What is non-thread-safety for?_问答_开发者_运维开发者技术经验分享

There are a lot of articles and discussions explaining why it is good to build thread-safe classes. It is said that if multiple threads access e.g. a field at the same time, there can only be some bad consequences. So, what is the point of keeping non thread-safe code? I'm focusing mostly on .NET, but I believe the main reasons are not language-dependent.

E.g. .NET static fields are not thread-safe. What would be the result if they were thread-safe by default? (without a need to perform "manual" locking). What are the benefits of using (actually defaulting to) non-thread-safety?

O开发者_开发百科ne thing that comes to my mind is performance (more of a guess, though). It's rather intuitive that, when a function or field doesn't need to be thread-safe, it shouldn't be. However, the question is: what for? Is thread-safety just an additional amount of code you always need to implement? In what scenarios can I be 100% sure that e.g. a field won't be used by two threads at once?

Writing thread-safe code:

Requires more skilled developers
Is harder and consumes more coding efforts
Is harder to test and debug
Usually has bigger performance cost

But! Thread-safe code is not always needed. If you can be sure that some piece of code will be accessed by only one thread the list above becomes huge and unnecessary overhead. It is like renting a van when going to neighbor city when there are two of you and not much luggage.

Thread safety comes with costs - you need to lock fields that might cause problems if accessed simultaneously.

In applications that have no use of threads, but need high performance when every cpu cycle counts, there is no reason to have safe-thread classes.

So, what is the point of keeping non thread-safe code?

Cost. Like you assumed, there usually is a penalty in performance.

Also, writing thread-safe code is more difficult and time consuming.

Thread safety is not a "yes" or "no" proposition. The meaning of "thread safety" depends upon context; does it mean "concurrent-read safe, concurrent write unsafe"? Does it mean that the application just might return stale data instead of crashing? There are many things that it can mean.

The main reason not to make a class "thread safe" is the cost. If the type won't be accessed by multiple threads, there's no advantage to putting in the work and increase the maintenance cost.

Writing threadsafe code is painfully difficult at times. For example, simple lazy loading requires two checks for '== null' and a lock. It's really easy to screw up.

[EDIT]

I didn't mean to suggest that threaded lazy loading was particularly difficult, it's the "Oh and I didn't remember to lock that first!" moments that come fast and hard once you think you're done with the locking that are really the challenge.

There are situations where "thread-safe" doesn't make sense. This consideration is in addition to the higher developer skill and increased time (development, testing, and runtime all take hits).

For example, List<T> is a commonly-used non-thread-safe class. If we were to create a thread-safe equivalent, how would we implement GetEnumerator? Hint: there is no good solution.

Turn this question on its head.

In the early days of programming there was no Thread-Safe code because there was no concept of threads. A program started, then proceeded step by step to the end. Events? What's that? Threads? Huh?

As hardware became more powerful, concepts of what types of problems could be solved with software became more imaginative and developers more ambitious, the software infrastructure became more sophisticated. It also became much more top-heavy. And here we are today, with a sophisticated, powerful, and in some cases unnecessarily top-heavy software ecosystem which includes threads and "thread-safety".

I realize the question is aimed more at application developers than, say, firmware developers, but looking at the whole forest does offer insights into how that one tree evolved.

So, what is the point of keeping non thread-safe code?

By allowing for code that isn't thread safe you're leaving it up to the programmer to decide what the correct level of isolation is.

As others have mentioned this allows for complexity reduction and improved performance.

Rico Mariani wrote two articles entitled "Putting your synchronization at the correct level" and Putting your synchronization at the correct level -- solution that have a nice example of this in action.

In the article he has a method called DoWork(). In it he calls other classes Read twice Write twice and then LogToSteam.

Read, Write, and LogToSteam all shared a lock and were thread safe. This is good except for the fact that because DoWork was also thread safe all the synchronizing work in each Read, Write and LogToSteam was a complete waste of time.

This is all related to the nature Imperative Programming. Its side effects cause the need for this.

However if you had an development platform where applications could be expressed as pure functions where there were no dependencies or side effects then it would be possible to create applications where the threading was managed without developer intervention.

So, what is the point of keeping non thread-safe code?

The rule of thumb is to avoid locking as much as possible. The Ideal code is re-entrant and thread safe with out any locking. But that would be utopia.

Coming back to reality, a good programmer tries his level best to have a sectional locking as opposed to locking the entire context. An example would be to lock few lines of code at a time in various routines than locking everything in a function.

So Also, one has to refactor the code to come up with a design that would minimize the locking if not get rid of it in entirity.

e.g. consider a foobar() function that gets new data on each call and uses switch() case on a type of data to changes a node in a tree. The locking can be mostly avoided (if not completely) As each case statement would touch a different node in a tree. This may be a more specific example but i think it elaborates my point.