[RFC] rte_ring: don't use always inline

[Konstantin Ananyev]

06/05/2022 17:39, Bruce Richardson пишет:
> On Fri, May 06, 2022 at 09:33:41AM -0700, Stephen Hemminger wrote:
>> On Fri, 6 May 2022 16:28:41 +0100
>> Bruce Richardson <bruce.richardson at intel.com> wrote:
>>
>>> 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

Just to note that apart from ring_perf_autotest, there also exist 
ring_stress_autotest which trying to do some stress-testing in hopefully
more realistic usage scenarios.
Might be worth to consider when benchmarking.

>>>
>>> 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
>>
>> I wonder if a mixed approach might help where some key bits were marked
>> as more important to inline? Or setting compiler flags in build infra?
> 
> Yep, could be a number of options.