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0 pointsimtringued23d ago0 comments

It's speculation because the author didn't show the byte code or even just what the code decompiles to in Java.

But even with speculation, it shouldn't be that surprising that dynamic dispatch and reflection [0] are quite expensive compared to a cast and a field access of the length property.

[0] https://bugs.openjdk.org/browse/JDK-8051447

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mkmccjr22d ago

These are both great points. When I wrote the post, it didn't occur to me that I could inspect the emitted bytecode. In hindsight, including that would have made the explanation much stronger.

To be honest, this is my first time really digging into performance on a JIT runtime. I learned to code as an astronomy researcher and the training I received from my mentors was "write Python when possible, and C or Fortran when it needs to be fast." Therefore I spent a lot of time writing C, and I didn't appreciate how aggressively something like HotSpot can optimize.

(I don't mean that as a dig against Python; it's simply the mental model I absorbed.)

The realization that I can have really good performance in a high-level language like Clojure is revolutionary for me.

I'm learning a ton from the comments here. Thanks to everyone sharing their knowledge -- it's genuinely appreciated.

zahlman22d ago

> The realization that I can have really good performance in a high-level language like Clojure is revolutionary for me.

I should try it out some time. The Lisp family takes a bit of a mental reset to work with, but I've done it before.

> ...the training I received from my mentors was "write Python when possible, and C or Fortran when it needs to be fast."... (I don't mean that as a dig against Python; it's simply the mental model I absorbed.)

Well, you know, I've been using Python for over 20 years and that really isn't a "dig" at all. The execution of Python is famously hard to optimize even compared to other languages you might expect to be comparable. (Seriously, the current performance of JavaScript engines seems almost magical to me.) PyPy is the "JIT runtime" option there; and you can easily create micro-benchmarks where it beats the pants off the reference (written in C with fairly dumb techniques) Python implementation, but on average the improvement is... well, still pretty good ("On average, PyPy is about 3 times faster than CPython 3.11. We currently support python 3.11 and 2.7"), but shrinking over time, and it's definitely not going to put you in the performance realm of native-compiled languages.

The problem is there's really just too much that can be changed at runtime. If you look at the differences between Python and its competitors like Mojo, and the subsets and variants of Python used for things like Shedskin and Cython (and RPython, used internally for PyPy) you quickly get a sense of it.

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