Re: [dpdk-dev] [PATCH v5 2/2] ring: move the atomic load of head above the loop

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

Hi,

On Sat, Nov 03, 2018 at 01:19:29AM +0000, Gavin Hu (Arm Technology China) wrote:
> 
> 
> > -----Original Message-----
> > From: Bruce Richardson <bruce.richardson@intel.com>
> > Sent: Friday, November 2, 2018 7:44 PM
> > To: Gavin Hu (Arm Technology China) <Gavin.Hu@arm.com>
> > Cc: dev@dpdk.org; thomas@monjalon.net; stephen@networkplumber.org;
> > olivier.matz@6wind.com; chaozhu@linux.vnet.ibm.com;
> > konstantin.ananyev@intel.com; jerin.jacob@caviumnetworks.com;
> > Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; stable@dpdk.org
> > Subject: Re: [PATCH v5 2/2] ring: move the atomic load of head above the
> > loop
> >
> > On Fri, Nov 02, 2018 at 07:21:28PM +0800, Gavin Hu wrote:
> > > In __rte_ring_move_prod_head, move the __atomic_load_n up and out
> > of
> > > the do {} while loop as upon failure the old_head will be updated,
> > > another load is costly and not necessary.
> > >
> > > This helps a little on the latency,about 1~5%.
> > >
> > >  Test result with the patch(two cores):
> > >  SP/SC bulk enq/dequeue (size: 8): 5.64  MP/MC bulk enq/dequeue (size:
> > > 8): 9.58  SP/SC bulk enq/dequeue (size: 32): 1.98  MP/MC bulk
> > > enq/dequeue (size: 32): 2.30
> > >
> > > Fixes: 39368ebfc606 ("ring: introduce C11 memory model barrier
> > > option")
> > > Cc: stable@dpdk.org
> > >
> > > Signed-off-by: Gavin Hu <gavin.hu@arm.com>
> > > Reviewed-by: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>
> > > Reviewed-by: Steve Capper <steve.capper@arm.com>
> > > Reviewed-by: Ola Liljedahl <Ola.Liljedahl@arm.com>
> > > Reviewed-by: Jia He <justin.he@arm.com>
> > > Acked-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> > > Tested-by: Jerin Jacob <jerin.jacob@caviumnetworks.com>
> > > ---
> > >  doc/guides/rel_notes/release_18_11.rst |  7 +++++++
> > >  lib/librte_ring/rte_ring_c11_mem.h     | 10 ++++------
> > >  2 files changed, 11 insertions(+), 6 deletions(-)
> > >
> > > diff --git a/doc/guides/rel_notes/release_18_11.rst
> > > b/doc/guides/rel_notes/release_18_11.rst
> > > index 376128f..b68afab 100644
> > > --- a/doc/guides/rel_notes/release_18_11.rst
> > > +++ b/doc/guides/rel_notes/release_18_11.rst
> > > @@ -69,6 +69,13 @@ New Features
> > >    checked out against that dma mask and rejected if out of range. If more
> > than
> > >    one device has addressing limitations, the dma mask is the more
> > restricted one.
> > >
> > > +* **Updated the ring library with C11 memory model.**
> > > +
> > > +  Updated the ring library with C11 memory model, in our tests the
> > > + changes  decreased latency by 27~29% and 3~15% for MPMC and SPSC
> > cases respectively.
> > > +  The real improvements may vary with the number of contending lcores
> > > + and the  size of ring.
> > > +
> > Is this a little misleading, and will users expect massive performance
> > improvements generally? The C11 model seems to be used only on some,
> > but not all, arm platforms, and then only with "make" builds.
> >
> > config/arm/meson.build: ['RTE_USE_C11_MEM_MODEL', false]]
> > config/common_armv8a_linuxapp:CONFIG_RTE_USE_C11_MEM_MODEL=y
> > config/common_base:CONFIG_RTE_USE_C11_MEM_MODEL=n
> > config/defconfig_arm64-thunderx-linuxapp-
> > gcc:CONFIG_RTE_USE_C11_MEM_MODEL=n
> >
> > /Bruce
> 
> Thank you Bruce for the review, to limit the scope of improvement, I rewrite the note as follows, could you help review? Feel free to change anything if you like.
> " Updated the ring library with C11 memory model, running ring_perf_autotest on Cavium ThunderX2 platform, the changes  decreased latency by 27~29% and 3~15% for MPMC and SPSC cases (2 lcores) respectively. Note the changes help the relaxed memory ordering architectures (arm, ppc) only when CONFIG_RTE_USE_C11_MEM_MODEL=y was configured, no impact on strong memory ordering architectures like x86. To what extent they help the real use cases depends on other factors, like the number of contending readers/writers, size of the ring, whether or not it is on the critical path."

I prefer your initial proposal which is more concise. What about
something like this?


* **Updated the C11 memory model version of ring library.**

  The latency is decreased for architectures using the C11 memory model
  version of the ring library.

  On Cavium ThunderX2 platform, the changes decreased latency by 27~29%
  and 3~15% for MPMC and SPSC cases respectively (with 2 lcores). The
  real improvements may vary with the number of contending lcores and
  the size of ring.


About the patch itself:
Acked-by: Olivier Matz <olivier.matz@6wind.com>

Thanks