Re: [RFC] parent in ->d_compare() arguments

[Al Viro <viro@ZenIV.linux.org.uk>]

On Sat, Jul 30, 2016 at 04:36:17PM -0700, Linus Torvalds wrote:

> So? Aren't they in the same cacheline?

Yes (unless lockdep map is stuck in between, in which case we are slow as
hell anyway).

> We've tried very hard to pack all those initial elements next to each other.
> 
> The first-order approximation is that number of cacheline accesses
> matter. And then the second order is to make code small and avoid
> extra instructions.
> 
> As far as I can tell, your change doesn't actually help the cacheline
> accesses, and it makes the code bigger and have extra instructions. So
> it doesn't appear to improve anything, and it does make things worse.
> 
> But numbers talk, bullshit walks. If you have numbers to show
> something different, that trumps my looking at code.

I'll profile and post the results; not today, though - it's not urgent anyway,
and right now I wouldn't trust my ability to do anything other than crawl to
the bed and hopefully get some sleep (RDU -> BOS -> RDU, on top of 4 hours of
sleep tonight and bloody two hours of delay in plane on the way back due to
bad weather; picking the younger kid from summer STEM camp had been...
interesting, for the lack of adequate printable words)