12 ms·

Let's say you write an Iterable class so that you can use foreach on your data structure. It's quite possible iteration won't be completely inlined and the over

Let's say you write an Iterable class so that you can use foreach on your data structure. It's quite possible iteration won't be completely inlined and the overhead of calling hasNext & next can become non-significant. An even worse case that made massive speedups is just inlining everything. I believe modern IDEs can automatically inline all invocations of a function. You'd be surprised how much performance you can get with that. As for writing efficient Java, it comes down to using primitive and value types. Although, my experience is mostly with Java 8. I have no idea how streams or other new syntax works. After optimizing a really massive Java code base I've never used it since.

iamcreasy 6y ago
But you still have to make the next and hasNext calls in a for loop. What guarantees that they would be inline in a for loop?
- knuthsat 6y ago
  I inline them manually.
  - iamcreasy 6y ago
    How much was the performance improvement?
    - knuthsat 6y ago
      The biggest improvement I measured was about 15x. I'd say you can get 3x most of the time (go down to 33% of previous runtime).
      - iamcreasy 6y ago
        That is insane speedup! Thanks for sharing. How do I find next and hasNext is the bottleneck? Should I run profiler to see if they are being called a large number of times?
        knuthsat 6y ago
        Unfortunately, any profiler I tried does not show these issues. Because the code you are profiling will be optimized completely differently. Although it makes sense to do the optimizations/inlining inside the top-level functions that profiler reports to take a lot of time (even if the iteration happens in deeper levels). You just have to try it and see. I remember it was quite easy in IntelliJ, I believe there's an option to inline all invocations of a function. I just went crazy with that option but did it systematically to really find where the bottleneck is and make a minimal change.

iamcreasy 6y ago

But you still have to make the next and hasNext calls in a for loop. What guarantees that they would be inline in a for loop?

knuthsat 6y ago
I inline them manually.
- iamcreasy 6y ago
  How much was the performance improvement?
  - knuthsat 6y ago
    The biggest improvement I measured was about 15x. I'd say you can get 3x most of the time (go down to 33% of previous runtime).
    - iamcreasy 6y ago
      That is insane speedup! Thanks for sharing. How do I find next and hasNext is the bottleneck? Should I run profiler to see if they are being called a large number of times?
      - knuthsat 6y ago
        Unfortunately, any profiler I tried does not show these issues. Because the code you are profiling will be optimized completely differently. Although it makes sense to do the optimizations/inlining inside the top-level functions that profiler reports to take a lot of time (even if the iteration happens in deeper levels). You just have to try it and see. I remember it was quite easy in IntelliJ, I believe there's an option to inline all invocations of a function. I just went crazy with that option but did it systematically to really find where the bottleneck is and make a minimal change.