Re: [RFC] ring: relax alignment constraint on ring structure

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

Hi,

On Fri, 30 Jun 2017 16:26:09 +0200, Olivier Matz <olivier.matz@6wind.com> wrote:
> The initial objective of
> commit d9f0d3a1ffd4 ("ring: remove split cacheline build setting")
> was to add an empty cache line betwee, the producer and consumer
> data (on platform with cache line size = 64B), preventing from
> having them on adjacent cache lines.
> 
> Following discussion on the mailing list, it appears that this
> also imposes an alignment constraint that is not required.
> 
> This patch removes the extra alignment constraint and adds the
> empty cache lines using padding fields in the structure. The
> size of rte_ring structure and the offset of the fields remain
> the same on platforms with cache line size = 64B:
> 
>   rte_ring = 384
>   rte_ring.name = 0
>   rte_ring.flags = 32
>   rte_ring.memzone = 40
>   rte_ring.size = 48
>   rte_ring.mask = 52
>   rte_ring.prod = 128
>   rte_ring.cons = 256
> 
> But it has an impact on platform where cache line size is 128B:
> 
>   rte_ring = 384        -> 768
>   rte_ring.name = 0
>   rte_ring.flags = 32
>   rte_ring.memzone = 40
>   rte_ring.size = 48
>   rte_ring.mask = 52
>   rte_ring.prod = 128   -> 256
>   rte_ring.cons = 256   -> 512
> 
> Link: http://dpdk.org/dev/patchwork/patch/25039/
> Suggested-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
> ---
> 
> I'm sending this patch to throw the discussion again, but since it
> breaks the ABI on platform with cache lines = 128B, I think we should
> follow the usual ABI breakage process.
> 
> If everybody agree, I'll send a notice and resend a similar patch after
> 17.08.
> 

If there is no comment, I'll send a deprecation notice in the
coming days.

Thanks,
Olivier