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[4]: RPC considered harmful
by Neolander on Mon 30th May 2011 11:41 UTC in reply to "RE[3]: RPC considered harmful"
Neolander
Member since:
2010-03-08

A call is something that doesn't return until it completes. A "non-blocking call" is something that defies logic.. ;)

I got the impression that your "non-blocking call" is a pair of normal/blocking calls, where (for e.g.) the address of the second call is passed as an argument to the first call (a callback). I also got the impression you're intending to optimise the implementation, so that blocking calls that return no data don't actually block (but that's an implementation detail rather than something that effects the conceptual model).

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 ?

I'm not sure where pipes were mentioned by anyone, but I don't really like them much because they force the receiver to do extra work to determine where each "piece of data" ends.

For me send_message() and get_message() was like pipe operation (you send messages to or receive messages from the pipe). Sorry if I didn't get it.

For an analogy (to summarise), think of email. Asynchronous messaging is like people writing emails and sending them to each other whenever they want while they do other things. Synchronous messaging and RPC is like people sending emails and then sitting there doing nothing for hours while they wait for a reply. Pipes are like people sending pieces of a conversation - "I just sent this email to say hell", "o and wish you a happy birth", "day.\n -Fred\000Dear sir, we are"...

Then what I do is definitely not RPC in the usual sense, as it is an asynchronous mechanism too. If the above description reminds you of some better name, please let me now.

I assumed IPC primitives would be implemented directly in the kernel because you can't implement IPC anywhere else. For example, if you have an "IPC service" implemented as a process/daemon, how would processes communicate with the "IPC service"?

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.

The other thing to consider is that usually IPC has a certain amount of control over the scheduler - tasks block when waiting for IPC, and tasks unblock (and then potentially preempt) when they receive IPC, so it makes sense to implement it near the scheduler.

Totally agree.

Edited 2011-05-30 11:57 UTC

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