[RFC] rte_ring: don't use always inline

[Bruce Richardson]

On Fri, May 06, 2022 at 03:12:32PM +0000, Honnappa Nagarahalli wrote:
> <snip>
> > 
> > On Thu, May 05, 2022 at 10:59:32PM +0000, Honnappa Nagarahalli wrote:
> > > Thanks Stephen. Do you see any performance difference with this change?
> > 
> > as a matter of due diligence i think a comparison should be made just to be
> > confident nothing is regressing.
> > 
> > i support this change in principal since it is generally accepted best practice to
> > not force inlining since it can remove more valuable optimizations that the
> > compiler may make that the human can't see.
> > the optimizations may vary depending on compiler implementation.
> > 
> > force inlining should be used as a targeted measure rather than blanket on
> > every function and when in use probably needs to be periodically reviewed and
> > potentially removed as the code / compiler evolves.
> > 
> > also one other consideration is the impact of a particular compiler's force
> > inlining intrinsic/builtin is that it may permit inlining of functions when not
> > declared in a header. i.e. a function from one library may be able to be inlined
> > to another binary as a link time optimization. although everything here is in a
> > header so it's a bit moot.
> > 
> > i'd like to see this change go in if possible.
> Like Stephen mentions below, I am sure we will have a for and against discussion here.
> As a DPDK community we have put performance front and center, I would prefer to go down that route first.
>

I ran some initial numbers with this patch, and the very quick summary of
what I've seen so far:

* Unit tests show no major differences, and while it depends on what
  specific number you are interested in, most seem within margin of error.
* Within unit tests, the one number I mostly look at when considering
  inlining is the "empty poll" cost, since I believe we should look to keep
  that as close to zero as possible. In the past I've seen that number jump
  from 3 cycles to 12 cycles due to missed inlining. In this case, it seem
  fine.
* Ran a quick test with the eventdev_pipeline example app using SW eventdev,
  as a test of an actual app which is fairly ring-heavy [used 8 workers
  with 1000 cycles per packet hop]. (Thanks to Harry vH for this suggestion
  of a workload)
  * GCC 8 build - no difference observed
  * GCC 11 build - approx 2% perf reduction observed

As I said, these are just some quick rough numbers, and I'll try and get
some more numbers on a couple of different platforms, see if the small
reduction seen is consistent or not. I may also test a few differnet
combinations/options in the eventdev test.  It would be good if others also
tested on a few platforms available to them.

/Bruce