Re: [dpdk-dev] [PATCH 1/6] ring: change head and tail to pointer-width size

["Eads, Gage" <gage.eads@intel.com>]

> -----Original Message-----
> From: Stephen Hemminger [mailto:stephen@networkplumber.org]
> Sent: Thursday, January 10, 2019 10:39 PM
> To: Eads, Gage <gage.eads@intel.com>
> Cc: dev@dpdk.org; olivier.matz@6wind.com; arybchenko@solarflare.com;
> Richardson, Bruce <bruce.richardson@intel.com>; Ananyev, Konstantin
> <konstantin.ananyev@intel.com>
> Subject: Re: [dpdk-dev] [PATCH 1/6] ring: change head and tail to pointer-width
> size
> 
> On Thu, 10 Jan 2019 15:01:17 -0600
> Gage Eads <gage.eads@intel.com> wrote:
> 
> > For 64-bit architectures, doubling the head and tail index widths
> > greatly increases the time it takes for them to wrap-around (with
> > current CPU speeds, it won't happen within the author's lifetime).
> > This is important in avoiding the ABA problem -- in which a thread
> > mistakes reading the same tail index in two accesses to mean that the
> > ring was not modified in the intervening time -- in the upcoming
> > non-blocking ring implementation. Using a 64-bit index makes the possibility of
> this occurring effectively zero.
> >
> > I tested this commit's performance impact with an x86_64 build on a
> > dual-socket Xeon E5-2699 v4 using ring_perf_autotest, and the change
> > made no significant difference -- the few differences appear to be system
> noise.
> > (The test ran on isolcpus cores using a tickless scheduler, but some
> > variation was stll observed.) Each test was run three times and the
> > results were averaged:
> >
> >                                   | 64b head/tail cycle cost minus
> >              Test                 |     32b head/tail cycle cost
> > ------------------------------------------------------------------
> > SP/SC single enq/dequeue          | 0.33
> > MP/MC single enq/dequeue          | 0.00
> > SP/SC burst enq/dequeue (size 8)  | 0.00 MP/MC burst enq/dequeue (size
> > 8)  | 1.00 SP/SC burst enq/dequeue (size 32) | 0.00 MP/MC burst
> > enq/dequeue (size 32) | -1.00
> > SC empty dequeue                  | 0.01
> > MC empty dequeue                  | 0.00
> >
> > Single lcore:
> > SP/SC bulk enq/dequeue (size 8)   | -0.36
> > MP/MC bulk enq/dequeue (size 8)   | 0.99
> > SP/SC bulk enq/dequeue (size 32)  | -0.40 MP/MC bulk enq/dequeue (size
> > 32)  | -0.57
> >
> > Two physical cores:
> > SP/SC bulk enq/dequeue (size 8)   | -0.49
> > MP/MC bulk enq/dequeue (size 8)   | 0.19
> > SP/SC bulk enq/dequeue (size 32)  | -0.28 MP/MC bulk enq/dequeue (size
> > 32)  | -0.62
> >
> > Two NUMA nodes:
> > SP/SC bulk enq/dequeue (size 8)   | 3.25
> > MP/MC bulk enq/dequeue (size 8)   | 1.87
> > SP/SC bulk enq/dequeue (size 32)  | -0.44 MP/MC bulk enq/dequeue (size
> > 32)  | -1.10
> >
> > An earlier version of this patch changed the head and tail indexes to
> > uint64_t, but that caused a performance drop on 32-bit builds. With
> > uintptr_t, no performance difference is observed on an i686 build.
> >
> > Signed-off-by: Gage Eads <gage.eads@intel.com>
> > ---
> >  lib/librte_eventdev/rte_event_ring.h |  6 +++---
> >  lib/librte_ring/rte_ring.c           | 10 +++++-----
> >  lib/librte_ring/rte_ring.h           | 20 ++++++++++----------
> >  lib/librte_ring/rte_ring_generic.h   | 16 +++++++++-------
> >  4 files changed, 27 insertions(+), 25 deletions(-)
> >
> > diff --git a/lib/librte_eventdev/rte_event_ring.h
> > b/lib/librte_eventdev/rte_event_ring.h
