Re: [dpdk-dev] [PATCH v1 01/14] ring: remove split cacheline build setting

[Olivier Matz <olivier.matz@6wind.com>]

On Wed, 1 Mar 2017 10:42:58 +0000, Bruce Richardson <bruce.richardson@intel.com> wrote:
> On Wed, Mar 01, 2017 at 11:17:53AM +0100, Olivier Matz wrote:
> > On Wed, 1 Mar 2017 09:47:03 +0000, Bruce Richardson
> > <bruce.richardson@intel.com> wrote:  
> > > So given that there is not much difference here, is the MIN_SIZE i.e.
> > > forced 64B, your preference, rather than actual cacheline-size?
>
> [...]
>
> > I don't quite like this macro CACHE_LINE_MIN_SIZE. For me, it does not
> > mean anything. The reasons for aligning on a cache line size are
> > straightforward, but when should we need to align on the minimum
> > cache line size supported by dpdk? For instance, in mbuf structure,
> > aligning on 64 would make more sense to me.
> > 
> > So, I would prefer using (RTE_CACHE_LINE_SIZE * 2) here. If we don't
> > want it on some architectures, or if this optimization is only for Intel
> > (or all archs that need this optim), I think we could have something
> > like:
> > 
> > /* bla bla */
> > #ifdef INTEL
> > #define __rte_ring_aligned __rte_aligned(RTE_CACHE_LINE_SIZE * 2)
> > #else
> > #define __rte_ring_aligned __rte_aligned(RTE_CACHE_LINE_SIZE)
> > #endif
> >   
> I would agree that CACHE_LINE_MIN_SIZE probably doesn't make any sense
> here, but I'm happy to put in any suitable scheme that others are happy
> with. The options are:
> 
> * Keep as-is: 
>     adv: simplest option, disadv: wastes 128B * 2 on some platforms
> * Change to MIN_SIZE: 
>     adv: no ifdefs, disadv: doesn't make much sense logically here
> * Use ifdefs:
>     adv: each platform gets what's best for it, disadv: untidy code, may
>     be harder to maintain
> * Use hard-coded 128:
>     adv: short and simple, disadv: misses any logical reason why 128 is
>     used, i.e. magic number
> 
> I'm ok with any option above.

I'd vote for "Keep as-is" or "Use ifdefs".

Olivier