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

[Ananyev, Konstantin]

> -----Original Message-----
> From: Richardson, Bruce
> Sent: Thursday, June 8, 2017 3:12 PM
> To: Olivier Matz <olivier.matz at 6wind.com>
> Cc: Ananyev, Konstantin <konstantin.ananyev at intel.com>; Verkamp, Daniel <daniel.verkamp at intel.com>; dev at dpdk.org
> Subject: Re: [dpdk-dev] [PATCH v2] ring: use aligned memzone allocation
> 
> 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.richardson at 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.richardson at 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 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
> > > >
> > > > 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

About would it be an ABI breakage to 17.05 - I think would...
Though for me the actual breakage happens in 17.05 when rte_ring
alignment was increased from 64B 128B.
Now we just restoring it.

> 
> 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?

Why 128B?
I thought we are discussing making rte_ring 64B aligned again?

Konstantin