guarantee that up-to-date value of variable is always visible to several threads on multi-processor system

Question

I'm using such configuration:

• .NET framework 4.5
• Windows Server 2008 R2
• HP DL360p Gen8 (2 * Xeon E5-2640, x64)

I have such field somewhere in my program:

protected int HedgeVolume;

I access this field from several threads. I assume that as I have multi-processor system it's possible that this threads are executing on different processors.

What should I do to guarantee that any time I use this field the most recent value is "read"? And to make sure that when I "write" value it become available to all other threads immediately?

What should I do?

• just leave field as is.
• declare it volatile
• use Interlocked class to access the field
• use .NET 4.5 Volatile.Read, Volatile.Write methods to access the field
• use lock

I only need simplest way to make my program work on this configuration I don't need my program to work on another computers or servers or operation systems. Also I want minimal latency so I'm looking for fastest solution that will always work on this standard configuration (multiprocessor intel x64, .net 4.5).

Answer 1

Your question is missing one key element... How important is the integrity of the data in that field?

volatile gives you performance, but if a thread is currently writing changes to the field, you won't get that data until it's done, so you might access out of date information, and potentially overwrite changes another thread is currently doing. If the data is sensitive, you might get bugs that would get very hard to track. However, if you are doing very quick update, overwrite the value without reading it and don't care that once in a while you get outdated (by a few ms) data, go for it.

lock guaranty that only one thread can access the field at a time. You can put it only on the methods that write the field and leave the reading method alone. The down side is, it is slow, and may block a thread while another is performing its task. However, you are sure your data stay valid.

Interlock exist to shield yourself from the scheduler context switch. My opinion? Don't use it unless you know exactly why you would be using it and exactly how to use it. It gives options, but with great options comes great problematic. It prevents a context switch while a variable is being update. It might not do what you think it does and won't prevent parallel threads from performing their tasks simultaneously.