multithreading - When to use the lock thread in C#?

Question

I have a server which handles multiple incoming socket connections and creates 2 different threads which store the data in XML format.

I was using the lock statement for thread safety almost in every event handler called asyncronously and in the 2 threads in different parts of code. Sadly using this approach my application significantly slows down.

I tried to not use lock at all and the server is very fast in execution, even the file storage seems to boost; but the program crashes for reasons I don't understand after 30sec - 1min. of work.

So. I thought that the best way is to use less locks or to use it only there where's strictly necessary. As such, I have 2 questions:

1. Is the lock needed when I write to the public accessed variables (C# lists) only or even when I read from them ?

2. Is the lock needed only in the asyncronous threads created by the socket handler or in other places too ?

Someone could give me some practical guidelines, about how to operate. I'll not post the whole code this time. It hasn't sense to post about 2500 lines of code.

Answer 1

You ever sit in your car or on the bus at a red light when there's no cross traffic? Big waste of time, right? A lock is like a perfect traffic light. It is always green except when there is traffic in the intersection.

Your question is "I spend too much time in traffic waiting at red lights. Should I just run the red light? Or even better, should I remove the lights entirely and just let everyone drive through the intersection at highway speeds without any intersection controls?"

If you're having a performance problem with locks then removing locks is the last thing you should do. You are waiting at that red light precisely because there is cross traffic in the intersection. Locks are extraordinarily fast if they are not contended.

You can't eliminate the light without eliminating the cross traffic first. The best solution is therefore to eliminate the cross traffic. If the lock is never contended then you'll never wait at it. Figure out why the cross traffic is spending so much time in the intersection; don't remove the light and hope there are no collisions. There will be.

If you can't do that, then adding more finely-grained locks sometimes helps. That is, maybe you have every road in town converging on the same intersection. Maybe you can split that up into two intersections, so that code can be moving through two different intersections at the same time.

Note that making the cars faster (getting a faster processor) or making the roads shorter (eliminating code path length) often makes the problem worse in multithreaded scenarios. Just as it does in real life; if the problem is gridlock then buying faster cars and driving them on shorter roads gets them to the traffic jam faster, but not out of it faster.