What's the status of / relation between "JEP 169: Value Objects" / "Value Types for Java" / "Object Layout"
brian.goetz at oracle.com
Thu Jan 29 17:05:23 UTC 2015
> Question: is JEP 169 still under active development or has it been
> merged into the more general "Value types for Java" proposal below?
It has been merged into the more general Value Types for Java proposal.
> The "Value types for Java" approach clearly seems to be the most
> general but also the most complex proposal.
For some meanings of "complex". It is certainly the most intrusive and
large; new bytecodes, new type signatures. But from a user-model
perspective, value types are actually fairly simple.
> It's out of scope for Java
> 9 and still questionable for Java 10 and above. The "PackedObject" and
> "ObjectLayout" approaches are clearly simpler and more limited in
> scope as they only concentrate on better object layout.
To your list, I'd add: Project Panama, the sister project to Valhalla.
Panama focuses on interop with native code and data, including layout
specification. A key goal of Packed was to be able to access off-heap
native data in its native format, rather than marshalling it across the
JNI boundary. Panama is focused on this problem as well, but aims to
treat it as a separate problem from Java object layout, resulting in
what we believe to be a cleaner decomposition of the two concerns.
Packed is an interesting mix of memory density (object embedding and
packed arrays) and native interop. But mixing the two goals also has
costs; our approach is to separate them into orthogonal concerns, and we
think that Valhalla and Panama do just that. So in many ways, while a
larger project, the combination of Valhalla+Panama addresses the problem
that Packed did, in a cleaner way.
> Question: is there a chance to get a some sort of Java-only but
> transparently optimizable structure package like "ObjectLayout" into
> Java early (i.e. Java 9)?
It would depend on a lot of things -- including the level of readiness
of the design and implementation, and the overlap with anticipated
future features. We've reviewed some of the early design of
ObjectLayout and provided feedback to the projects architects;
currently, I think it's in the "promising exploration" stage, but I
think multiple rounds of simplification are needed before it is ready to
be considered for "everybody's Java." But if the choice is to push
something that's not ready into 9, or to wait longer -- there's not
actually a choice to be made there.
I appreciate the desire to "get something you can use now", but we have
to be prepared to support whatever we push into Java for the next 20
years, and deal with the additional constraints it generates -- which
can be an enormous cost. (Even thought the direct cost is mostly borne
by Oracle, the indirect cost is borne by everyone, in the form of slower
progress on everything else.) So I am very wary of the motivation of
"well, something better is coming, but this works now, so can we push it
in?" I'd prefer to focus on answering whether this is right thing for
Java for the next 20 years.
> In my eyes this wouldn't contradict with a more general solution like
> the one proposed in the "Value types for Java" approach while still
> offering quite significant performance improvements for quite a big
> range of problems.
The goals of the ObjectLayout effort has overlap with, but also differs
from, the goals of Valhalla. And herein is the problem; neither
generalizes the other, and I don't think we do the user base a great
favor by pursuing two separate neither-coincident-nor-orthogonal
approaches. I suspect, though, that after a few rounds of
simplification, ObjectLayout could morph into something that fit either
coincidently or orthogonally with the Valhalla work -- which would be
great. But, as you know, our resources are limited, so we (Oracle)
can't really afford to invest in both. And such simplification takes
time -- getting to that "aha" moment when you realize you can simplify
something is generally an incompressible process.
> Question: what would be the right place to propose something like the
> "ObjectLayout" library for Java 9/10? Would that fit within the
> umbrella of the Valhalla project or would it be done within its own
> project / under it's own JEP?
Suggesting a version number at this point would be putting the cart
before the horse (you'll note that we've not even proposed a version
number for Valhalla; the closest we've gotten to that is "after 9".)
OpenJDK Projects are a tool for building a community around a body of
work; JEPs are a project-management tool for defining, scoping, and
tracking the progress of a feature. Given where OL is, it would be
reasonable to start a Project, which would become the nexus of
collaboration that could eventually produce a JEP.
Hope this helps,
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