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Re: [microblaze-uclinux] Re: [PATCH 6/6] Add support for __read_mostly to linux/cache.h

On Fri, May 01, 2009 at 09:52:18AM -0400, Christoph Lameter wrote:
> On Fri, 1 May 2009, Sam Ravnborg wrote:
> 
> > Are there any specific reason why we do not support read_mostly on all
> > architectures?
> 
> Not that I know of.
> 
> > read_mostly is about grouping rarely written data together
> > so what is needed is to introduce this section in the remaining
> > archtectures.
> >
> > Christoph - git log says you did the inital implmentation.
> > Do you agree?
> 
> Yes.
> 
> There is some concern that __read_mostly is needlessly applied to
> numerous variables that are not used in hot code paths. This may make
> __read_mostly ineffective and actually increase the cache footprint of a
> function since global variables are no longer in the same cacheline. If
> such a function is called and the caches are cold then two cacheline
> fetches have to be done instead of one.

FWIW I think that's a valid concern. Also, I think one can question the
value of __read_mostly for write-through caches, given the mentioned
concern it probably makes things worse for those. IMO there should be
a way to turn it off for arch's that know it's no good for them.

Cheers
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