On Thu, Jun 08, 2017 at 04:05:26PM +0200, Olivier Matz wrote:
> On Thu, 8 Jun 2017 14:20:52 +0100, Bruce Richardson
> <bruce.richard...@intel.com> wrote:
> > On Thu, Jun 08, 2017 at 02:45:40PM +0200, Olivier Matz wrote:
> > > On Tue, 6 Jun 2017 15:56:28 +0100, Bruce Richardson
> > > <bruce.richard...@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.anan...@intel.com>
> > > > > > Cc: Verkamp, Daniel <daniel.verk...@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?
> > >
> > >
> > Is it an ABI break so long as we keep the resulting size and field
> > placement of the structures the same? The alignment being reduced should
> > not be a problem, as 128byte alignment is also valid as 64byte
> > alignment, after all.
>
> I'd say yes. Consider the following example:
>
> ---8<---
> #include <stdio.h>
> #include <stdlib.h>
>
> #define ALIGN 64
> /* #define ALIGN 128 */
>
> /* dummy rte_ring struct */
> struct rte_ring {
> char x[128];
> } __attribute__((aligned(ALIGN)));
>
> struct foo {
> struct rte_ring r;
> unsigned bar;
> };
>
> int main(void)
> {
> struct foo array[2];
>
> printf("sizeof(ring)=%zu diff=%u\n",
> sizeof(struct rte_ring),
> (unsigned int)((char *)&array[1].r - (char *)array));
>
> return 0;
> }
> ---8<---
>
> The size of rte_ring is always 128.
> diff is 192 or 256, depending on the value of ALIGN.
>
>
>
> Olivier
Yes, the diff will change, but that is after a recompile. If we have
rte_ring_create function always return a 128-byte aligned structure,
will any already-compiled apps fail to work if we also change the alignment
of the rte_ring struct in the header?
/Bruce
