This is very technical, most likely way past what 99% of developers both need to care about and have to care about.
Given that most developers work at best with embarrassingly parallel problems they wouldn't need to know much of these details.
But if you're up for it: It's a very rewarding feeling when you're both allowed to figure these things out and someone are also paying you while you figure it out.
Was the English version of this talk recorded? I found the talk in Russian[1], but with my understanding of the Russian language, it's impossible to watch.
In the section about nasal demons, did anyone else think the right answer was 12 too? I figured the parens make the context local so the ++ had to occur after the add.
I know that was the point, but I thought the spec on ++ is that it occurs after the statement. I thought the spec on parens is that they are sub statement. Since there are two ++ occurring, it is odd, but I thought by spec the 12 is right.
The spec says "The value of the operand of the prefix ++ operator is incremented. The result is the new value of the operand after incrementation. The expression ++E is equivalent to (E+=1)."
I think the problem in the example from the article is that the spec doesn't say what happens to i when it appears twice in the same expression. What is the "current" value of i each increment sees?
Maybe they both see the initial value 5, and the result is (5+1) + (5+1) = 12 like you say. But compiling the example with gcc 4.6.3 gives me 14 as a result! It seems as if gcc is seeing the two updates and then replacing each occurrence of i with the final value 7 (but I'm guessing). I guess the point is that, since it isn't in the spec, each implementation of C is free to do whatever the implementors decided.
The memory model seems to be the real achilles heel for concurrency in Java. In the end with a only little bit of work one can master the concurrency primitives and they are quite usable. However understanding the full ramifications of the memory model is pretty much beyond mere mortals. As a result it is hard to do anything else than stick to established patterns and / or toolkits and frameworks if you don't want to set yourself up for insidious issues that are nearly impossible to reproduce.
Au contraire, the fact that Java actually has a memory model means one can reason about concurrency in a way impossible on other platforms.
As you note you rarely need to concern yourself with the details of the memory model as most of us are users, not designers, of concurrency primitives. But you should you need to design a new primitive you can actually have some guarantees it will work in a cross-platform manner, which is more than you an say about any other platform I know of.
Having it defined is very important. However part of the problem is that definition actually takes away guarantees that many CPUs (eg. x86) give you. For example double checked locking works in x86 but famously not in Java. So the fact is that Java is giving you in some ways a less sequential model than what you started with is what makes it particularly insidious.
You are free to rely on the guarantees made by a particular hardware architecture. The JVM implementation doesn't "take them away" -- the spec merely doesn't guarantee them. But if you do, then you might lose portability.
My impression is that by not guaranteeing them it does take them away, because the implementation is free to implement them in any way they choose including in the compiler. So it will not matter if your code is executing on a processor that does not reorder instructions, Javac may already have done it to the byte code.
Even a C compiler is allowed to reorder instructions. How is any compiler supposed to know if you prefer performance (and therefore reordering) or to to rely on a processor-specific feature? That's why any language that runs on more than one architecture needs to define a language-level memory model.
The memory model is actually relatively straightforward on the user side; it's significantly more complicated on the implementer side.
In most cases you can get away with coarser constructs (which are higher-performing anyway, eg task parallelism vs. trying to decompose an individual operation) and only dive into the language lawyering semantics when you, eg, really really need a lockfree concurrent linked list.
> The memory model seems to be the real achilles heel for concurrency in Java.
I'd expand it beyond Java to shared-everything models in general. Any language with a shared-everything model is going to incur all this complexity. At least Java goes through this pain to ensure memory safety in the presence of unsynchronized accesses from multiple threads.
Given that most developers work at best with embarrassingly parallel problems they wouldn't need to know much of these details.
But if you're up for it: It's a very rewarding feeling when you're both allowed to figure these things out and someone are also paying you while you figure it out.