The OS-periment: RPC-Based Daemon Model Goes 'RC'
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[3]: RPC considered harmful
by Brendan on Mon 30th May 2011 10:08 UTC in reply to "RE[2]: RPC considered harmful"
Brendan
Member since:
2005-11-16

Hi,

I probably shouldn't use the "RPC" term, you too got confused into thinking that I was talking about blocking calls, while I am in fact doing nonblocking calls.


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).

As for RPC being too high level, well... I'm tempted to say that pipes are too low level.


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.

Pipes can also make scheduling less efficient. For e.g. if a task unblocks when it receives IPC (like it should), then a task can unblock, look at what it received, realise it hasn't received enough data to do anything useful, and block again; which is mostly a waste of time (and task switches).

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"...

I personally think that if an IPC primitive is to be very frequently used, it's better to implement it directly in the kernel (or at least parts of it), due to the extra control it gives over the communication process. The kernel executes trusted code, but library code can be compromised.


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"?

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.

- Brendan

Reply Parent Score: 2

RE[4]: RPC considered harmful
by Neolander on Mon 30th May 2011 11:41 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

Reply Parent Score: 1

RE[5]: RPC considered harmful
by Brendan on Tue 31st May 2011 02:44 in reply to "RE[4]: RPC considered harmful"
Brendan Member since:
2005-11-16

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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