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

[Mike Travis <travis@sgi.com>]

Andrew Morton wrote:
> On Thu, 29 May 2008 20:56:20 -0700 Christoph Lameter <clameter@sgi.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