Re: [gentoo-user] Mounting /dev/sdaX on boot does not work

[Alan McKinnon]

On Wednesday 30 January 2008, Mateusz Mierzwinski wrote:
> Alan McKinnon pisze:

> > One could try listing these drivers in /etc/autoload.d/kernel-2.6
> > but the easiest is probably to compile them into the kernel
>
> You have right. Standard unix kernel was designed to have all inside.
> I don't know why some people still prefer modules than monolith
> kernel. If you have modules, you must recompile all of them when new
> ABI comes out. On monolithic kernel its all there. Also i don't know
> if modules are not slower than monolith Kernel because of User space
> to Kernel space connection. Compiled in modules makes kernel run
> faster, and if it's server, then even 0.00001 sec makes the different
> on functions execution and internal core communication. Partly beyond
> of that problem is IPC module communication. But if there are
> benefits then also are problems with error code execution or beta
> drivers installed. Also if You have Molnar's Real Time Preemption
> Model on Your kernel, You should choose monolithic kernel. PS: some
> drivers are don't work as modules like they should. You see Windows -
> there are drivers like Linux modules - what it makes? Blue screens of
> death ;), but this is another story... ;)

That's an easy answer :-)

Standard Unix was designed for systems where you know exactly what 
hardware you have up front and there are no nasty surprises. Take for 
example an SGI box. How many PCI chipsets could it possibly have? 
Exactly one. So you know exactly which driver you will need.

Apple are still lucky in this regard, as the only hardware that goes in 
them is Apple's hardware and kernel configurationis then quite easy.

But we use the worst possible design that demented designers could ever 
come up with - PCs. The range of stuff available is staggering. The 
amount of dodgy hardware that claims to conform to spec but doesn't is 
even more staggering. So now exactly which modules are you going to 
compile in monolithically? What about hotpluggable hardware? As a 
vendor you have no way of knowing which funky hardware the user will 
ever plug into a notebook, and you simply cannot compile everything in 
(never mind the driver conflicts you will have).

Gentoo expects their users to compile their own kernels so to a large 
extent we can make stuff monolithic, apart from the hot-pluggable 
devices.

Binary distros cannot do this. To conserve memory they must load only 
the drivers for hardware that is actually present, and to do that one 
needs modules. I don;t know of a binary commercial distro that will 
gladly still support users who compile their own kernels - they usually 
stuff it all up gloriously. It's a no-brainer really.

I don't buy the speedup argument either and have never seena benchmark 
that proves modules are slower. The Linux kernel module loader is 
essentially self-modifying code and inserts modules into kernel space 
as if they had been there monolithically (within reason of course). 
Some drivers do behave differently between being modular and 
monolithic, but this is a function of a crappy coder and not a function 
of modularity :-)   If a piece of kernel code causes a BSOD, then it 
will probably do it either way it is loadable as the code is probably 
crappy.

A Real Time kernel is not suitable for a PC either - I can't think of 
any RT application where a PC would be a *GOOD* choice. For that I 
would be using the embedded arches with a board where as designer I 
know exactly what is present and what isn't. Monolithic makes more 
sense in that case. I can imagine why Ingo made that choice - with RT 
he has to supply certain guarantees and probbaly can't do that with 
modules coming and going all the time.

Basically. modern Linux is essentially unusable without kernel modules.


-- 
Alan McKinnon
alan dot mckinnon at gmail dot com
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