> 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
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