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

Aleksey Shipilev aleksey.shipilev at oracle.com
Mon Nov 26 10:40:28 PST 2012

On 11/26/2012 09:35 PM, Remi Forax wrote:

On 11/26/2012 05:44 PM, Aleksey Shipilev wrote:

On 11/26/2012 08:22 PM, Doug Lea wrote:

One small suggestion to slightly appease the nanny-state folks. How about burying the annotation one lever deeper to java.util.concurrent.atomic. +1

Still fail to see the rationale about this.

I think you should re-write the javadoc to not use 'hot' (hot -> optimize -> I should use it) and talk about false sharing (with a link to the wikipedia article http://en.wikipedia.org/wiki/Falsesharing).

Yup, will do.

You should mention that marking @Contented a field which is not volatile is useless unless there is proper fences.

Um. Actually, there is a lot of sense with marking plain fields with @Contended, especially when visibility is guaranteed elsewhere. You probably mean "marking the field @Contended does not turn it into volatile"? ...does it really worth mentioning?

And also mention that it may consume a lot of memory thus it should only be used if there is a known issue.

Yes, good idea.

Also, I'm not in favour of allowing to use @Contented on class, if you want all fields to be marked as @Contented, just mark them as is. Given that this annotation is here to solve a corner case, using the annotation in a class wide way in my opinion a door open to stupid usages. You don't mark a class volatile if all their fields are volatile.

From the layout perspective, class-level @Contended is equivalent to all-field @Contended with the same contention group. Do you think we don't need this ask the shortcut for tuple/value/struct classes?

I.e.

@Contended
public class ValueClass {
     private int field1;
     private int field2;
     private int field3;
     private int field4;
}

is the shortcut for:

public class ValueClass {
     @Contended("theSame") private int field1;
     @Contended("theSame") private int field2;
     @Contended("theSame") private int field3;
     @Contended("theSame") private int field4;
}

That is, all the fields would be densely-packed, but will be padded as the group. Note that it is not the same as:

public class ValueClass {
     @Contended private int field1;
     @Contended private int field2;
     @Contended private int field3;
     @Contended private int field4;
}

Maybe that is already confusing enough to drop class-level annotation? What do others feel about this?

-Aleksey.

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