[dpdk-stable] [PATCH v4 2/2] ring: move the atomic load of head above the loop

[Gavin Hu (Arm Technology China)]

> -----Original Message-----
> From: Thomas Monjalon <thomas at monjalon.net>
> Sent: Friday, November 2, 2018 5:37 PM
> To: Gavin Hu (Arm Technology China) <Gavin.Hu at arm.com>; Honnappa
> Nagarahalli <Honnappa.Nagarahalli at arm.com>
> Cc: Stephen Hemminger <stephen at networkplumber.org>; dev at dpdk.org;
> olivier.matz at 6wind.com; chaozhu at linux.vnet.ibm.com;
> bruce.richardson at intel.com; konstantin.ananyev at intel.com;
> jerin.jacob at caviumnetworks.com; stable at dpdk.org; nd <nd at arm.com>
> Subject: Re: [PATCH v4 2/2] ring: move the atomic load of head above the
> loop
> 
> 02/11/2018 08:15, Gavin Hu (Arm Technology China):
> >
> > > -----Original Message-----
> > > From: Honnappa Nagarahalli
> > > Sent: Friday, November 2, 2018 12:31 PM
> > > To: Gavin Hu (Arm Technology China) <Gavin.Hu at arm.com>; Stephen
> > > Hemminger <stephen at networkplumber.org>
> > > Cc: dev at dpdk.org; thomas at monjalon.net; olivier.matz at 6wind.com;
> > > chaozhu at linux.vnet.ibm.com; bruce.richardson at intel.com;
> > > konstantin.ananyev at intel.com; jerin.jacob at caviumnetworks.com;
> > > stable at dpdk.org; nd <nd at arm.com>
> > > Subject: RE: [PATCH v4 2/2] ring: move the atomic load of head above
> > > the loop
> > >
> > > <Fixing this to make the reply inline, making email plain text>
> > >
> > >
> > > On Thu, 1 Nov 2018 17:53:51 +0800
> > > Gavin Hu <mailto:gavin.hu at arm.com> wrote:
> > >
> > > > +* **Updated the ring library with C11 memory model.**
> > > > +
> > > > + Updated the ring library with C11 memory model including the
> > > > + following
> > > changes:
> > > > +
> > > > + * Synchronize the load and store of the tail
> > > > + * Move the atomic load of head above the loop
> > > > +
> > >
> > > Does this really need to be in the release notes? Is it a user
> > > visible change or just an internal/optimization and fix.
> > >
> > > [Gavin] There is no api changes, but this is a significant change as
> > > ring is fundamental and widely used, it decreases latency by 25% in
> > > our tests, it may do even better for cases with more contending
> > > producers/consumers or deeper depth of rings.
> > >
> > > [Honnappa] I agree with Stephen. Release notes should be written
> > > from DPDK user perspective. In the rte_ring case, the user has the
> > > option of choosing between c11 and non-c11 algorithms. Performance
> > > would be one of the criteria to choose between these 2 algorithms.
> > > IMO, it probably makes sense to indicate that the performance of c11
> > > based algorithm has been improved. However, I do not know what DPDK
> > > has followed historically regarding performance optimizations. I
> > > would prefer to follow whatever has been followed so far.
> > > I do not think that we need to document the details of the internal
> > > changes since it does not help the user make a decision.
> >
> > I read through the online guidelines for release notes, besides API and new
> features, resolved issues which are significant and not newly introduced in
> this release cycle, should also be included.
> > In this case, the resolved issue existed for long time, across multiple
> release cycles and ring is a core lib, so it should be a candidate for release
> notes.
> >
> > https://doc.dpdk.org/guides-18.08/contributing/patches.html
> > section 5.5 says:
> > Important changes will require an addition to the release notes in
> doc/guides/rel_notes/.
> > See the Release Notes section of the Documentation Guidelines for details.
> > https://doc.dpdk.org/guides-18.08/contributing/documentation.html#doc-
> > guidelines "Developers should include updates to the Release Notes
> > with patch sets that relate to any of the following sections:
> > New Features
> > Resolved Issues (see below)
> > Known Issues
> > API Changes
> > ABI Changes
> > Shared Library Versions
> > Resolved Issues should only include issues from previous releases that
> have been resolved in the current release. Issues that are introduced and
> then fixed within a release cycle do not have to be included here."
> >
> >      Suggested order in release notes items:
> >      * Core libs (EAL, mempool, ring, mbuf, buses)
> >      * Device abstraction libs and PMDs
> 
> I agree with Honnappa.
> You don't need to give details, but can explain that performance of
> C11 version is improved.
> 

V5 was submitted to indicate the improvement by the change, without giving more technical details, please have a review, thanks!