You use smaller scale locks as and when you need them, and you spend a lot of time to carefully implement core threading constructs and similar to make sure the

You use smaller scale locks as and when you need them, and you spend a lot of time to carefully implement core threading constructs and similar to make sure they are correct. Most of the time in an interpreter you’re iterating through the byte code for a function, and mutating variables. It doesn’t matter if two threads are executing that same byte code, it only matters if they are mutating the same object. That’s not to say there aren’t some operations that need coordination across all threads. For that you implement safe points so you signal that something has to be done, wait for all threads to reach a safe point, do the work, and then release the threads again. You can often be even smarter about this and avoid stopping all threads at once but instead stagger things. Allocation and GC you can make more thread local to avoid stopping the world too often.