Re: [PATCH] 2.5.21 Nonlinear CPU support

[Robert Love <rml@tech9.net>]

On Tue, 2002-06-11 at 03:57, Anton Altaparmakov wrote:

> This is crazy! It means you are allocating 2MiB of memory instead of just 
> 128kiB on a 2 CPU system, which will be about 99% of the SMP systems in 
> use, at my guess. So your change is throwing away 1920kiB of kernel ram for 
> no reason at all. And that is just ntfs...
>
> CPU hot plugging is an extremely specialised corner case so can you please 
> make it a config option and not get rid of smp_num_cpus? If people enable 
> the option make smp_num_cpus be the same as NR_CPUS and if not leave it be 
> as it is now.

I agree.  One can argue these rants are just for "micro optimizations"
(although I disagree the size issue is "micro") but someone has to stay
on top of these issues...

Hot swappable CPUs is incredibly specialized and corner-cased.
 
> Anything else penalizes the majority of users just to allow a tiny minority 
> to do strange things like swap cpus without rebooting...

It is by no means a solution, but I just posted a patch to configure
NR_CPUS... so setting it to, say, 2 on your dual box should help you
out.  On the converse, however, it introduces a default of 64 on 64-bit
boxen so it compounds the problem for users who don't tweak the
setting... something still needs to be done with the hotplug code.

	Robert Love