Reading an int that's updated by Interlocked on other threads

Question

(This is a repeat of: How to correctly read an Interlocked.Increment'ed int field? but, after reading the answers and comments, I'm still not sure of the right answer.)

There's some code that I don't own and can't change to use locks that increments an int counter (numberOfUpdates) in several different threads. All calls use:

Interlocked.Increment(ref numberOfUpdates);

I want to read numberOfUpdates in my code. Now since this is an int, I know that it can't tear. But what's the best way to ensure that I get the latest value possible? It seems like my options are:

int localNumberOfUpdates = Interlocked.CompareExchange(ref numberOfUpdates, 0, 0);

Or

int localNumberOfUpdates = Thread.VolatileRead(numberOfUpdates);

Will both work (in the sense of delivering the latest value possible regardless of optimizations, re-orderings, caching, etc.)? Is one preferred over the other? Is there a third option that's better?

Answer 1

It seems like my options are:

int localNumberOfUpdates = Interlocked.CompareExchange(ref numberOfUpdates, 0, 0);

Or

int localNumberOfUpdates = Thread.VolatileRead(numberOfUpdates);

Starting from the .NET Framework 4.5, there is a third option:

int localNumberOfUpdates = Volatile.Read(ref numberOfUpdates);

The Interlocked methods impose full fences, while the Volatile methods impose half fences¹. So using the static methods of the Volatile class is a potentially more economic way of reading atomically the latest value of an int variable or field.

Alternatively, if the numberOfUpdates is a field of a class, you could declare it as volatile. Reading a volatile field is equivalent with reading it with the Volatile.Read method.

I should mention one more option, which is to simply read the numberOfUpdates directly, without the help of either the Interlocked or the Volatile. We are not supposed to do this, but demonstrating an actual problem caused by doing so might be impossible. The reason is that the memory models of the most commonly used CPUs are stronger than the C# memory model. So if your machine has such a CPU (for example x86 or x64), you won't be able to write a program that fails as a result of reading directly the field. Nevertheless personally I never use this option, because I am not an expert in CPU architectures and memory protocols, nor I have the desire to become one. So I prefer to use either the Volatile class or the volatile keyword, whatever is more convenient in each case.

¹ _{With some exceptions, like reading/writing an Int64 or double on a 32-bit machine.}

Answer 2

Not sure why nobody mentioned Interlocked.Add(ref localNumberOfUpdates, 0), but seems the simplest way to me...

Answer 3

I'm a firm believer in that if you're using interlocked to increment shared data, then you should use interlocked everywhere you access that shared data. Likewise, if you use insert you favorite synchronization primitive here to increment shared data, then you should use insert you favorite synchronization primitive here everywhere you access that shared data.

int localNumberOfUpdates = Interlocked.CompareExchange(ref numberOfUpdates, 0, 0);

Will give you exactly what your looking for. As others have said interlocked operations are atomic. So Interlocked.CompareExchange will always return the most recent value. I use this all the time for accessing simple shared data like counters.

I'm not as familiar with Thread.VolatileRead, but I suspect it will also return the most recent value. I'd stick with interlocked methods, if only for the sake of being consistent.

---

Additional info:

I'd recommend taking a look at Jon Skeet's answer for why you may want to shy away from Thread.VolatileRead(): Thread.VolatileRead Implementation

Eric Lippert discusses volatility and the guarantees made by the C# memory model in his blog at http://blogs.msdn.com/b/ericlippert/archive/2011/06/16/atomicity-volatility-and-immutability-are-different-part-three.aspx. Straight from the horses mouth: "I don't attempt to write any low-lock code except for the most trivial usages of Interlocked operations. I leave the usage of "volatile" to real experts."

And I agree with Hans's point that the value will always be stale at least by a few ns, but if you have a use case where that is unacceptable, its probably not well suited for a garbage collected language like C# or a non-real-time OS. Joe Duffy has a good article on the timeliness of interlocked methods here: http://joeduffyblog.com/2008/06/13/volatile-reads-and-writes-and-timeliness/

Answer 4

Well, any value you read will always be somewhat stale as Hans Passant said. You can only control a guarantee that other shared values are consistent with the one you've just read using memory fences in the middle of code reading several shared values without locks (ie: are at the same degree of "staleness")

Fences also have the effect of defeating some compiler optimizations and reordering thus preventing unexpected behavior in release mode on different platforms.

