Beginner question: Why MultiProducerSequencer.next uses compareAndSet instead of addAndGet.

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Andrii Merkushev

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Oct 1, 2019, 9:21:22 PM10/1/19

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Good day, This is my first experiment with the Disruptor.

I have a question why implementation of MultiProducerSequencer.next uses Sequence.compareAndSet to advance cursor instead of Sequence.addAndGet?

Theoretically, fetch_and_add must be faster than compare_and_swap because because it's a single operation and no retries are required. Is compare_and_swap is crucial there?

Thank you,

Andrii

Tom Lee

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Oct 1, 2019, 11:42:03 PM10/1/19

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You mean here, right? https://github.com/LMAX-Exchange/disruptor/blob/0f3f827d937b65f7fbeef137b9f21713b59dc6b4/src/main/java/com/lmax/disruptor/MultiProducerSequencer.java#L142-L145 (and similar logic in tryNext)

Entirely possible I'm misunderstanding your suggestion, but pretty sure the CAS is necessary because the whole while loop needs to be retried if another thread progresses the cursor while we're in the middle of checking ring buffer invariants etc.

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