Linked by Hadrien Grasland on Sun 29th May 2011 09:42 UTC
OSNews, Generic OSes It's funny how trying to have a consistent system design makes you constantly jump from one area of the designed OS to another. I initially just tried to implement interrupt handling, and now I'm cleaning up the design of an RPC-based daemon model, which will be used to implement interrupt handlers, along with most other system services. Anyway, now that I get to something I'm personally satisfied with, I wanted to ask everyone who's interested to check that design and tell me if anything in it sounds like a bad idea to them in the short or long run. That's because this is a core part of this OS' design, and I'm really not interested in core design mistakes emerging in a few years if I can fix them now. Many thanks in advance.
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RE[5]: RPC considered harmful
by Brendan on Tue 31st May 2011 02:44 UTC in reply to "RE[4]: RPC considered harmful"
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What I want to do is...

1/Process A gives work to do to process B through a "fast" system call, that in turn calls a function of B in a new thread using a stack of parameters given by A.
2/Process A forgets about it and goes doing something else.
3/When process B is done, it sends a callback to process A through the same mechanism using which A has given B work to do (running a function of A). Callbacks may have parameters, the "results" of the operation.

Does it remind you of something ?

While I can see some similarities between this and asynchronous messaging, there's also significant differences; including the overhead of creating (and eventually destroying) threads, which (in my experience) is the third most expensive operation microkernels do (after creating and destroying processes).

On top of that, (because you can't rely on the queues to serialise access to data structures) programmers would have to rely on something else for reentrancy control; like traditional locking, which is error-prone (lots of programmers find it "hard" and/or screw it up) and adds extra overhead (e.g. mutexes with implied task switches when under lock contention).

I also wouldn't underestimate the effect that IPC overhead will have on the system as a whole (especially for "micro-kernel-like" kernels). For example, if IRQs are delivered to device drivers via. IPC, then on a server under load (with high speed ethernet, for e.g.) you can expect thousands of IRQs per second (and expect to be creating and destroying thousands of threads per second). Once you add normal processes communicating with each other, this could easily go up to "millions per second" under load. If IPC costs twice as much as it does on other OSs, then the resulting system as a whole can be 50% slower than comparable systems (e.g. other micro-kernels) because of the IPC alone.

If you have something like a pipe or message queue, you can implement higher-level IPC protocols on top of it, and use user-space libraries to implement a new IPC mechanism that uses these protocols. That's what I was talking about. But except when trying to make the kernel unusually tiny, I'm not sure it's a good idea either.

In general, any form of IPC can be implemented on top of any other form of IPC. In practice it's not quite that simple because you can't easily emulate the intended interaction with scheduling (blocking/unblocking, etc) in all cases; and even in cases where you can there's typically some extra overhead involved.

The alternative would be if the kernel has inbuilt support for multiple different forms of IPC; which can lead to a "Tower of Babel" situation where it's awkward for different processes (using different types of IPC) to communicate with each other.

Basically, you want the kernel's inbuilt/native IPC to be adequate for most purposes, with little or no scaffolding in user-space.

- Brendan

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