[dpdk-dev] [PATCH v2] ring: use aligned memzone allocation

[Bruce Richardson]

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 at intel.com>
> > Cc: Verkamp, Daniel <daniel.verkamp at intel.com>; dev at 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