linux-kernel - Re: [git pull] cpus4096 fixes

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Message-ID: <488E0EBD.7070304@sgi.com>
Date:	Mon, 28 Jul 2008 11:23:57 -0700
From:	Mike Travis <travis@....com>
To:	Rusty Russell <rusty@...tcorp.com.au>
CC:	Ingo Molnar <mingo@...e.hu>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [git pull] cpus4096 fixes

Rusty Russell wrote:
> On Monday 28 July 2008 18:16:39 Ingo Molnar wrote:
>> * Rusty Russell <rusty@...tcorp.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
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