Project Valhalla: Goals
brian.goetz at oracle.com
Fri Oct 14 19:23:21 UTC 2016
I've heard a number of people describe Valhalla recently as being
"primarily about performance". While it is understandable why people
might come to that conclusion -- many of the motivations for Valhalla
are, in fact, rooted in performance considerations -- this
characterization misses something very important. Yes, performance is
an important part of the story -- but so are safety, abstraction,
encapsulation, expressiveness, maintainability, and compatible library
The major goals of Valhalla are:
- Align JVM memory layout behavior with the cost model of modern hardware;
- Extend generics to allow abstraction over all types, including
primitives, values, and even void;
- Enable existing libraries -- especially the JDK -- to compatibly
evolve to fully take advantage of these features.
Let's take these in turn.
*1. **Align JVM memory layout behavior with the cost model of modern
Java's approach of "(almost) everything is an object" was a reasonable
match for the hardware and compilation technology of the mid-nineties,
when the cost of a memory fetch and an arithmetic operation were of
comparable magnitude. But since then, these costs have diverged by a
factor of several hundred. At the same time, memory costs have come to
dominate application provisioning costs. These two considerations
combine to make the graph-of-small-objects data representation, which
typical Java programs result in, suboptimal in both program performance
and provisioning cost.
The root cause of this is a partly accidental one: object identity.
Every object has an identity, but not all objects /need/ an identity --
many objects represent values, such as decimal numbers, currency
amounts, cursors, or dates and times, and do not need this identity.
This need to preserve identity foils many otherwise powerful optimizations.
Our solution for this is /value types/; value types are aggregates, like
traditional Java classes, that renounce their identity. In return, this
enables us to create data structures that are /flatter/ (because values
can be inlined into objects, arrays, and other values, just as
primitives are today) and /denser/ (because we don't waste space on
object headers and pointers, which can increase memory usage by up to
4x), with a programming model more like objects -- supporting nominal
substructure, behavior, subtyping, and encapsulation. /Codes like a
class, works like an int./
If you view values as being "faster objects", you could indeed view this
fundamental aspect of Valhalla as being primarily about efficiency. But
equally, you could view them as "programmable primitives", in which case
it also becomes about better abstraction, encapsulation, readability,
maintainability, and type safety.
Which is the real point -- that we need not force users to choose
between abstraction/encapsulation/safety and performance. We can have
both. Whether you call that "cheaper objects" or "richer primitives",
the end result is the same.
*2. **E**xtend generics to allow abstraction over all types, including
primitives, values, and even void.**
Generics are currently limited to abstracting only over reference
types. Sometimes this is merely a performance cost (one can always
appeal to boxing), but in reality this not only increases the cost, but
decreases the expressiveness, of libraries.
Methods like Arrays.fill() have to be written nine times; this is nine
methods to write, nine methods to test, and nine methods for the user to
wade through in the docs.
Real-world libraries like Streams often resort to hand-coded
specializations like IntStream; not only is this an inconvenience for
the writer, but was also a significant constraint in the design of the
stream library, forcing some undesirable tradeoffs. And this approach
rarely provides total coverage; users complain that we have IntStream
and LongStream but not CharStream. (For generic types with multiple
type variables, like Map, the number of specializations starts to
explode out of control.) The functional interfaces introduced in Java 8
are another consequence of the limitations of generics; because generics
are boxed and erased, we had to provide a large number of
hand-specialized versions (and still, not all the ones people want.)
You don't need Predicate if you have can simply say Function<T, boolean>
and not suffer boxing or erasure.
Everyone would be better off if we could write a generic class or method
once -- and abstract over all the possible data types, not just
reference types. (This includes not only primitives and values, but
also void. Treating void uniformly is no mere "abstract all the
things"; HashSet is based on HashMap, for implementation convenience,
but suffers a lot of wasted space as a result; we could use a HashMap<T,
void>. And the XxxConsumer functional interfaces are really just
Function<T, void> -- we don't need separate abstractions here.)
Being able to write things once -- rather than having an ad-hoc
explosion of types and implementations (which often propagates into
further explosion at the client site) -- means simpler, more expressive,
more regular, more testable, more composible libraries. Without giving
up performance when dealing with primitives and values, as boxing does
Which is to say, again, just at the data-abstraction layer instead of
the data-modeling layer: we need not force users to choose between
abstraction/encapsulation/safety and performance.
*3. **Enable existing libraries -- especially the JDK -- to compatibly
evolve to fully take advantage of these features.**
The breadth and quality of Java libraries is one of the core assets of
the Java ecosystem. We don't want people to have to replace their
libraries to migrate to a value-ful world; nor do we want existing
libraries to be "frozen in time." (Imagine if we did lambdas and
streams, but didn't do default methods, that allowed the Collection
classes to evolve to take advantage of them -- Collections would have
instantly looked ten years older.) There should be a straightforward
path to extending existing libraries -- especially core JDK libraries --
to supporting values and enhanced generics, in a way that makes them
"look built in". This may require additional linguistic tools to allow
libraries to evolve while providing compatibility with older clients and
subclasses that have not yet migrated.
Just providing these features for new libraries is not enough; if widely
used libraries can't compatibly evolve to take advantage of this new
world, they are effectively consigned to a slow death. This might be OK
for some classes -- deprecating OldX and replacing it with NewX -- but
only when uses of the OldX types aren't strewn through lots of other
libraries. That rules out starting fresh with Collections, Streams,
JSR-310, and many other abstractions, without rewriting the whole JDK
(and many third party libraries). So instead, we have to provide a
compatible path for such libraries (and their clients) to modernize.
So, to summarize: Valhalla may be motivated by performance
considerations, but a better way to view it as enhancing abstraction,
encapsulation, safety, expressiveness, and maintainability -- /without/
giving up performance.
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