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0 pointsemn137y ago0 comments

Value types aren't "necessary", but they would have been valueable at day 1. The GC heap is simply inefficient; not necessarily because of the GC (which indeed is harder with massive multicore), but simply because of the per-object memory overhead.

There's a reason java had built-in value types from day 1, because it made sense even back then.

Frankly, I think both java and C# kind of got this wrong. There was an overreaction against the C/C++ of the day, and whereas the GC turned out brilliant, the idea that it's not even necessary to express the notion of references/pointers/values etc. was too much; and the idea of a single type system root (object) is similarly dubious, and then particularly the idea that that root type isn't the empty type. Object has semantics, and that was a mistake, because it contributes to the bloat. I'm totally happy with ignoring those features 99.9% of the time, but having them completely unavailable makes those 0.1% cases extremely expensive. (I mean, I think those things are slightly changing, but it's slow going).

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pron7y ago

> because it made sense even back then.

That was necessary for performance back then. User-defined value types weren't, and Java has done well without them.

> Object has semantics, and that was a mistake, because it contributes to the bloat.

I think most of the RAM bloat is due to the GC trading off extra RAM for speed rather than object headers, and I'm not sure trading off complexity for headers was right 25 years ago (JS is doing fine on the client without value types). What changed was the performance characteristics.

As to object semantics, it may be a fixable mistake. The goal is to get value types without today's object semantics while preserving a single class hierarchy at the same time. The Valhalla team thinks that's achievable.

emn13OP7y ago

Heap (over)use by the GC is effectively a scaling factor. How large the underlying objects are remains remains relevant: if your objects are twice the size necessary, the GC will "bloat" that further - and this tradeoff isn't entirely GC specific, other allocators such as those used to implement malloc/free have related tradeoffs to make; free() won't release memory to the OS either (and memory, released or not, may end up evicted from RAM anyhow).

pron7y ago

Of course it is relevant. I'm just saying it isn't the decisive factor that makes this an absolute necessity, as evidenced by the fact that much the backbone of the largest software services in the world is Java. There are lots and lots of tradeoffs in runtime design, and it's important to look at the whole rather than at one decision in isolation and point out that it's important. As a whole, the criticality of value types for Java is relatively recent.

pjmlp7y ago

I advise you to read Mesa/Cedar report on the impact of garbage collection algorithms available at Xerox PARC bitsavers archive, EthOS or SpinOS experience with Modula-3.

All of them refer that having value types alongside GC had a relevant impact improving performance.

All systems designed before Java was a thing.

Or since you refer to JS, the paper about SELF's design.

Even Dylan was designed with AOT/JIT and value types support, which is relevant here given that its domain was being a systems language for the Newton. That politics killed it is another matter.

pron7y ago

Oh, I don't deny that value types would have helped performance back in '95, just that they were absolutely essential for Java. Smalltalk/Self and Scheme/CL didn't have them, and those were probably Java's greatest influences; I don't think VB had them, either. Also, in its first four years, before HotSpot was ready, Java was interpreted, so it had bigger performance problems.

I don't know why there was no emphasis on AOT back then. I guess they started with interpreter/JIT, and then there just wasn't much demand for AOT until now.

pjmlp7y ago

Microsoft Basics have support for value types since MS-DOS.

QuickBasic supported value types and AOT to native code, and while Visual Basic used P-Code, version 6 introduced a proper AOT native compiler.

Modula-3 was also a big influence, at least accordingly to some papers.

There was surely demand for AOT, given that most commercial JVMs had it in some form or the other since 2000.

Even Sun actually supported it in Java Embedded variant for OEMs, probably grudgingly.

Common Lisp certainly has support for value types.

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pjmlp7y ago

C# got it right from day one, regarding value types.

AOT compilation not so much, given the NGEN constraints.

However they got it both wrong, considering CLU, Modula-3, Delphi and Eiffel are considered influencial languages on their design.

pron7y ago

Mutable structs are not exactly value types, but Microsoft has always preferred control over simplicity (after all, they pushed C++ really hard). I won't say whether that philosophy is right or wrong, but it is very different from Java's.

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pron7y ago

> because it made sense even back then.

That was necessary for performance back then. User-defined value types weren't, and Java has done well without them.

> Object has semantics, and that was a mistake, because it contributes to the bloat.

emn13OP7y ago

pron7y ago

pjmlp7y ago

I advise you to read Mesa/Cedar report on the impact of garbage collection algorithms available at Xerox PARC bitsavers archive, EthOS or SpinOS experience with Modula-3.

All of them refer that having value types alongside GC had a relevant impact improving performance.

All systems designed before Java was a thing.

Or since you refer to JS, the paper about SELF's design.

Even Dylan was designed with AOT/JIT and value types support, which is relevant here given that its domain was being a systems language for the Newton. That politics killed it is another matter.

pron7y ago

I don't know why there was no emphasis on AOT back then. I guess they started with interpreter/JIT, and then there just wasn't much demand for AOT until now.

pjmlp7y ago

Microsoft Basics have support for value types since MS-DOS.

QuickBasic supported value types and AOT to native code, and while Visual Basic used P-Code, version 6 introduced a proper AOT native compiler.

Modula-3 was also a big influence, at least accordingly to some papers.

There was surely demand for AOT, given that most commercial JVMs had it in some form or the other since 2000.

Even Sun actually supported it in Java Embedded variant for OEMs, probably grudgingly.

Common Lisp certainly has support for value types.

1 more reply

pjmlp7y ago

C# got it right from day one, regarding value types.

AOT compilation not so much, given the NGEN constraints.

However they got it both wrong, considering CLU, Modula-3, Delphi and Eiffel are considered influencial languages on their design.

pron7y ago

j / k navigate · click thread line to collapse