Is there any advantage of using volatile keyword in contrast to use the Interlocked class?

Question

In other words, can I do something with a volatile variable that could not also be solved with a normal variable and the Interlocked class?

Decretal

I know what volatile is good for. The question is if there is any situation where you can't replace volatile with an interlocked method? – Decretal

Answer 1

EDIT: question largely rewritten

To answer this question, I dived a bit further in the matter and found out a few things about volatile and Interlocked that I wasn't aware of. Let's clear that out, not only for me, but for this discussion and other people reading up on this:

• volatile read/write are supposed to be immune to reordering. This only means reading and writing, it does not mean any other action;
• volatility is not forced on the CPU, i.e., hardware level (x86 uses acquire and release fences on any read/write). It does prevent compiler or CLR optimizations;
• Interlocked uses atomic assembly instructions for CompareExchange (cmpxchg), Increment (inc) etc;
• Interlocked does use a lock sometimes: a hardware lock on multi processor systems; in uni-processor systems, there is no hardware lock;
• Interlocked is different from volatile in that it uses a full fence, where volatile uses a half fence.
• A read following a write can be reordered when you use volatile. It can't happen with Interlocked. VolatileRead and VolatileWrite have the same reordering issue as `volatile (link thanks to Brian Gideon).

Now that we have the rules, we can define an answer to your question:

• Technically: yes, there are things you can do with volatile that you cannot do with Interlocked:
  1. Syntax: you cannot write a = b where a or b is volatile, but this is obvious;
  2. You can read a different value after you write it to a volatile variable because of reordering. You cannot do this with Interlocked. In other words: you can be less safe with volatile then you can be with Interlocked.
  3. Performance: volatile is faster then Interlocked.

• Semantically: no, because Interlocked simply provides a superset of operations and is safer to use because it applies full fencing. You can't do anything with volatile that you cannot do with Interlocked and you can do a lot with Interlocked that you cannot do with volatile:

  static volatile int x = 0;
  x++;                        // non-atomic
  static int y = 0;
  Interlocked.Increment(y);   // atomic
  

• Scope: yes, declaring a variable volatile makes it volatile for every single access. It is impossible to force this behavior any other way, hence volatile cannot be replaced with Interlocked. This is needed in scenarios where other libraries, interfaces or hardware can access your variable and update it anytime, or need the most recent version.

If you'd ask me, this last bit is the actual real need for volatile and may make it ideal where two processes share memory and need to read or write without locking. Declaring a variable as volatile is much safer in this context then forcing all programmers to use Interlocked (which you cannot force by the compiler).

---

EDIT: The following quote was part of my original answer, I'll leave it in ;-)

A quote from the the C# Programming Language standard:

For nonvolatile fields,optimization techniques that consider that reorder instructions can lead to unexpected and unpredictable results in multithreaded programs that access fields without synchronization such as that provided by the lock-statement. These optimizationscan be performed by the compiler, by the runtime system, or by hardware. For volatile fields, such reordering optimizations are restricted:

• A read of a volatile field is called a volatile read. A volatile read has :acquire semantics"; that is, it is guaranteed to occur prior to any references to memory that occur after it in the instruction sequence.

• A write of a volatile field is called a volatile write. A volatile write has "release semantics"; that is, it is guaranteed to happen after any memory references prior to the write instruction in the instruction sequence.

Update: question largely rewritten, corrected my original response and added a "real" answer

Crystallize

The Interlocked class doesn't have to do much with a lock and it has (in contrast to lock) not much overhead assigned with it - on most CPUs the Interlocked methods are implemented as single instruction. – Decretal

You are correct, thanks for pointing at it. Looking into CompareExchange (thinking it wouldn't be atomic) SSCLI (shared source CLI) shows that it ultimately translates to the cmpxchg assembly instruction on x86 processors, and that instruction is — as anything else in Interlocked — indeed atomic. – Crystallize

That, and other corrections, plus rewrite are now in the answer :) – Crystallize

You siad, you cannot enforce threadsafe access to a variable without the volatile keyword. But you can define a property which uses Interlocked.Read() and Interlocked.Exchange() in its getter/setter. – Decretal

That's disguising a variable. Of course you can do that. But it doesn't change the fact that you can't have a variable or a field that way. Hidden behind a property makes the field less prone to error, but inside the class and from outside (breaking OO through reflection), it's just accessible, with thread-unsafety and all. OTOH, it isn't possible to give the address of a "volatiled" property to some external function (API, hardware, otherwise), you need to hand in the address of the field and no way Interlocked can help then. – Crystallize

Passing a reference of a volatile variables is loosing it's volatility, the compiler will warn you if you try it. However, your answer is very good and exhausting :) – Decretal

Agreed, but isn't that a warning about the reference, which means that reading/writing the variable still has the same semantics? Otherwise, how would the CLR allow using volatile for variables that can be written to by other libs or even hardware? – Crystallize

Answer 2

This is a fairly complex topic. I find Joseph Albahari's writeup to be one of the more definitive and accurate sources for multithreading concepts in the .NET Framework that might help answer your question.

But, to quickly summarizes there is a lot of overlap between the volatile keyword and the Interlocked class as far as how they can be used. And of course both go way above and beyond what you can do with a normal variable.

Jamshid

Very great article! It will take a while to read it and a much longer while to fully understand it – Decretal

Answer 3

Yes - you can look at the value directly.

As long as you ONLY use the Interlocked class to access the variable then there is no difference. What volatile does is it tells the compiler that the variable is special and when optimizing it shouldn't assume that the value hasn't changed.

Takes this loop:

bool done = false;

...

while(!done)
{
... do stuff which the compiler can prove doesn't touch done...
}

If you set done to true in another thread you would expect the loop to exit. However - if done is not marked as volatile then the compiler has the option to realize that the loop code can never change done and it can optimize out the compare for exit.

This is one of the difficult things about multithread programming - many of the situations which are problems only come up in certain situations.

Tove

You can do the same with Interlocked.Read(). – Decretal

Your description of volatile is slightly wrong. You describe c++ volatile (no good for multithreading). C# (and Java) volatile make the compiler insert sync operations when accessing the variable. Because of the syncs, it is forced to reread the variable everytime, which has an effect similar to c++'s volatile. – Frydman

Answer 4

I won't attempt to be an authority on this subject but I would highly recommend that you take a look at this article by the vaunted Jon Skeet.

Also take a look at the final part of this answer which details what volatile should be used for.

Drawtube

Skeet's article is good but just coveres the basics. I know what volatile is good for, but the question was if there is any situation where you can't replace volatile with an interlocked method? – Decretal

Answer 5

Yes, you can gain some performance by using a volatile variable instead of a lock.

Lock is a full memory barrier which can give you the same characteristics of a volatile variable as well as many others. As has already been said volatile just ensures that in multi-threaded scenarios if a CPU changes a value in its cache line, the other CPUs sees the value immediately but do not ensure any locking semantic at all.

The thing is lock is a lot more powerful than volatile and you should use volatile when you can to avoid unnecessary locks.

Kolb

I wasn't talking about the lock statement, but the Interlocked class, but yes what you said is true – Decretal