[raspasov]

Yes and no. There's real production cases where even default Clojure can result in the same or faster performance than Java. One hypothetical case is a system that does a lot of complex in-memory reads and a few writes to persistent data structures. That kinda of system could be faster than the Java equivalent, out of the box, in Clojure.

A write-heavy system would benefit Java mutable collections out of the box. Clojure can get pretty close to Java with transients and a good dose of type hinting in all the right places.

When we say "faster" or "slower" it's equally important to specify "faster" or "slower" when and where. It's a complex question with no easy answer.

[eternalban]

> One hypothetical case is a system that does a lot of complex in-memory reads and a few writes to persistent data structures. That kinda of system could be faster than the Java equivalent, out of the box, in Clojure.

Why would Java be slower in a read-mostly regime? Your hypothetical is not convincing. Btw, you mention "real" and then move on to "hypothetical" as an example.

Are there actually OSS "production" cases of this subset of systems where Java lags behind Clojure in performance?

> When we say "faster" or "slower" it's equally important to specify "faster" or "slower" when and where. It's a complex question with no easy answer.

These sort of subtle distinctions only matter to langauge wars and debates like this. For actuals systems that need to do work and need to be maintained, we can in fact have metrics on efficiency.

That said, fundamentally, Java affords much greater facilities to "optimize" and approach white hot performance than Clojure.

[raspasov]

> Why would Java be slower in a read-mostly regime? Your hypothetical is not convincing. Btw, you mention "real" and then move on to "hypothetical" as an example.

Because of Clojure's default persistent in-memory data structures which allow you to obtain a stable reference to your in-memory data even when the data is being updated live. With Java's default mutable data structures, you'd have to use locking or copying to obtain a stable reference, not to mention the huge complexity of those solutions. I said real because I've built similar solutions in Clojure. Substitute "hypothetical" with "example", sorry for the word confusion.

> That said, fundamentally, Java affords much greater facilities to "optimize" and approach white hot performance than Clojure.

Taken to the extreme, the resulting Clojure would not be idiomatic, it would be effectively Java-in-Clojure, but Clojure has all the facilities that Java has, by definition.

[eternalban]

In read-mostly regimes, the performance considerations are typically systems-level consideration. Data locality, page swaps, and cache-line misses are typical concerns of high performance systems engineering.

Regarding "complexity", not sure what you mean. IIRC, someone, possibly even me ;), may have pointed out to Rich Hickey in the early days that Java code could also use those (Java or was it Scala) STM libraries. So, there is your "complexity" behind an API, just like Clojure.

> Clojure has all the facilities that Java has, by definition.

Ergo, a situation where Java can -not- be made as fast or faster than Clojure code seems unlikely.

I think all we have here is you claim of "complexity". I remember Rob Pike making a comment along the lines of "prefer libraries over language semantics" or something like that in context of Go language design. I find it a very compelling argument, based on my overall experience to date.