CODEWITHSUNDEEP
c#multithreadinglocking
0x49D1
0x49D1

Reputation: 8704

What is the proper way to lock code areas

What is better:
to have large code area in lock statement
or
to have small locks in large area?..
exchanges in this sample are not changable. 

lock (padLock)
{
  foreach (string ex in exchanges)
  {
     sub.Add(x.ID, new Subscription(ch, queue.QueueName, true));
.........
}

or 

foreach (string ex in exchanges)
{
  lock (padLock) 
  {
     sub.Add(x.ID, new Subscription(ch, queue.QueueName, true));
  }
.....

Upvotes: 5

Views: 650

Answers (5)

ugoren
ugoren

Reputation: 16441

I think there are two different questions:
1. Which would be correct?
2. Which would give better performance?

The correctness question is complicated. It depends on your data structures, and how you intend the lock to protect them. If the "sub" object is not thread-safe, you definitely need the big lock.

The performance question is simpler and less important (but for some reason, I think you're focused on it more).
Many small locks may be slower, because they just do more work. But if you manage to run a large portion of the loop code without lock, you get some concurrency, so it would be better.

Upvotes: 1

Sign
Sign

Reputation: 1959

You can't effectively judge which is "right" with the given code snippets. The first example says it is not OK for people to see sub with only part of the content from exchanges. The second example says it is OK for people to see sub with only part of the content from exchanges.

Upvotes: 0

Tudor
Tudor

Reputation: 62439

In this particular case, the best option is the first one, because otherwise you're just wasting time locking/unlocking since you have to execute the entire loop anyway. So there's not much opportunity for parallelism in a loop that executes individually atomic operations anyway.

For more general advice on critical section sizes check this article: http://software.intel.com/en-us/articles/managing-lock-contention-large-and-small-critical-sections/

Upvotes: 1

Oleg Dok
Oleg Dok

Reputation: 21756

The wider lock - the less you get from multithreading, and vice versa So, use of locks completely depends on logic. Lock only things and places which changing and have to run only by one thread in a time

If you lock for using the collection sub - use smaller lock, but if you run multiple simultaneous foreach loops in parallel

Upvotes: 1

Haris Hasan
Haris Hasan

Reputation: 30097

Good practise would be to only lock that area which you want to be executed by only one thread at a given time

If that area is whole foreach loop then first approach is fine

but if that area is just one line as you have shown I second approach then go for the second approach

Upvotes: 1

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