Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above. For example, when we have a large number of reading operations and a small number of write operations, we want to use the volatile keyword for better performance. What does 'immediately visible' mean? That doesn't sound a correct explain. Trending is based off of the highest score sort and falls back to it if no posts are trending. More specifically, if two actions share a happens-before relationship, they do not necessarily have to appear to have happened in that order to any code with which they do not share a happens-before relationship. In this article, I would like to share a different approach to this subject that maybe helpful for you, and also to talk about some particularities and misconceptions about volatile. In a multi-threaded context, we usually must resort to explicit synchronization. No at all: volatile, synchronized, etc. Now the other thread is not aware of the modified value which leads to data inconsistency. And then you would either have to run a byte code interpreter without a JIT or the JIT issuing memory reads without reordering the code too much. @pveentjer You've never heard of a JVM using the physical CPU's registers as a cache to avoid memory accesses? That sounds a very vague definition to me. A lot of nonsense is written and 'flushing volatile writes to main memory instead of using the cache' is the most common misunderstanding I'm seeing. To illustrate the risks of an improperly synchronized program, lets consider the following example borrowed from JCIP book. MESI) anyway, then why do we need. Or my biggest question is: If compiler does not do the magic you mentioned, and CPU cache coherence can guarantee single variable's visibility between threads, why do we need volatile for that? While the volatile keyword is simple and easy to use, it performs better than the locking mechanism. I forgot to mention about compiler's magic. A correctly synchronized program is one with no data races (from here). that is not correct and the articles are wrong. Some people say that is incorrect for CPUs that implement a cache coherency protocol. When we have a simple flag (like a completion, interruption, or status flag) or other primitive item of data that is accessed (read) by multiple threads. The opposite. That may allow your machine to use the caches and still have the memory visibility required by the Java specification. Its yet another reason for using the volatile keyword. So nobody can observe some in between state. In some situations, it is seen that the volatile performs much better than the synchronized. To better understand the situation where we can use the volatile keyword safely and effectively, we should watch out for the following conditions. Opinions expressed by DZone contributors are their own. Is there a difference between truing a bike wheel and balancing it? The purpose of volatile is to ensure that when one thread writes a value to a field, the value written is "immediately available" to any thread that subsequently reads it. In this particularcase, making ready volatile is enough to make that class thread-safe because after main() updates ready, ReaderThread will see the new value for both ready and number (happens-before relationship). However, the volatile reads are generally observed to be very good at performance and comparable to the normal reads. Doing so would mean eliminating a significant number of extremely valuable optimizations. On the other hand, volatile writes are always cheaper than acquiring locks. generate link and share the link here. For example, a simple increment statement such as x = x + 1; or x++ seems to be a single operation but is really a compound read-modify-write sequence of operations that must execute atomically. If one action happens before another, then the first is visibleto and orderedbefore the second (for example, the write of a default value to every field of an object constructed by a thread need not happen before the beginning of that thread, as long as no read ever observes that fact). rev2022.7.21.42639. So lets assume that your diagram shows physical time and represents threads running on different physical cores, reading and writing a cache-coherent memory cell via their respective caches. The synchronized keyword is used to achieve the locking mechanism in Java, and the lock provides visibility and atomicity. https://download.oracle.com/otndocs/jcp/memory_model-1.0-pfd-spec-oth-JSpec/, change to ready on publisher thread is immediately visible to other threads. Therefore, the use of volatile keywords is very restricted. You can find a lot more information about the examples and their internal working there. In this case, not making INSTANCE volatile could lead getInstance() to return a non-fully initialized object, breaking the Singleton pattern. But I haven't seen it being mentioned in the JMM. There is only one copy of them in the main memory. Sorry I did not state clearly. Because the subscriber may not ever see the results of the publisher's write. This relationship is established by either: To understand the importance of this relationship, let's review the concept of data race. Is Java "pass-by-reference" or "pass-by-value"? And all sorts of other stuff that is hard to explain. Mutable objects may not be fully constructed when their references are made visible. Somethread-safe alternatives: synchronized,AtomicInteger.
Making statements based on opinion; back them up with references or personal experience. Difference between volatile and transient keywords in Java, Difference Between Atomic, Volatile and Synchronized in Java, Comparison of static keyword in C++ and Java, Using the super Keyword to Call a Base Class Constructor in Java, super keyword for Method Overloading in Java, Access Super Class Methods and Instance Variables Without super Keyword in Java, Difference Between java.sql.Time, java.sql.Timestamp and java.sql.Date in Java. That's not to say this would always happen. Whether the extra effort is worthwhile depends on the size and complexity of the application. The above example would work perfectly fine if ready would make use of a store release and an acquire load; would not work any different compared to SC. You can now choose to sort by Trending, which boosts votes that have happened recently, helping to surface more up-to-date answers. SC (and coherence since coherence is a suborder of SC), do not provide real-time guarantees. @pveentjer When you write Java code, you are writing it for a. I mostly agree with you, but unfortunately not everybody does. 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 (from JCIP book). On the other hand, if the updated value for ready is visible to ReaderThreadbefore number is (code reordering/caching), ReaderThread would see that number is 0 (default value). But it also depends on how long it takes for the writer thread's cache invalidation to propagate to the others' caches. An atomic action in concurrent programming is one that either happens completely, or it doesn't happen at all. A volatile read is guaranteed to see the most recent write before it in the memory order, but isn't guaranteed to see the most recent write before it. This article will also provide a basic example code to help you understand the volatile keyword. Can a read in another thread that happens very shortly after the write starts but before the write finishes see the change? -Can we use volatile without using synchronized? How do I efficiently iterate over each entry in a Java Map? You might have come across the parallel threads and the synchronized keyword while working with Java.
Although the volatile keyword is not as powerful as the synchronized locking mechanism, it is still used in practice. Let's simplify the above example: If ready is not defined volatile, is it possible that subscriber get stuck infinitely in the while loop? Using volatile on those types ensures the read and write operations are atomic (accessing the rest of the primitivevariables and referencevariables is atomic by default). @hangyuan The meaning of "doesn't do any optimization" may be a bit complex in java. volatile keyword: https://docs.oracle.com/javase/specs/jls/se16/html/jls-8.html#jls-22.214.171.124, memory model: https://docs.oracle.com/javase/specs/jls/se16/html/jls-17.html#jls-17.4. For more info see this answer. This applies for volatile variables, and for various other things. It is helpful to understand what these people are saying and the distinction between the physical CPU in your computer and the virtual CPU you are targeting with your code.