@Contended and inline classes

[Frederic Parain]

Here’s the basic question: how the JVM should handle the @Contended
annotation with respect to inline classes?

Here’s some thoughts:

First scenario: the @Contended annotation is used within an inline class:

The @Contended annotation aims to solve the problem of false sharing,
but false sharing requires that the data in the cache are altered. Because
inline classes are immutable, no write operation can be performed on their fields,
so no false sharing is possible between fields of a single inline class.
So there’s no point to change the layout of the inline class to put fields on
different cache lines.

In addition to that, each use the @Contended adds 256 bytes to the size
of the instance (at least on the current HotSpot implementation), which is
well above the threshold we have considered reasonable so far to flatten
an inline classes instance.

So the proposal would be to simply ignore @Contended when used
in an inline class. The javadoc of @Contended describes the annotation
as a hint to the JVM, so there’s no strict  requirement to alter the layout
because of the annotation.

Second scenario: A regular class has a field which is an inline class, and
                             this field is annotated with @Contended

This use case still makes sense, to isolate this field (which could be
flattened or not) from other fields. This use of @Contended impacts
the layout of the container (the regular class) but not the layout of
the inline class.

In this case, the JVM should use the hint to compute a layout isolating
the inline class field.

Does this analysis make sense to you? Feedback and comments
are welcome.

Regards,

Fred