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Comment by stmw | Hacker News Reader

Impressive and interesting, but it is really a great example of how in larger production systems, Garbage Collection turns into "manual memory management" akin to that required to manage malloc() and free() correctly -- little performance traps, ever-changing library behaviors, tuning parameters, etc. (This is not specific to Ruby, the JVM has the same kind of "here is a whole blog post about person-weeks or person-months that went into getting the GC to behave better).

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The thing is, the more I work with network based reactive systems, the more a GC seems like an inefficient hack in an application, arising from the fact that we had nothing better at the time it was invented and grew big.

I mean a reactive application needs three things:

- a fast network connection to persistence. If you can't talk quickly to your database, latency eats your response time no matter what you do.

- Maybe some LRU caches across requests, but caches inside an application tend to become a maintenance nightmare quickly. Stales reads are fun, and stalled cache writes are too.

- And beyond that, it should be possible to handle most request data by stack allocation/deallocation with references to a cache version if you have a cache with little overhead, depending on your security requirements.

I understand the ease of use and simplicity of a GC'd language, especially given the time GC'd languages came around big time. But I've always been wondering if you really need a GC if you think properly about request and object lifetimes.

> handle most request data by stack allocation/deallocation

I just write crappy code for microcontrollers. Over 30 years I've seen the amount of ram available for stack grow. From maybe 20-30 bytes to a couple of k. That said it appears to me that stack allocation is severely under utilized for historical reasons. The idea that stack space is a precious resource. Which doesn't fly on machines with 10's of gigabytes of memory.

Think programs that put vast quantities of ephemeral objects and short strings on the heap. Using call tree analysis would allow you to put a lot of that on the stack resulting is much better performance and bounded latency.

Java performs escape analysis and may allocate non-escaping objects on the stack.

Quoting https://www.beyondjava.net/#/blog/escape-analysis-java

> Moving objects from the heap to the stack

> Many sources report that escape analysis moves Java objects from the heap to the stack. As Aleksey Shipilёv points out in his article about scalar replacement, the JVM does not do this implementation. It's just a misconception. But it's an interesting one, and I wonder why the JVM doesn't implement it.

Interesting article, but the distinction you're making is quite subtle. Using escape analysis, Java can avoid heap allocations by removing the creation of the Object altogether, treating its fields as if they were local variables which live in registers/the stack. The practical difference between that and simply allocating the object on the stack is that no space is required for the object header, and some reference-based operations are not possible.

But the article states "Anything you create with new is always created on the heap."

Yes, but it then explains that escape analysis allows the runtime to avoid creating the Object entirely with scalar replacement. It's written in a confusing way, which is why I felt the need to clarify.

Ugh, you're right, of course.

In my defence the sentence I quoted isn't so much deliberately misleading, as outright false. It essentially states that 'new' always results in a heap allocation, which just isn't true.

A profoundly unhelpful writing style, going out of its way to punish skim-readers. I'm reminded of this TV Tropes page: http://tvtropes.org/pmwiki/pmwiki.php/Main/ByNoIMeanYes

Edit: Turns out that BeyondJava article is just poorly written, and tries so hard to emphasise its subtle distinctions that it ends up being actively harmful to the reader's understanding. I'll leave my comment anyway.

From that article:

> Java doesn't store any object in the stack. It does store local variables on the stack: primitive types like int or boolean. It also store the pointers to objects on the stack. But it doesn't store the objects themselves on the stack. Anything you create with new is always created on the heap.

Is that true? For years now there have been articles from serious sources discussing JVM escape-analysis-based optimisations.

Is there something mistaken in the analysis in this DZone article?

https://dzone.com/articles/do-not-let-your-java-objects-esca...

How about this StackOverflow answer, which even goes into the detail of distinguishing escape-analysis, stack allocation of objects, and object deconstruction+scalar replacement:

https://stackoverflow.com/a/43002529/

Link to Shipilev's article: https://shipilev.net/jvm-anatomy-park/18-scalar-replacement

If stack allocation was really done, it would allocate the entire object storage on the stack, including the header and the fields, and reference it in the generated code. The caveat in this scheme is that once the object is escaping, we would need to copy the entire object block from the stack to the heap, because we cannot be sure current thread stays in the method and keeps this part of the stack holding the object alive. Which means we have to intercept stores to the heap, in case we ever store stack-allocated object — that is, do the GC write barrier.

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