Re: A Layered Kernel: Proposal

From: Timothy Miller
Date: Tue Mar 09 2004 - 15:51:17 EST

Grigor Gatchev wrote:

On Mon, 8 Mar 2004, Theodore Ts'o wrote:

On Mon, Mar 08, 2004 at 02:23:43PM +0200, Grigor Gatchev wrote:

Also, does "think API changes, nothing generalised *at all*" mean anything
different from "think code, no design *at all*"? If this is some practical
joke, it is not funny. (I can't believe that a kernel programmer will not
know why design is needed, where is its place in the production of a
software, and how it should and how it should not be done.)

So give us a design. Make a concrete proposal. Tell us what the
API's --- with C function prototypes --- should look like, and how we
should migrate what we have to your new beautiful nirvana.

Aside from the beautiful nirvana irony, that is what I am trying not to
do. For the following reason:

When you have to produce a large software, you first design it at a global
level. Then you design the subsystems. After that, their subsystems. And
so on, level after level. At last, you design the code itself. Only then
you write the code actually.


As one of the people who has been told "show me the code" before, let me try to help you understand what the kernel developers are asking of you.

First of all, they are NOT asking you to do the bottom-up approach that you seem to think they're asking for. They're not asking you to show them code which was not the result of careful design. No. Indeed, they all agree with you that careful planning is always a good idea, in fact critical.

Rather, what they are asking you to do is to create the complete top-down design _yourself_ and then show it to them. If you do a complete design that is well-though-out and complete, then code (ie. function prototypes) will naturally and easily fall out from that. Present your design and the resultant code for evaluation.

Only then can kernel developers give you meaningful feedback. You'll notice that the major arguments aren't about your design but rather about there being a lack of anything to critique. If you want feedback, you must produce something which CAN be critiqued.

Follow the scientific method:
1) Construct a hypothesis (the document you have already written plus more detail).
2) Develop a means to test your hypothesis (write the code that your design implies).
3) Test your hypothesis (present your code and design for criticism).
4) If your hypothesis is proven wrong (someone has a valid criticism), adjust the hypothesis and then goto step (2).

Perhaps you have not done this because you feel that your "high level" design (which you have presented) is not complete. The problem is that, based on what you have presented, no one can help you complete it. Therefore, the thing to do is to complete it yourself, right or wrong. Only when you have actually done something which is wrong can you actually go about doing things correctly. Actually wrong is better than hypothetically correct.

Then, you may be thinking that this will result in more work, because you'll create a design and write come code just to find out that it needs to be rewritten. But this would be poor reasoning. It would be extremely unrealistic to think that you could create a design a priori that was good and correct, before you've ever done anything to test its implications.

Mostly likely, you would go through several iterations of your spec and the implied code before it's acceptable to anyone, regardless of how good it is to begin with. Just think about how many iterations Con and Nick have gone through for their interativity schedulers; they've had countless good ideas, but only experimentation and user criticism could tell them what really worked and what didn't. And these are just the schedulers -- you're talking about the architecture of the whole kernel!

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