What is the difference between compare-and-swap and test-and-set?

What is the difference between compare-and-swap and test-and-set?

Compare-and-swap (CAS) and test-and-set (TAS) are both atomic operations used in concurrency control. CAS checks if a value is equal to a given value and, if so, updates it to a new value. TAS, on the other hand, sets a value to true and returns the previous value, effectively locking a resource.

Can you give me an example of how each one works?

Sure! For CAS, imagine you have a variable 'x' with a value of 10. If you want to change 'x' to 20 only if it is still 10, you would use CAS. If 'x' is 10, it changes to 20; if not, it does nothing. For TAS, if you want to lock a resource, you would set a lock variable to true. If it was already true, you know the resource is locked.

Which one is more efficient?

CAS is generally more efficient in scenarios with low contention because it allows multiple threads to proceed without blocking. TAS can lead to more contention and potential deadlocks since it locks the resource.

Are there any drawbacks to using CAS?

Yes, CAS can lead to a problem called 'livelock' where threads keep retrying without making progress if they keep failing to update the value. It also requires a way to handle retries, which can complicate the implementation.