[vitalyd]

99.99% is perhaps your estimate, but not necessarily others. This is also not likely what people would consider "layout control" if they're coming from a language that allows field-level layout control. Being able to place frequently used together fields manually is quite useful in quite a few circumstances.

[pcwalton]

> Being able to place frequently used together fields manually is quite useful in quite a few circumstances.

I have almost never seen this make a difference outside of, say, GPU programming. The fact that Java's optimizer is much better than that of Go will make a much larger difference in execution speed.

[vitalyd]

This is mostly an issue for large objects (i.e. span multiple cache lines), but have different access patterns for various fields (i.e. clusters of fields accessed together).

The other aspect of layout control is cacheline padding, which is also not present in the JVM. There's @Contended, but it's a blunt tool and not currently a public API (it's in sun.misc).

>The fact that Java's optimizer is much better than that of Go will make a much larger difference in execution speed

Yes, but that's orthogonal.

[pron]

Both grouping and padding are possible with Java's value types. The only thing you don't have full guaranteed (i.e. on multiple JVMs) control over is ordering and alignment.

[vitalyd]

Padding is possible with today's java (at least Hotspot), although it's ugly and relies on Hotspot's existing layout implementation.

Yes, you can group but as I mentioned in the other reply, you get "blob" granularity/control. This is unlikely to be an issue for a value type instance itself, but say you want to have multiple value types (each with several fields) embedded into a heap allocated object. Assume this total embedded blog spans multiple cachelines, but you have clustered access patterns. So you've grouped some data together into value types, but you have no control over how that will be laid out in the "host" object.

Again, I'm happy that Oracle will be adding value types -- they will definitely help. I contend, however, that calling it "layout control" is a slight stretch in the general sense; it's perhaps fair to call it that when comparing against today's java, but not against languages that allow explicit field-by-field layout (along with padding, alignment, etc).

[pron]

> it's perhaps fair to call it that when comparing against today's java, but not against languages that allow explicit field-by-field layout (along with padding, alignment, etc).

It's fair to call it that when comparing against any language or runtime targetting general-purpose development, but not compared to languages designed for different purposes. I agree with that.

However, we're not discussing those languages here so I don't see the relevance. I'm also sure that many statements are wrong when compared to quantum computing, but I don't think there's any need to make that qualification. It's pretty clear from the context.

[vitalyd]

>It's fair to call it that when comparing against any language or runtime targetting general-purpose development

Unless you consider, e.g., C, C++, and Rust as not general-purpose. Even C# has StructLayout where you can control the struct layout. Heck, Go and C# allow you to embed a fixed size array into a struct (this facility is not in the works for Java at the moment).

[pron]

But value types do let you group fields (with sub-component values).

Also, I've noticed that whenever I say Java does X, you say, "Oh, no! It does X - ε!" Now, to me, that's nitpicking, especially considering that a perfect general-purpose language/runtime designed to be simple (for some definition of simple) should give you 90+% performance in 99% of general-purpose use cases (or 95% in 95% etc.). If it does any better then one of two possibilities is true: 1/ it's magic, or 2/ it's not a perfect simple language/runtime because it could have been made simpler (by whatever definition of simple it's chosen).

Anyone who can't settle for anything less than 100% performance or does something that's outside 95% of the use cases knows not to use such a general-purpose language/runtime, and, instead, uses a more domain-specific language/runtime or one that's not designed to be simple.

[vitalyd]

>But value types do let you group fields (with sub-component values).

They let you treat the fields of a value type as a "blob", you have no control over how they're laid out within that blob.

>Also, I've noticed that whenever I say Java does X, you say, "Oh, no! It does X - ε!" Now, to me, that's nitpicking, especially considering that a perfect general-purpose language/runtime designed to be simple (for some definition of simple) should give you 90+% performance in 99% of general-purpose use cases (or 95% in 95% etc.). If it does any better then one of two possibilities is true: 1/ it's magic, or 2/ it's not a perfect simple language/runtime because it could have been made simpler (by whatever definition of simple it's chosen).

Nothing personal, but I find your JVM related posts as borderline fanboyism (and I say this as someone that greatly respects the engineering in Hotspot, despite certain things bugging me). It's not about 100% or 95% performance; it's about not making exaggerated claims since, as you say, there's no magic.

[pron]

> I find your JVM related posts as borderline fanboyism

I think they're much less fanboyish than any other language/runtime discussion on HN.

> it's about not making exaggerated claims

I am not making exaggerated claims because perfect for me (and, by definition 95% of people) is precisely how I've defined the requirements from a runtime like Java (or Go, or Python, for that matter). So, if I say "it gives you all the layout control you need", I don't mean all the layout control you need to write a DSP, or all the layout control you need to get 100% performance -- only 99% performance. I think most people make the same assumption (because they're not writing DSPs), and I find your posts nitpicky and possibly misleading for the target audience (who, by and large, don't write DSPs or medical devices or particle accelerator beam controllers -- at least certainly on threads not discussing those interesting but extreme use cases). I don't think it's reasonable to qualify every statement for some negligible (possibly nonexistent) portion of the participants, especially considering that that minority already knows the statements don't completely apply to them. Not doing so doesn't qualify as exaggeration IMO, but reasonable expressiveness, or else every discussion will be bogged down in irrelevant detail that will only distract from the main point.

So, yes, I stand behind my claim that value types give you all the control you need to get 99% performance for 99% of the use cases people would normally use Java for.

[TheLoneWolfling]

X - ε, for very large values of ε.

[TheLoneWolfling]

X - ε, for very large values of ε.