[pron]

Sure, using arenas is very often straightforward, but it also very often isn't. For example, say you have a server. It's very natural to have an arena for the duration of some request. But then things could get complicated. Say that in the course of handling the transaction, you need to make multiple outgoing calls to services. They have to be concurrent to keep latency reasonable. Now arenas start posing some challenges. You could use async/coroutine IO to keep everything on the same thread, but that imposes some limitations on what you can do. If you use multiple threads, then either you need to synchronise the arena (which is no longer as efficient) or use "cactus stacks" of arenas and figure out a way to communicate values from the "child" tasks to the parent one, which isn't always simple (and may not even be super efficient).

In lots of common cases, arenas work great; in lots of common cases they don't.

There are also other advantages unrelated to memory management. In this talk by Andrew Kelley (https://youtu.be/f30PceqQWko) he shows how Zig, despite its truly spectacular partial evaluation, still runs into an abstraction/performance tradeoff (when he talks about what should go "above" or "below" the vtable). When you have a really good JIT, as Java does, this tradeoff is gone (instead, you trade off warmup time) as the "runtime knowns" are known at compile time (since compilation is done at runtime).

[chpill]

  When you have a really good JIT, as Java does, this tradeoff is gone

Is there a way to visualize the machine code generated by the JVM when optimizing the same kind of code as the examples shown in the talk you mention? I tried putting the following into godbolt.org, but i'm not sure I'm doing it right:

  public class DontForgetToFlush {
      public static void example(java.io.BufferedWriter w) throws java.io.IOException {
          w.write("a");
          w.write("b");
          w.write("c");
          w.write("d");
          w.write("e");
          w.write("f");
          w.write("g");
          w.flush();
      }
      public static void main(String... args) throws java.io.IOException {
          var os = new java.io.OutputStreamWriter(System.out);
          var writer = new java.io.BufferedWriter(os, 100);
          example(writer);
      }
  }

[pron]

You can ask the JVM to dump the actual instructions (https://javanexus.com/blog/printing-assembly-code-hotspot-ji...).

Just note that there may be differences between the very old APIs (as in your example), and the newer NIO (https://docs.oracle.com/en/java/javase/24/docs/api/java.base...), and you need to pay attention to text output that undergoes characeter set encoding (as in your example) vs binary output.

[vips7L]

You can use jit watch: https://github.com/AdoptOpenJDK/jitwatch