CACHE_LINE_SIZE macro.

[Warner Losh]

On Nov 5, 2012, at 10:01 AM, Eitan Adler wrote:

> On 5 November 2012 11:49, Warner Losh <imp at bsdimp.com> wrote:
>>> There has been some discussion recently about padding lock mutexs to
>>> the cache line size in order to avoid false sharing of CPUs. Some have
>>> claimed to see significant performance increases as a result.
>> 
>> Is that an out-of-kernel interface?
>> 
>> If we did that, we'd have to make it run-time settable, because there's no one right answer for arm and MIPS cpus: they are all different.
> 
> The discussion ended up with using a special parameter
> CACHE_LINE_SIZE_LOCKS which is different than CACHE_LINE_SIZE. This is
> necessary for other reasons as well (CACHE_LINE_SIZE_LOCKS may take
> into account prefetching of cache lines, but CACHE_LINE_SIZE
> wouldn't).
> 
> I think the "correct" thing to do here is choose a reasonable, but
> not-always-correct CACHE_LINE_SIZE_LOCKS and make CACHE_LINE_SIZE a
> per-board constant (or run time setting, or whatever works).  You
> can't make it run-time settable as the padding is part of the ABI:
> 
> For more details see
> http://comments.gmane.org/gmane.os.freebsd.devel.cvs/483696
> which contains the original discussion.
> 
> Note - I was not involved.

this is a kernel-only interface, so compile time constants are fine there.  What user-land visible interfaces are affected by this setting?  The answer should be 'none'

Warner