How java volatile works and why.

4. Double checked locking
public class Keeper {
private Data data = null;
public Data getData() {
if(data == null) {
synchronized(this) {
if(data == null) {
data = new Data();
}
}
}
return data;
}
}

5. CPU

7. Non coherent caches

8. Cache coherence

9. Memory Barrier
• No reads/writes can cross the Barrier
• Forces cache to flush (it depends)

10. X86 (or any cache-coherent)
performance
• Volatile-read: very close to a
normal read
• Volatile-write: about 1/3 time of
synchronization write

11. False-sharing problem

12. public class SomePopularObject {
public volatile long usefulVal;
public volatile long t1, t2, t3, t4, t5, t6, t7;
}

13. Dead code elimination

14. public class SomePopularObject {
public volatile long usefulVal;
public volatile long t1, t2, t3, t4, t5, t6, t7 = 1L;
public long preventOptimisation(){
return t1 + t2 + t3 + t4 + t5 + t6 + t7;
}
}

15. Java 8 @Contended
public class SomePopularObject {
@Contended
public volatile long usefulVal;
public volatile long anotherVal;
}

16. Java Object Layout tool

17. – Herb Sutter, chair of the ISO C++ standards committee, Microsoft.
"Everybody who learns concurrency thinks they
understand it, ends up finding mysterious races they
thought weren’t possible, and discovers that they
didn’t actually understand it yet after all."