@Contended

@Contended Overview

The Java SE 8 platform introduced [0] a new annotation called @Contended [1] to reduce "False Sharing" [2],[3],[4]. False sharing is a term to describe performance degradation when several threads are modifying independent variable(s) sharing the same cache line.

@Contended is applied as a conventional Java language annotation to fields in a class. It specifies that some field(s) in an object are probably highly contended across processor cores.

At runtime the VM optimizes field layout. At the VM’s discretion, those fields annotated as contended are given extra padding so they do not share cache lines with other fields that are likely to be independently accessed. This helps to avoid cache contention across multiple threads.

Zing provides an implementation of this feature for JDK 7.

Using @Contended Annotation

To use the @Contended annotation:

Marking a Class as Contended

To mark a class as contended, precede the class with @Contended. In this example, both field1 and field2 are padded from the both sides.

 
@Contended
public class ContendedClass {
    private int field1;
    private Object field2;
}

Marking an Instance Field as Contended

To mark an instance field as contended, precede the field with @Contended. In this example, the padding is applied to field1 only.

 
public class ContendedField {
    @Contended
    private int field1;
    private Object field2;
}

Marking Multiple Fields as Contended

To mark multiple fields as contended, precede each field with @Contended. In this example, both fields have separate padding.

 
public class ContendedFields {
    @Contended
    private int field1;
    @Contended
    private Object field2;
}

Marking Groups as Contended

To mark a group as contended, include the optional @Contended parameter in the @Contended annotation to describe the equivalence class for contention group. In this example, The contendedField1 and contendedField2 fields are padded from everything else, but are still densely packed with each other.

 
public class ContendedGroups {
    @Contended("updater1")
    private Object contendedField1;

    @Contended("updater1")
    private Object contendedField2;

    @Contended("updater2")
    private Object contendedField3;

    private Object plainField5;
    private Object plainField6;
}

Zing @Contended Options

After applying @Contended annotation to your Java application, include the Zing launch and configuration options in your Java command. Zing @Contended options and flags are:

@Contended References

The following links provide additional information about the general use of @Contended.