Re: [dpdk-dev] [PATCH v3 1/3] lib/lpm: not inline unnecessary functions

["Medvedkin, