When you change from one user process to another, the virtual page table must change since each process has its own virtual address space. That's the main thing I can think of that makes in-process context switches faster.
You're right in a sense; a cooperative event loop avoids OS-managed context switches entirely by running all tasks in the same thread context. So the only time that event loop has to give up CPU time to the OS is when the OS preempt the entire event loop thread, or some task in the event loop performs a system call. It's a technicality, but Linux will still preempt the event loop periodically to run other user processes, handle interrupts, and run kernel threads. The event loop just keeps all the tasks that it owns from fragmenting into multiple user threads.
This is a good point, though I think in Linux at least, it only applies to actual process switching. That is, if we're switching between threads that are all running under the same process (as would be the case for Apache process running on a given core), I believe the memory isn't switched out the way it is when we switch from one process to another. I am not familiar with the details though, just that heap memory is generally shared, so I may be missing something.
I would encourage finding some reading material on operating systems if you're interested in this kind of stuff, I personally love it. Here is the one my OS professor wrote.
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When you change from one user process to another, the virtual page table must change since each process has its own virtual address space. That's the main thing I can think of that makes in-process context switches faster.
You're right in a sense; a cooperative event loop avoids OS-managed context switches entirely by running all tasks in the same thread context. So the only time that event loop has to give up CPU time to the OS is when the OS preempt the entire event loop thread, or some task in the event loop performs a system call. It's a technicality, but Linux will still preempt the event loop periodically to run other user processes, handle interrupts, and run kernel threads. The event loop just keeps all the tasks that it owns from fragmenting into multiple user threads.
This is a good point, though I think in Linux at least, it only applies to actual process switching. That is, if we're switching between threads that are all running under the same process (as would be the case for Apache process running on a given core), I believe the memory isn't switched out the way it is when we switch from one process to another. I am not familiar with the details though, just that heap memory is generally shared, so I may be missing something.
I would encourage finding some reading material on operating systems if you're interested in this kind of stuff, I personally love it. Here is the one my OS professor wrote.