Multi-thread state visibility in Java: is there a way to turn the JVM into the worst case scenario?

Question

Suppose our code has 2 threads (A and B) have a reference to the same instance of this class somewhere:

public class MyValueHolder {

    private int value = 1;
         


        

Answer 1

I think you are refering to the principle called "false sharing" where different CPUs must synchronize their caches or else face the possibility that data such as you describe could become mismatched. There is a very good article on false sharing on Intel's website. Intel describes some useful tools in their article for diagnosing this problem. This is a relevant quote:

The primary means of avoiding false sharing is through code inspection. Instances where threads access global or dynamically allocated shared data structures are potential sources of false sharing. Note that false sharing can be obscured by the fact that threads may be accessing completely different global variables that happen to be relatively close together in memory. Thread-local storage or local variables can be ruled out as sources of false sharing.

Although methods described in the article are not what you have asked for (forcing worst-case behavior from the JVM), as already stated this isn't really possible. The methods described in this article are the best way I know to try to diagnose and avoid false sharing.

There are other resources addressing this problem around the web. For example, this article has a suggestion for a way to avoid false sharing in Java. I have not tried this method, so I cannot vouch for it, but I think the author's idea is sound. You might consider trying out his suggestion.