linux-kernel - Re: [patch 00/41] cpu alloc / cpu ops v3: Optimize per cpu access

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Date:	Wed, 04 Jun 2008 08:07:59 -0700
From:	Mike Travis <travis@....com>
To:	Andrew Morton <akpm@...ux-foundation.org>
CC:	Christoph Lameter <clameter@....com>, linux-arch@...r.kernel.org,
	linux-kernel@...r.kernel.org, David Miller <davem@...emloft.net>,
	Eric Dumazet <dada1@...mosbay.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [patch 00/41] cpu alloc / cpu ops v3: Optimize per cpu access

Andrew Morton wrote:
> On Thu, 29 May 2008 20:56:20 -0700 Christoph Lameter <clameter@....com> wrote:
> 
>> In various places the kernel maintains arrays of pointers indexed by
>> processor numbers. These are used to locate objects that need to be used
>> when executing on a specirfic processor. Both the slab allocator
>> and the page allocator use these arrays and there the arrays are used in
>> performance critical code. The allocpercpu functionality is a simple
>> allocator to provide these arrays.
> 
> All seems reasonable to me.  The obvious question is "how do we size
> the arena".  We either waste memory or, much worse, run out.
> 
> And running out is a real possibility, I think.  Most people will only
> mount a handful of XFS filesystems.  But some customer will come along
> who wants to mount 5,000, and distributors will need to cater for that,
> but how can they?
> 
> I wonder if we can arrange for the default to be overridden via a
> kernel boot option?
> 
> 
> Another obvious question is "how much of a problem will we have with
> internal fragmentation"?  This might be a drop-dead showstopper.

One problem with variable sized cpu_alloc area is this comment in bitmap.h:

 * Note that nbits should be always a compile time evaluable constant.
 * Otherwise many inlines will generate horrible code.

I'm guessing since this will be of low use and not performance critical,
then we can ignore the "horrible code"?  ;-)

Thanks,
Mike

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