Volatile variable in Java

Question

So I am reading this book titled Java Concurrency in Practice and I am stuck on this one explanation which I cannot seem to comprehend without an example. This is the quote:

When thread A writes to a volatile variable and subsequently thread B reads that same variable, the values of all variables that were visible to A prior to writing to the volatile variable become visible to B after reading the volatile variable.

Can someone give me a counterexample of why "the values of ALL variables that were visible to A prior to writing to the volatile variable become visible to B AFTER reading the volatile variable"?

I am confused why all other non-volatile variables do not become visible to B before reading the volatile variable?

Answer 1

Declaring a volatile Java variable means:

• The value of this variable will never be cached thread-locally: all reads and writes will go straight to "main memory".
• Access to the variable acts as though it is enclosed in a synchronized block, synchronized on itself.

Just for your reference, When is volatile needed ?

When multiple threads using the same variable, each thread will have its own copy of the local cache for that variable. So, when it's updating the value, it is actually updated in the local cache not in the main variable memory. The other thread which is using the same variable doesn't know anything about the values changed by the another thread. To avoid this problem, if you declare a variable as volatile, then it will not be stored in the local cache. Whenever thread are updating the values, it is updated to the main memory. So, other threads can access the updated value.

From JLS §17.4.7 Well-Formed Executions

We only consider well-formed executions. An execution E = < P, A, po, so, W, V, sw, hb > is well formed if the following conditions are true:

1. Each read sees a write to the same variable in the execution. All reads and writes of volatile variables are volatile actions. For all reads r in A, we have W(r) in A and W(r).v = r.v. The variable r.v is volatile if and only if r is a volatile read, and the variable w.v is volatile if and only if w is a volatile write.

2. Happens-before order is a partial order. Happens-before order is given by the transitive closure of synchronizes-with edges and program order. It must be a valid partial order: reflexive, transitive and antisymmetric.

3. The execution obeys intra-thread consistency. For each thread t, the actions performed by t in A are the same as would be generated by that thread in program-order in isolation, with each write wwriting the value V(w), given that each read r sees the value V(W(r)). Values seen by each read are determined by the memory model. The program order given must reflect the program order in which the actions would be performed according to the intra-thread semantics of P.

4. The execution is happens-before consistent (§17.4.6).

5. The execution obeys synchronization-order consistency. For all volatile reads r in A, it is not the case that either so(r, W(r)) or that there exists a write win A such that w.v = r.v and so(W(r), w) and so(w, r).

Useful Link : What do we really know about non-blocking concurrency in Java?

Answer 2

Thread B may have a CPU-local cache of those variables. A read of a volatile variable ensures that any intermediate cache flush from a previous write to the volatile is observed.

For an example, read the following link, which concludes with "Fixing Double-Checked Locking using Volatile":

http://www.cs.umd.edu/~pugh/java/memoryModel/DoubleCheckedLocking.html

Answer 3

If a variable is non-volatile, then the compiler and the CPU, may re-order instructions freely as they see fit, in order to optimize for performance.

If the variable is now declared volatile, then the compiler no longer attempts to optimize accesses (reads and writes) to that variable. It may however continue to optimize access for other variables.

At runtime, when a volatile variable is accessed, the JVM generates appropriate memory barrier instructions to the CPU. The memory barrier serves the same purpose - the CPU is also prevent from re-ordering instructions.

When a volatile variable is written to (by thread A), all writes to any other variable are completed (or will atleast appear to be) and made visible to A before the write to the volatile variable; this is often due to a memory-write barrier instruction. Likewise, any reads on other variables, will be completed (or will appear to be) before the read (by thread B); this is often due to a memory-read barrier instruction. This ordering of instructions that is enforced by the barrier(s), will mean that all writes visible to A, will be visible B. This however, does not mean that any re-ordering of instructions has not happened (the compiler may have performed re-ordering for other instructions); it simply means that if any writes visible to A have occurred, it would be visible to B. In simpler terms, it means that strict-program order is not maintained.

I will point to this writeup on Memory Barriers and JVM Concurrency, if you want to understand how the JVM issues memory barrier instructions, in finer detail.

Related questions

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Answer 4

Threads are allowed to cache variable values that other threads may have since updated since they read them. The volatile keyword forces all threads to not cache values.

Answer 5

This is simply an additional bonus the memory model gives you, if you work with volatile variables.

Normally (i.e. in the absence of volatile variables and synchronization), the VM can make variables from one thread visible to other threads in any order it wants, or not at all. E.g. the reading thread could read some mixture of earlier versions of another threads variable assignments. This is caused by the threads being maybe run on different CPUs with their own caches, which are only sometimes copied to the "main memory", and additionally by code reordering for optimization purposes.

If you used a volatile variable, as soon as thread B read some value X from it, the VM makes sure that anything which thread A has written before it wrote X is also visible to B. (And also everything which A got guaranteed as visible, transitively).

Similar guarantees are given for synchronized blocks and other types of locks.