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SubjectRe: Compatible with i386/UP but optimised for i686/SMP [was Re: test_and_set_bit() not atomic forever?]
Hi,

> > >The point was that lots of people are running SMP kernels on UP machines,
> > >where it's gratuitous overhead.
> >
> > Then they shouldn't do that.
>
> Unless they're a distributor who wants to run one kernel per
> architecture, + modules.

You cannot avoid significant slowdown with SMP kernel on UP machines
due to extra locking needed.

> Or a sysadmin, in a situation where managing different kernels for that
> extra bit of efficiency isn't worth the administrative overhead.

When you need to have a single kernel for lots of similar machines,
they are usually UP, so compiling a SMP kernel doesn't make sense.

> These points apply also to the i386/i486/i586/i686 optimisation issue.
> Is it worth making a kernel nearly optimised for i686 but compatible
> with i386? (Perhaps using fixups in the same way to blank out calls to
> do the "verify put_user" type stuff, vs. blanking out flush_tlb and so
> forth?)

It would make sense to distribute three kernels:

- i386

- i586

- i586 SMP

Have a nice fortnight
--
Martin `MJ' Mares <mj@ucw.cz> http://atrey.karlin.mff.cuni.cz/~mj/
Faculty of Math and Physics, Charles University, Prague, Czech Rep., Earth
"The computer is mightier than the pen, the sword, and usually, the programmer."

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