Re: [git pull] cpus4096 fixes

[Mike Travis <travis@sgi.com>]

Rusty Russell wrote:
> On Monday 28 July 2008 18:16:39 Ingo Molnar wrote:
>> * Rusty Russell <rusty@rustcorp.com.au> wrote:
>>> Mike: I now think the right long-term answer is Linus' dense cpumap
>>> idea + a convenience allocator for cpumasks.  We sweep the kernel for
>>> all on-stack vars and replace them with one or the other.  Thoughts?
>> The dense cpumap for constant cpumasks is OK as it's clever, compact and
>> static.
>>
>> All-dynamic allocator for on-stack cpumasks ... is a less obvious
>> choice.
> 
> Sorry, I was unclear.  "long-term" == "more than 4096 CPUs", since I thought 
> that was Mike's aim.  If we only want to hack up 4k CPUS and stop, then I 
> understand the current approach.
> 
> If we want huge cpu numbers, I think cpumask_alloc/free gives the clearest 
> code.  So our approach is backwards: let's do that *then* put ugly hacks in 
> if it's really too slow.
> 
> Cheers,
> Rusty.

Well, yes, "long-term" is not really that long and the system will be capable
of supporting 16k cpus.  With the limit on clock scalability, core count is
going through the roof.  Fortunately, we have a whole new release cycle to
rethink some basic ideas.

I did bring up a number of suggestions on how to replace cpumask_t, but they
all seemed to hamper small systems in one way or another.  And the goal, again
was to minimize impact for 99.99% of the systems that won't have a thousand
or more cpus.  (Though it only takes 8 Larrabee chips to attain that.)

Thanks,
Mike