Saturday, October 30, 2010

More volatility

Wondering why one of our legacy apps seems to be having thread issue, I continued digging into the recesses of the Java Memory Model.

Volatile is a fascinating keyword since many excellent Java programmers don't appreciate its finer points. An engineer I much admire told me that he only used it for boolean primitives when telling threads to stop running. Fair enough, but there is more to it than that. Here are some:

1. What if we're not using volatile primitives? What if we're using, say, an array? Can I be sure that setting an element of the array is volatile too and will be seen by other threads? Apparently not, according to Google's Jeremy Manson in this tutorial (about 40' into the video) where he says: "there is no way to make the elements of an array volatile... the reference to the array is volatile not the elements of the array itself".

2. Furthermore, the performance of volatile is generally pretty good. "On an x86 there is no cost in reading a volatile variable. There is a cost to writing to a volatile variable but not reading... as of 2007".

3. On some hardware, 64-bit read/writes are not atomic but "writes and reads of volatile long and double values are always atomic" (Java Language Specification 17.7).

4. The order of the lines of your code that a JVM executes can also be changed by volatile (you did know the order of execution may differ to how you wrote your code, right?). Brian Goetz explains:

"There is no guarantee that operations in one thread will be performed in the order given by the program, as long as the reordering is not detectable within that thread - even if the reordering is apparent to other threads"

(Java Concurrency in Practise, p34).

However, when we introduce volatile :

"Under the old memory model, accesses to volatile variables could not be reordered with each other, but they could be reordered with nonvolatile variable accesses. [...]

Under the new memory model, it is still true that volatile variables cannot be reordered with each other. The difference is that it is now no longer so easy to reorder normal field accesses around them... In effect, because the new memory model places stricter constraints on reordering of volatile field accesses with other field accesses, volatile or not, anything that was visible to thread A when it writes to volatile field f becomes visible to thread B when it reads f."

(JSR-133 FAQ)

Incidentally, this does mean that if you write code that is fine in JDK 1.5+, it might not be fine running on older machines. If you really want to write-once-run-anywhere Java, you should assume you're writing for the old JMM and make all variables volatile for whom the order of execution of read/write operations is important.

So, did all of this help me to fix the buggy legacy app? No. Every time I have cast a suspicious eye at the finer points of the JMM, it has actually been a much more mundane bug that was the culprit. In this case, it turns out that somebody, somewhere was introspectively calling one of our methods that starts threads - a call that is very hard to find with most IDEs.


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