https://itnext.io/understanding-the-lmax-disruptor-caaaa2721496 I was going thru this article... i have a trouble in understanding this paragraph :

In our example, because sharedCounter is updated by two CPU cores at the same time, it will bounce the variable between the two local L1 caches which will slow down drastically the execution.

bouncing the variable between two local L1 caches makes it slower ? how ?

CodePudding user response：

Coherent L1 caches are implemented with some version or variant of the MESI protocol. In this protocol, a given core has to "own" a cache line before being able to write to it. If the cache line is already owned by another core, there is a mechanism to request that ownership be transferred and the updated data copied over, but this takes time.

If two cores are frequently writing to the same variable (or to two different variables in the same cache line, in the case of false sharing), then most of the writes will need such a transfer, slowing everything down. But if only one core needs to write it, the transfer is not required, so it is faster.

A comprehensive writeup of this and other memory-related topics is Ulrich Drepper's long article "What every programmer should know about memory" (HTML PDF).