"SMP Kernel Running in Uniprocssor mode"
or
"Uniprocessor Kernel"
On Tue, 9 Sep 1997, David Woodhouse wrote:
>
> This will save me an awful lot of time on the all-too-frequent occasions when
> I forget to edit the Makefile before compiling a new kernel. I've built SMP
> kernels for other peoples' machines, and I've built uniprocessor kernels for
> my own. Once I ran my machine on a single CPU for a week before I noticed!
>
> One other thing I'd like to see is a message on booting which says
> "SMP architecture not found" if you run an SMP kernel on a single-CPU box.
> Otherwise people may never know that they're running an SMP kernel when they
> didn't mean to. I submitted a patch many moons ago, and will do so again after
> 2.1.55 is out (pre-55-1 has modifications to exactly the code I want to change)
>
> If the objection to the Makefile changes is that more files will depend on
> config.h and hence get recompiled more often than they do at the moment, I
> agree with Michael that they aren't really that much of a problem; after all -
> there's lots of files which get recompiled unnecessarily anyway, so a few more
> is just pi^H^Hurinating in the wind.
>
> What I'd like to see, in fact, is a new approach to the dependencies - if a
> file includes config.h, then don't just recompile it if config.h is modified -
> only recompile it if the #defines that it uses are changed. We could define
> each CONFIG_xxx option in autoconf.h with the date and time that it was set,
> rather than just defining it to 1. So autoconf.h becomes:
>
> #undef CONFIG_MATH_EMULATION 19970909102900
> #define CONFIG_NET 19970909102900
> etc...
>
>
> Then the dependency checking involves scanning the source file (& included
> headers) for references to CONFIG_xxxx options, and marking it in need of
> recompilation iff the change time of any of the used options is later than the
> stamp of the object file.
>
> This will make the dependency checking more complex, hence slower, but will
> mean that we no longer need to recompile the whole kernel when only one option
> is changed.
>
> I'm not even sure if what I'm suggesting is possible, but if so, and if people
> like the idea, I'm happy to look into implementing it (although possibly next
> month after I finish here and go back to College, where I'm not paid, so can
> work on what I like).
>
>
> My initial thought was to separate each CONFIG_ option into a separate file,
> and that source files should only include the files for the options they
> depended on:
>
> #include <linux/config/config_arcnet.h>
> #ifdef CONFIG_ARCNET
> etc...
>
> I don't like this because it means an explosion of small files in the
> linux/config directory, which will waste a lot of space on some filesystems,
> and generally not look nice, but it's a simple way of implementing finer
> grained dependencies without overhauling the dependency code.
>
> Comments, flames, abuse, money orders, etc. are as usual welcome.
>
> --
> David Woodhouse, CB3 9AN http://dwmw2.robinson.cam.ac.uk/
> dwmw2@cam.ac.uk Tel: 0976 658355
> D.W.Woodhouse@nortel.co.uk Tel: 01279 402332
>
>