> > index 827a3209e..eae70f904 100644
> > --- a/lib/librte_eventdev/rte_event_ring.h
> > +++ b/lib/librte_eventdev/rte_event_ring.h
> > @@ -1,5 +1,5 @@
> >  /* SPDX-License-Identifier: BSD-3-Clause
> > - * Copyright(c) 2016-2017 Intel Corporation
> > + * Copyright(c) 2016-2019 Intel Corporation
> >   */
> >
> >  /**
> > @@ -88,7 +88,7 @@ rte_event_ring_enqueue_burst(struct rte_event_ring *r,
> >  		const struct rte_event *events,
> >  		unsigned int n, uint16_t *free_space)  {
> > -	uint32_t prod_head, prod_next;
> > +	uintptr_t prod_head, prod_next;
> >  	uint32_t free_entries;
> >
> >  	n = __rte_ring_move_prod_head(&r->r, r->r.prod.single, n, @@ -129,7
> > +129,7 @@ rte_event_ring_dequeue_burst(struct rte_event_ring *r,
> >  		struct rte_event *events,
> >  		unsigned int n, uint16_t *available)  {
> > -	uint32_t cons_head, cons_next;
> > +	uintptr_t cons_head, cons_next;
> >  	uint32_t entries;
> >
> >  	n = __rte_ring_move_cons_head(&r->r, r->r.cons.single, n, diff --git
> > a/lib/librte_ring/rte_ring.c b/lib/librte_ring/rte_ring.c index
> > d215acecc..b15ee0eb3 100644
> > --- a/lib/librte_ring/rte_ring.c
> > +++ b/lib/librte_ring/rte_ring.c
> > @@ -1,6 +1,6 @@
> >  /* SPDX-License-Identifier: BSD-3-Clause
> >   *
> > - * Copyright (c) 2010-2015 Intel Corporation
> > + * Copyright (c) 2010-2019 Intel Corporation
> >   * Copyright (c) 2007,2008 Kip Macy kmacy@freebsd.org
> >   * All rights reserved.
> >   * Derived from FreeBSD's bufring.h
> > @@ -227,10 +227,10 @@ rte_ring_dump(FILE *f, const struct rte_ring *r)
> >  	fprintf(f, "  flags=%x\n", r->flags);
> >  	fprintf(f, "  size=%"PRIu32"\n", r->size);
> >  	fprintf(f, "  capacity=%"PRIu32"\n", r->capacity);
> > -	fprintf(f, "  ct=%"PRIu32"\n", r->cons.tail);
> > -	fprintf(f, "  ch=%"PRIu32"\n", r->cons.head);
> > -	fprintf(f, "  pt=%"PRIu32"\n", r->prod.tail);
> > -	fprintf(f, "  ph=%"PRIu32"\n", r->prod.head);
> > +	fprintf(f, "  ct=%"PRIuPTR"\n", r->cons.tail);
> > +	fprintf(f, "  ch=%"PRIuPTR"\n", r->cons.head);
> > +	fprintf(f, "  pt=%"PRIuPTR"\n", r->prod.tail);
> > +	fprintf(f, "  ph=%"PRIuPTR"\n", r->prod.head);
> >  	fprintf(f, "  used=%u\n", rte_ring_count(r));
> >  	fprintf(f, "  avail=%u\n", rte_ring_free_count(r));  } diff --git
> > a/lib/librte_ring/rte_ring.h b/lib/librte_ring/rte_ring.h index
> > af5444a9f..12af64e13 100644
> > --- a/lib/librte_ring/rte_ring.h
> > +++ b/lib/librte_ring/rte_ring.h
> > @@ -1,6 +1,6 @@
> >  /* SPDX-License-Identifier: BSD-3-Clause
> >   *
> > - * Copyright (c) 2010-2017 Intel Corporation
> > + * Copyright (c) 2010-2019 Intel Corporation
> >   * Copyright (c) 2007-2009 Kip Macy kmacy@freebsd.org
> >   * All rights reserved.
> >   * Derived from FreeBSD's bufring.h
> > @@ -65,8 +65,8 @@ struct rte_memzone; /* forward declaration, so as
> > not to require memzone.h */
> >
> >  /* structure to hold a pair of head/tail values and other metadata */
> > struct rte_ring_headtail {
> > -	volatile uint32_t head;  /**< Prod/consumer head. */
> > -	volatile uint32_t tail;  /**< Prod/consumer tail. */
> > +	volatile uintptr_t head;  /**< Prod/consumer head. */
> > +	volatile uintptr_t tail;  /**< Prod/consumer tail. */
> >  	uint32_t single;         /**< True if single prod/cons */
> >  };
> 
> Isn't this a major ABI change which will break existing applications?

Correct, and this patch needs to be reworked with the RTE_NEXT_ABI ifdef, as described in the versioning guidelines. I had misunderstood the ABI change procedure, but I'll fix this in v2.