I was trying to speed up a log processing service running on PyPy by rewriting it in Java. I was surprised that the result was about twice slower (I know Java quite well and I didn't see obvious optimizations; most of the time was spent in GC). So it can be quite fast even in more absolute terms (VM languages), at least for some types of code.

If more than 1% of your time is in GC you are doing something very wrong.

The fact that a singular implementation was better than java says less about the languages and more about the particular software.

I know that it’s asking a lot, but any chance that you can post a minimum reproducible sample? From what I know it is quite smelly...

I don't have access to this codebase now but I'll try to write some benchmark.

If you manage to do it would be awesome, otherwise thanks a lot anyway for the effort :) I’m just curious to understand why it happens because it’s exactly the opposite of what I would expect. The only explanation that comes to my mind is excessive gc as someone else already mentioned, but it would be interesting to see the original code.

I started doing it but yes, it's too much effort to get two full benchmarks, I'm sorry :). But I think it went down to the inefficiency of String.split(): https://stackoverflow.com/questions/37007189/string-split-te... and generally the Java's String built-in methods not being GC-friendly: https://stackoverflow.com/questions/20336459/garbage-friendl.... I'm guessing that when such parts can be coded in a non-VM environment (CPython/PyPy runtime) they can be made much faster, and Java to these days has the standard library coded in pure Java?

Yes, Java has nothing like C# Span to avoid these kind of problems, but I thought that also python would be affected in a similar way... Anyway, thanks for sharing.

CPython and PyPy are both VMs, similarly to the JVM.

I meant the inside of a VM (its implementation), which is coded in C/RPython. Java's VM is coded in C++, but I don't think C++ is used for any regular library functions, while C is used heavily for Python's stdlib.

Not all JVMs are implemented in C++.

It is a specification with multiple implementations, some of them are even bootstrapped in Java.

I would have expected it: https://stackoverflow.com/questions/9371238/why-is-reading-l...

I had a binary parser written in Python that took around 30 seconds on typical input on CPython. PyPy took that down to about 10 seconds. Rewriting it in C# took it down to 200 ms.

If this was using a loop processing a single byte in an iteration I would expect a greater speedup on PyPy. I've seen 100x speedup in such cases.

Not single byte, but individual fields (float32/int32/string etc). Yes, I expected a much more significant speed-up as well. It's probably because a lot of that code was driven by reflection-type techniques.

Curiously, IronPython did better than anything (but still slow). Haven't tried Jython.

Compiling the whole thing with Cython was less effective than PyPy.

Yes, reflection voids many JIT paths in PyPy, AFAIK. Maybe it was worth rewriting the Python code to get rid of the reflection?