Event based handling is inherently more efficient because it runs in the context of the caller, instead of requiring its own context like in process-based applications.
This is the main reason why file system code in the kernel is more efficient than file system servers running in a different process, eg via FUSE. No context switching
I'd rather see this as a way to ingest performance critical code pieces into the kernel space more easily, with virtualization and verification options providing safety within an otherwise dangerous/complicated domain.
I would not agree with the article that this kind of paradigm is new - neither inside not outside of kernel land.
What's new here is that this is being made available to user-level custom applications.
> No, you need not just the blocks, you need the actual cache chain data structures themselves. For doing things like good read-ahead, you need to be able to (efficiently) look up trivial things like "is that block already in the cache".
> So you need not only the data, you need the _tags_ too.
> In other words, your filesystem needs to have access to the whole disk cache layer, not just the contents. Or it will not perform well.
https://yarchive.net/comp/microkernels.html
Edit: and the context of this discussion was fairly ancient systems with tagged TLBs and simple in order cores with syscalls nearly as cheap as regular user space call instructions. They were still ungodly slow with microkernels, and his explanation is the meat of his view as to why. It's all about having the data in the right place with as little synchronization required.
I actually have a lot of experience in this area, and I can say that effective readahead is a bit of a crapshoot. Only really works in trivial cases. Ultimately if IO latency sucks, nothing can save you.
His particular point doesn’t fully make sense either. It’s easy to kick off readahead when you only have access to block data, the kernel won’t issue redundant IO requests for blocks already in the cache. Also mlock/madvise give a lot of control in terms of dictating eviction strategies for special blocks.
All thins equal (costless syscall/mm swapping, IO), I still think inter-task synchronization is the largest overhead, but I have no numbers to back it up. Something tells me Marshalling all IO syscalls to a kernel thread would be about as slow as a user-space FUSE task.