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> [-Ofast will] be "faster" but completely break applications Except for the jvm, apparently. And everything else I've tried it with. > It also suggests usin

> [-Ofast will] be "faster" but completely break applications Except for the jvm, apparently. And everything else I've tried it with. > It also suggests using omit frame pointer which destroys debugability. Which is completely useless except for jvm developers. > As I understand it the arch that Java is compiled with is not the same as the one that gets used for JIT compiling. The performance of the compiler itself matters, not just the performance of the generated code, because, since it's a JIT, compiler code continues to run.

rjsw 7y ago
The Hotspot JIT reads the CPU configuration at runtime to choose which optimizations are best or which instruction extensions are available.
- chrisseaton 7y ago
  That doesn't help the performance of the runtime code, which is C++ or AOT-compiled Java.
- pron 7y ago
  Right, but the OpenJDK VM (HotSpot) uses three JITs -- C1, C2 and Graal -- and two of them are written in C++, so C++ compiler flags could affect the performance of the JIT compilers, although not of the code they generate. Because the performance of the emitted code is far more important than the performance of the compiler, I doubt that will make a difference, but there are other important parts of the OpenJDK VM that are written in C++ and whose performance might be affected, most notably the GCs.
  - rjsw 7y ago
    I would like gcc to compile OpenJDK correctly for AArch64 before I started messing with any performance settings.
    - pron 7y ago
      I agree that messing with compiler flags is not the most effective use of time and risk as fat as improving the JDK's performance goes.
- msbarnett 7y ago
  That's great for the Java code being JITed, but does nothing for the JIT compiler itself, or the garbage collectors, or any other runtime components that are written in C or C++ and compiled (and optimized) ahead of time.
  - simpsond 7y ago
    Yeah, I suspect a good chunk of the performance improvement is related to GC performance improvement. The netty benchmark with pooled buffers compared to unpooled buffers hints at that (although I don't know for sure).
paulddraper 7y ago
> omit frame pointer which destroys debugability. IIRC frame pointers were necessary for a Linux flame graph tool I used on the JVM.
- umanwizard 7y ago
  Based on `perf`? If so, try passing `--call-graph dwarf` in your `perf record` invocation. Then things should work fine despite lack of frame pointers.

rjsw 7y ago

The Hotspot JIT reads the CPU configuration at runtime to choose which optimizations are best or which instruction extensions are available.

chrisseaton 7y ago
That doesn't help the performance of the runtime code, which is C++ or AOT-compiled Java.
pron 7y ago
Right, but the OpenJDK VM (HotSpot) uses three JITs -- C1, C2 and Graal -- and two of them are written in C++, so C++ compiler flags could affect the performance of the JIT compilers, although not of the code they generate. Because the performance of the emitted code is far more important than the performance of the compiler, I doubt that will make a difference, but there are other important parts of the OpenJDK VM that are written in C++ and whose performance might be affected, most notably the GCs.
- rjsw 7y ago
  I would like gcc to compile OpenJDK correctly for AArch64 before I started messing with any performance settings.
  - pron 7y ago
    I agree that messing with compiler flags is not the most effective use of time and risk as fat as improving the JDK's performance goes.
msbarnett 7y ago
That's great for the Java code being JITed, but does nothing for the JIT compiler itself, or the garbage collectors, or any other runtime components that are written in C or C++ and compiled (and optimized) ahead of time.
- simpsond 7y ago
  Yeah, I suspect a good chunk of the performance improvement is related to GC performance improvement. The netty benchmark with pooled buffers compared to unpooled buffers hints at that (although I don't know for sure).

paulddraper 7y ago

> omit frame pointer which destroys debugability. IIRC frame pointers were necessary for a Linux flame graph tool I used on the JVM.

umanwizard 7y ago
Based on `perf`? If so, try passing `--call-graph dwarf` in your `perf record` invocation. Then things should work fine despite lack of frame pointers.