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I'm reading the Book Java Concurrency in Practice. In chapter 15, they are talking about the nonblocking algorithms and the compare-and-swap (CAS) method.
It is written that CAS perform much better than the locking methods. I want to ask the people who already worked with both of these concepts and would like to hear when you are preferring which one of these concepts? Is it really so much faster?
For me, the usage of locks is much clearer and easier to understand and maybe even better to maintain (please correct me if I am wrong). Should we really focus on creating our concurrent code related to CAS than locks to get a better performance boost or is sustainability more important?
I know there is maybe not a strict rule when to use what. But I just would like to hear some opinions, experiences with the new concept of CAS.
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CAS stands for “Compare and Swap”. This is a technique used when designing concurrent algorithms. The approach is to compare the actual value of the variable to the expected value of the variable and if the actual value matches the expected value, then swap the actual value of the variable for the new value passed in.
Lock framework works like synchronized blocks except locks can be more sophisticated than Java's synchronized blocks. Locks allow more flexible structuring of synchronized code.
Major difference between lock and synchronized: with locks, you can release and acquire the locks in any order. with synchronized, you can release the locks only in the order it was acquired.
In computer science, compare-and-swap (CAS) is an atomic instruction used in multithreading to achieve synchronization. It compares the contents of a memory location with a given value and, only if they are the same, modifies the contents of that memory location to a new given value.
CAS is generally much faster than locking, but it does depend on the degree of contention. Because CAS may force a retry if the value changes between reading and comparing, a thread can theoretically get stuck in a busy-wait if the variable in question is being hit hard by many other threads (or if it is expensive to compute a new value from the old value (or both)).
The main issue with CAS is that it is much more difficult to program with correctly than locking. Mind you, locking is, in turn, much harder to use correctly than message-passing or STM, so don't take this as a ringing endorsement for the use of locks.
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The relative speed of the operations is largely a non-issue. What is relevant is the difference in scalability between lock-based and nonblocking algorithms. And if you're running on a 1 or 2 core system, stop thinking about such things.
Nonblocking algorithms generally scale better because they have shorter "critical sections" than lock-based algorithms.
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