Simple #define for cache line size

[John Baldwin]

On Monday 13 April 2009 7:05:20 pm Max Laier wrote:
> On Saturday 11 April 2009 18:11:55 Robert Watson wrote:
> > Dear all:
> >
> > We have a number of __aligned() qualifiers scattered around the kernel
> > intended to space objects to improve alignment with respect to cache lines.
> > This is important for a number of reasons, not least avoiding cache line
> > thrashing when using arrays of foo[MAXCPU].  What I'd like to do is provide
> > a single compile-time constant, CACHE_LINE_SIZE, defined in
> > machine-dependent param.h, to use for spacing such objects, rather than
> > hard-coding various values around.  Here are some examples of existing
> > spacing attempts:
> 
> How much does __aligned(FOO) even achieve?  If we allocate these structs at 
> runtime the alignment requirements have to be passed to the allocator as well 
> (eg. uma(9)'s align parameter in uma_zcreate).  If my assumption is correct it 
> would make sense to have a global symbol that is initialized early in the boot 
> process instead of a compile time #define.
> 
> In addition it seems that we need to make the define a worst case value to 
> assure correctness for static arrays as below.

Actually, we need both the boot-time tunable that UMA has I think and a
worst-case constant for static arrays.  UMA already has the tunable and I
think it should remain (though we may want to provide a way for the MD code
to adjust its value).  I think the constant is still needed for static arrays
and should also be used to initialize the constant in UMA.

-- 
John Baldwin