Things you didn't know about multithreaded programming

http://www.ibm.com/developerworks/java/library/j-5things15/index.html?ca=drs-

by fogus • 16 years ago

39 33 16 years ago

ibm.com

Jabbles2• 16 years ago

...in Java. Or at least, the JVM.

Does anyone know the limits of the JVM for multithreading? e.g. Max number of cores

tomjen3• 16 years ago

properly 2^32-1.

mzl8• 16 years ago

Any limits depend on the particular JVM that you are using. In practice one needs to experiment.

A few years back I did some comparisons between Java and Erlang with respect to the number of threads the system could handle. Somewhere around a thousand threads was sufficient to make the JVM go very slowly.

Jabbles2• 16 years ago

Surely the number of threads you can handle will depend on your hardware specs, in particular your cache sizes?

Also, were these threads proper OS threads, or more lightweight coroutines?

Thanks

lemming5• 16 years ago

See Kilim (http://www.malhar.net/sriram/kilim) for a lightweight thread solution for Java that easily outpaces Erlang for thread creation and message passing. It's the basis for the Erjang project which runs Erlang bytecode on the JVM. While it's faster at a low level, it obviously doesn't provide everything that Erlang does (supervisor hierarchies etc). It's still pretty impressive that this can be achieved on the JVM though.

lemming5• 16 years ago

This is a pretty crappy article in my opinion, and certainly won't do anything to help the problem that the author describes - that developers' knowledge of concurrency comes piecemeal without any structure.

I challenge anyone to gain an understanding of volatile variables from his description. His example of volatile vs. synchronised contains an example of incorrect code but not an example of how it should be fixed, and doesn't even contain a particularly clear explanation of why it's bad.

The explanation of the bytecode generated for a synchronised method compared to a synchronised block is curious if you're into bytecode but totally useless for a working programmer, and offers no guidance on choosing between them. Worse, it actually makes it sound like the option you almost certainly don't want (the synchronised method) is somehow more efficient than the almost universally better method (synchronising on a lock object, which provides much better lock granularity, prevents deadlock in the case of some external object claiming your object lock and generally makes lock ownership much clearer and easier to track).

Finally, his example of the AtomicReferenceFieldUpdater ignores the javadoc (This class is designed for use in atomic data structures in which several reference fields of the same node are independently subject to atomic updates) and proceeds to show a contrived example of a compareAndSet that is really only a more confusing set, and doesn't actually achieve anything at all.

There are much better sources to learn this stuff if you really need to know it, starting with Java Concurrency In Practice - no Java programmer should be writing concurrent code without having read the first 7 chapters of this.

motxilo2• 16 years ago

Agreed. It's hard to discern the real value of using synchronized methods vs synchronized blocks when you end up reading: "Creating the synchronized block yielded 16 lines of bytecode, whereas synchronizing the method returned just 5". As you point out, synchronization on a lock object is, as a rule of thumb, a much better option than synchronizing on the intrinsic lock, and thus losing control over it.

The explanation of volatile is woeful, to say the least. He doesn't even mention the main issue that volatile addresses (the only one?): visibility. Anyone really interested in this should read the following article by the author of JCIP: http://www.ibm.com/developerworks/java/library/j-jtp06197.ht...

I liked the section regarding ThreadLocal though (an example would've helped a bit).

larsberg• 16 years ago

For people actually interested in parallel programming, particularly on Java, Herlihy's _The Art of Multiprocessor Programming_ is a fantastic introduction. On top of the grab-bag of techniques you'd expect, it boasts enough formal treatment to help folks learn to reason about correctness in their own programs.

barfoomoo2• 16 years ago

Reference field update operations in Java are not thread safe?

jbellis• 16 years ago

reference updates are atomic, but without volatile (or more heavyweight synchronization) there may be an arbitrary delay before the change is visible to other threads.

barfoomoo2• 16 years ago

Thanks for that.

known• 16 years ago

Kernel locking http://www.linuxjournal.com/article/5833

jbarham• 16 years ago

Articles like this make me appreciate Go's approach to concurrency (http://golang.org/doc/effective_go.html#concurrency).

CodeMage• 16 years ago

The only part I found interesting was the first part, but only because it raises an interesting question that the author (unfortunately) didn't address:

Supposing you want to synchronize only one part of your method, which approach would cause the JVM to do less work: putting that part in a synchronized block (thus generating extra bytecode instructions) or extracting it into a synchronized method (thus generating an additional call)?

Not that it's really important -- micro-optimizations like that are silly -- but it does make me curious.

drblast• 16 years ago

The article really fails here, this is a horrible example and horrible reasoning.

I'm no Java expert, but the reason you'd synchronize a smaller block of code, or use any finer grain lock, rather than a whole method is that you'd have a shorter time to wait if many threads are competing for the lock. You'd increase utilization.

Lines of bytecode doesn't really tell you anything, especially since the synchronized method is a stub that does nothing. Not only that, but the synchronized method does all its synchronization work implicitly; the JVM still has to acquire the lock somehow, it's just not written in the bytecode.

What really matter is what happens when the bytecode is compiled to native code, so you could see the synchronized method call overhead. If the article analyzed that, it would be helpful.

ddlatham• 16 years ago

Synchronized methods and blocks generate different bytecode, but according to the spec, have "exactly the same effect".

http://java.sun.com/docs/books/jls/third_edition/html/classe...

http://stackoverflow.com/questions/417285/equivalent-code-fo...

rbranson• 16 years ago

This article should be renamed to "beginners guide to multithreaded programming in Java."

Things you didn't know about multithreaded programming

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