获取和释放 Java 监视器锁（同步块、重入锁等）是否需要上下文切换到内核空间？

Question

AFAIK, Every object in Java has a mark word. The first word (the mark word) is used for storing locking information, either through a flag if only one thread is acquiring the lock or pointing to a lock monitor object if there is contention between different threads, and in both the cases, compare and swap construct is used for acquiring the lock.

But according to this link - https://www.baeldung.com/lmax-disruptor-concurrency

To deal with the write contention, a queue often uses locks, which can cause a context switch to the kernel. When this happens the processor involved is likely to lose the data in its caches.

What am I missing ?

Answer 1

Neither, synchronized nor the standard Lock implementations, require a context switch into the kernel when locking uncontended or unlocking. These operations indeed boil down to an atomic cas or write.

The performance critical aspect is the contention , ie when trying to acquire the monitor or lock and it's not available. Waiting for the availability of the monitor or lock implies putting the thread into a waiting state and reactivating it when the resource became available. The performance impact is so large, that you don't need to worry about CPU caches at all.

For this reason, typical implementations perform some amount of spinning, rechecking the availability of the monitor or lock in a loop for some time, when there is a chance of becoming available in that time. This is usually tied to the number of CPU cores. When the resource becomes available in that time, these costs can be avoided. This, however, usually requires the acquisition to be allowed to be unfair , as a spinning acquisition may overtake an already waiting thread.

Note that the linked article says before your cited sentence:

Queues are typically always close to full or close to empty due to the differences in pace between consumers and producers.

In such a scenario, the faster threads will sooner or later enter a condition wait, waiting for new space or new items in a queue, even when they acquired the lock without contention. So in this specific scenario, the associated costs are indeed there and unavoidable when using a simple queue implementation.