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

[Thomas Monjalon <thomas@monjalon.net>]

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.