[concurrency-interest] RFR (S): JEP-142: Reduce Cache Contention on Specified Fields (original) (raw)

Aleksey Shipilev aleksey.shipilev at oracle.com
Fri Nov 23 12:07:16 PST 2012

On 11/23/2012 11:43 PM, Doug Lea wrote:

(Warning: cross-posting, so replies may bounce to one or both lists.)

However, this still has a drawback that the subclass annotation will only affect the fields in the subclass, and not in the superclasses (laying the superclass fields on the same offsets in the subclass is important to get cheap polymorphism). Considering that the only usages with predictable effects will be on leaf/final classes anyway, perhaps the @Contended annotation should only have a defined effect on layout when applied to final classes (plus statics). This also seems to be the only case when a developer can have an empirical basic for using the annotation. And if there someday turns out to be some reason to lift this restriction, it would be possible to do so.

I don't think so. My comment was that you can't change the padding for the superclass fields, but the contended safety for the superclass fields should actually stay intact. I.e.,

class Base { @Contended private int mySuperImportantField; }

...should NOT be compromised with

class Rogue extends Base { private int wannaKillYourPadding; }

...and it doesn't in the present version, and it sticks with the same principle: once superclass fields are laid out, you stick with that layout in all the subclasses.

-Aleksey.

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