Re: [dpdk-dev] [RFC] ring: make ring implementation non-inlined

["Ananyev, Konstantin" <konstantin.ananyev@intel.com>]

> 
> > From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Honnappa
> > Nagarahalli
> > Sent: Wednesday, July 1, 2020 1:16 AM
> >
> > <snip>
> >
> > > Subject: Re: [dpdk-dev] [RFC] ring: make ring implementation non-
> > inlined
> > >
> > > 26/03/2020 09:04, Morten Brørup:
> > > > From: Jerin Jacob
> > > > > On Fri, Mar 20, 2020 Konstantin Ananyev wrote:
> > > > > >
> > > > > > As was discussed here:
> > > > > > http://mails.dpdk.org/archives/dev/2020-February/158586.html
> > > > > > this RFC aimed to hide ring internals into .c and make all ring
> > > > > > functions non-inlined. In theory that might help to maintain
> > ABI
> > > > > > stability in future.
> > > > > > This is just a POC to measure the impact of proposed idea,
> > proper
> > > > > > implementation would definetly need some extra effort.
> > > > > > On IA box (SKX) ring_perf_autotest shows ~20-30 cycles extra
> > for
> > > > > > enqueue+dequeue pair. On some more realistic code, I suspect
> > > > > > the impact it might be a bit higher.
> > > > > > For MP/MC bulk transfers degradation seems quite small, though
> > for
> > > > > > SP/SC and/or small transfers it is more then noticable (see
> > exact
> > > > > > numbers below).
> > > > > > From my perspective we'd probably keep it inlined for now to
> > avoid
> > > > > > any non-anticipated perfomance degradations.
> > > > > > Though intersted to see perf results and opinions from other
> > > > > > interested parties.
> > > > >
> > > > > +1
> > >
> > > Konstantin, thank you for doing some measures
> > >
> > >
> > > > > My reasoning is a bit different, DPDK is using in embedded boxes
> > too
> > > > > where performance has more weight than ABI stuff.
> > > >
> > > > As a network appliance vendor I can confirm that we certainly care
> > > > more about performance than ABI stability.
> > > > ABI stability is irrelevant for us;
> > > > and API instability is a non-recurring engineering cost each time
> > we
> > > > choose to switch to a new DPDK version, which we only do if we
> > cannot
> > > > avoid it, e.g. due to new drivers, security fixes or new features
> > that
> > > > we want to use.
> > > >
> > > > For us, the trend pointed in the wrong direction when DPDK switched
> > > > the preference towards runtime configurability and deprecated
> > compile
> > > > time configurability. I do understand the reasoning behind it, and
> > the
> > > > impact is minimal, so we accept it.
> > >
> > > The code can be optimized by removing some instructions with #ifdef.
> > > But the complexity of managing #ifdef enabling/disabling, depending
> > on the
> > > platform and the use case, would be huge.
> > > We try to have a reasonable code "always enabled" which performs well
> > in all
> > > cases. This is a design choice which makes DPDK a library, not a pool
> > of code
> > > to cherry-pick.
> > >
> > > > However, if DPDK starts sacrificing performance of the core
> > libraries
> > > > for the benefits of the GNU/Linux distributors, network appliance
> > > > vendors may put more effort into sticking with old DPDK versions
> > > > instead of updating.
> > >
> > > The initial choice regarding ABI compatibility was "do not care".
> > > Recently, the decision was done to care about ABI compatibility as
> > priority
> > > number 2. The priority number 1 remains the performance.
> > > That's a reason for allowing some ABI breakages in some specific
> > releases
> > > announced in advance.
> > >
> > > > > I think we need to focus first on slow path APIs ABI stuff.
> > >
> > > Yes we should not degrade fast path performance for the sake of
> > avoiding
> > > uncertain future ABI issues.
> > >
> > > Morten, Jerin, thank you for the feedback.
> > I think we have a consensus here not to make any changes to inline
> > functions for now.
> > Should we mark this as 'Deferred or Rejected'?
> 
> Rejected.
> 
> There is no need for this modification now, and no actual use cases for it in the road map. In other words: This modification has no use
> cases; it is purely academic. Many other suggestions have been rejected for the reason that they have no current use cases.
> 
> As Thomas mentioned, DPDK has transitioned towards being a library, rather than a pool of code to cherry-pick from. I have learned to live
> with this.
> 
> Being a library doesn't mean that functions cannot be exposed as inline code in the library header files. DPDK is mainly a high performance
> library with a tradition of exposing many of its internals in its API, and we should keep it this way. We certainly don't want an opaque API
> hiding all of its internals, passing around void pointers.
> 
> However, it was still an interesting experiment to investigate the performance cost.

Yes, please reject it.
Konstantin