Thread.VolatileRead will cause a full memory fence to be emitted so that no reads or writes can be reordered around your read of the int (in the method that's reading it). Obviously if you're only reading a single shared value (and you're not reading something else shared and the order and consistency of them both is important), then it may not seem necessary...

But I think that you will need it anyway to defeat some optimizations by the compiler or CPU so that you don't get the read more "stale" than necessary.

A dummy Interlocked.CompareExchange will do the same thing as Thread.VolatileRead (full fence and optimization defeating behavior).

There is a pattern followed in the framework used by CancellationTokenSource http://referencesource.microsoft.com/#mscorlib/system/threading/CancellationTokenSource.cs#64

//m_state uses the pattern "volatile int32 reads, with cmpxch writes" which is safe for updates and cannot suffer torn reads. 
private volatile int m_state;

public bool IsCancellationRequested
{
    get { return m_state >= NOTIFYING; } 
}

// ....
if (Interlocked.CompareExchange(ref m_state, NOTIFYING, NOT_CANCELED) == NOT_CANCELED) {
}
// ....

The volatile keyword has the effect of emitting a "half" fence. (ie: it blocks reads/writes from being moved before the read to it, and blocks reads/writes from being moved after the write to it).

Answer 5

Will both work (in the sense of delivering the latest value possible regardless of optimizations, re-orderings, caching, etc.)?

No, the value you get is always stale. How stale the value might be is entirely unpredictable. The vast majority of the time it will be stale by a few nanoseconds, give or take, depending how quickly you act on the value. But there is no reasonable upper-bound:

• your thread can lose the processor when it context-switches another thread onto the core. Typical delays are around 45 msec with no firm upper-bound. This does not mean that another thread in your process also gets switched-out, it can keep motoring and continue to mutate the value.
• just like any user-mode code, your code is subjected to page-faults as well. Incurred when the processor needs RAM for another process. On a heavily loaded machine that can and will page-out active code. As sometimes happens to the mouse driver code for example, leaving a frozen mouse cursor.
• managed threads are subject to near-random garbage collection pauses. Tends to be the lesser problem since it is likely that another thread that's mutating the value will be paused as well.

Whatever you do with the value needs to take this into account. Needless to say perhaps, that's very, very difficult. Practical examples are hard to come by. The .NET Framework is a very large chunk of battle-scarred code. You can see the cross-reference to usage of VolatileRead from the Reference Source. Number of hits: 0.

Answer 6

Thread.VolatileRead(numberOfUpdates) is what you want. numberOfUpdates is an Int32, so you already have atomicity by default, and Thread.VolatileRead will ensure volatility is dealt with.

If numberOfUpdates is defined as volatile int numberOfUpdates; you don't have to do this, as all reads of it will already be volatile reads.

---

There seems to be confusion about whether Interlocked.CompareExchange is more appropriate. Consider the following two excerpts from the documentation.

From the Thread.VolatileRead documentation:

Reads the value of a field. The value is the latest written by any processor in a computer, regardless of the number of processors or the state of processor cache.

From the Interlocked.CompareExchange documentation:

Compares two 32-bit signed integers for equality and, if they are equal, replaces one of the values.

In terms of the stated behavior of these methods, Thread.VolatileRead is clearly more appropriate. You do not want to compare numberOfUpdates to another value, and you do not want to replace its value. You want to read its value.

---

Lasse makes a good point in his comment: you might be better off using simple locking. When the other code wants to update numberOfUpdates it does something like the following.

lock (state)
{
    state.numberOfUpdates++;
}

When you want to read it, you do something like the following.

int value;
lock (state)
{
    value = state.numberOfUpdates;
}

This will ensure your requirements of atomicity and volatility without delving into more-obscure, relatively low-level multithreading primitives.