Re: [PATCH v2] ring: use aligned memzone allocation

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

On Tue, 6 Jun 2017 15:56:28 +0100, Bruce Richardson <bruce.richardson@intel.com> wrote:
> On Tue, Jun 06, 2017 at 02:19:21PM +0100, Ananyev, Konstantin wrote:
> > 
> >   
> > > -----Original Message-----
> > > From: Richardson, Bruce
> > > Sent: Tuesday, June 6, 2017 1:42 PM
> > > To: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> > > Cc: Verkamp, Daniel <daniel.verkamp@intel.com>; dev@dpdk.org
> > > Subject: Re: [dpdk-dev] [PATCH v2] ring: use aligned memzone allocation
> > > 
> > > On Tue, Jun 06, 2017 at 10:59:59AM +0100, Ananyev, Konstantin wrote:  
> > > >  
> > > > > >
> > > > > >
> > > > > >  
> > > > > > >
> > > > > > > The PROD/CONS_ALIGN values on x86-64 are set to 2 cache lines, so members  
> > > > > > of struct rte_ring are 128 byte aligned,  
> > > > > > >and therefore the whole struct needs 128-byte alignment according to the ABI  
> > > > > > so that the 128-byte alignment of the fields can be guaranteed.
> > > > > >
> > > > > > Ah ok, missed the fact that rte_ring is 128B aligned these days.
> > > > > > BTW, I probably missed the initial discussion, but what was the reason for that?
> > > > > > Konstantin  
> > > > >
> > > > > I don't know why PROD_ALIGN/CONS_ALIGN use 128 byte alignment; it seems unnecessary if the cache line is only 64 bytes.  An  
> > > alternate  
> > > > > fix would be to just use cache line alignment for these fields (since memzones are already cache line aligned).  
> > > >
> > > > Yes, had the same thought.
> > > >  
> > > > > Maybe there is some deeper  reason for the >= 128-byte alignment logic in rte_ring.h?  
> > > >
> > > > Might be, would be good to hear opinion the author of that change.  
> > > 
> > > It gives improved performance for core-2-core transfer.  
> > 
> > You mean empty cache-line(s) after prod/cons, correct?
> > That's ok but why we can't keep them and whole rte_ring aligned on cache-line boundaries?
> > Something like that:
> > struct rte_ring {
> >    ...
> >    struct rte_ring_headtail prod __rte_cache_aligned;
> >    EMPTY_CACHE_LINE   __rte_cache_aligned;
> >    struct rte_ring_headtail cons __rte_cache_aligned;
> >    EMPTY_CACHE_LINE   __rte_cache_aligned;
> > };
> > 
> > Konstantin  
> 
> Sure. That should probably work too. 
> 
> /Bruce

I also agree with Konstantin's proposal. One question though: since it
changes the alignment constraint of the rte_ring structure, I think it is
an ABI breakage: a structure including the rte_ring structure inherits
from this constraint.

How could we handle that, knowing this is probably a rare case?