568253/difference-between-jvm-and-hotspot https://stackoverflow.com/questions/16568253/difference-betw...

chrisseaton 6y ago
> I always understood (since Hotspot anyway) that not all bytecode is necessarily JIT'd to native code Who are you disagreeing with? I didn’t say that’s the only way they execute bytecode, did I? The question was how it’s executed ‘after they warm up’. > Then of course there are compilers (eg. gcj) which compile to native up-front. But I was replying to someone who asked specifically about how ‘production JVMs’ do it, not how discontinued compilers do it.
- nineteen999 6y ago
  > Who are you disagreeing with? I didn’t say that’s the only way they execute bytecode, did I? Nope, my bad. Thanks for clarifying.
bitcharmer 6y ago
Only hot code paths get compiled. That's after 10000 executions on "server" JVM and 1000 executions on a "client" JVM by default. This is by design, and if you need everything compiled right away you can set the compilation threshold to 1. I don't see any value in compiling parts of code that only gets executed during bootstrap.
- nineteen999 6y ago
  > I don't see any value in compiling parts of code that only gets executed during bootstrap. Not disagreeing with you there, since stopping to compile code/optimize at runtime contributes to sluggish interactive performance.
  - bitcharmer 6y ago
    Compilation is performed concurrently to the application in specialised compilation threads. Only OSR (on-stack-replacement) requires stopping execution.
  - chrisseaton 6y ago
    > Not disagreeing with you there, since stopping to compile code/optimize at runtime contributes to sluggish interactive performance. No JVM I am aware of stops to compile - the compiler runs on a background thread while the application continues to run as normal.
- MaxBarraclough 6y ago
  Nitpick: the HotSpot 'client' JVM's default JIT threshold is 1500, not 1000. https://www.oracle.com/java/technologies/javase/vmoptions-jsp.html https://www.oracle.com/java/technologies/javase/vmoptions-js...
  - bitcharmer 6y ago
    You're right, thank you for correcting me. For more popular archs (x86, arm, aarch64) it's 1500. It's only 1000 by default for ppc, s390 and sparc. https://chriswhocodes.com/hotspot_options_jdk14.html https://chriswhocodes.com/hotspot_options_jdk14.html
    - MaxBarraclough 6y ago
      So we're both right! Didn't know it varied between architectures.

chrisseaton 6y ago

> I always understood (since Hotspot anyway) that not all bytecode is necessarily JIT'd to native code Who are you disagreeing with? I didn’t say that’s the only way they execute bytecode, did I? The question was how it’s executed ‘after they warm up’. > Then of course there are compilers (eg. gcj) which compile to native up-front. But I was replying to someone who asked specifically about how ‘production JVMs’ do it, not how discontinued compilers do it.

nineteen999 6y ago
> Who are you disagreeing with? I didn’t say that’s the only way they execute bytecode, did I? Nope, my bad. Thanks for clarifying.

bitcharmer 6y ago

Only hot code paths get compiled. That's after 10000 executions on "server" JVM and 1000 executions on a "client" JVM by default. This is by design, and if you need everything compiled right away you can set the compilation threshold to 1. I don't see any value in compiling parts of code that only gets executed during bootstrap.

nineteen999 6y ago
> I don't see any value in compiling parts of code that only gets executed during bootstrap. Not disagreeing with you there, since stopping to compile code/optimize at runtime contributes to sluggish interactive performance.
- bitcharmer 6y ago
  Compilation is performed concurrently to the application in specialised compilation threads. Only OSR (on-stack-replacement) requires stopping execution.
- chrisseaton 6y ago
  > Not disagreeing with you there, since stopping to compile code/optimize at runtime contributes to sluggish interactive performance. No JVM I am aware of stops to compile - the compiler runs on a background thread while the application continues to run as normal.
MaxBarraclough 6y ago
Nitpick: the HotSpot 'client' JVM's default JIT threshold is 1500, not 1000. https://www.oracle.com/java/technologies/javase/vmoptions-jsp.html https://www.oracle.com/java/technologies/javase/vmoptions-js...
- bitcharmer 6y ago
  You're right, thank you for correcting me. For more popular archs (x86, arm, aarch64) it's 1500. It's only 1000 by default for ppc, s390 and sparc. https://chriswhocodes.com/hotspot_options_jdk14.html https://chriswhocodes.com/hotspot_options_jdk14.html
  - MaxBarraclough 6y ago
    So we're both right! Didn't know it varied between architectures.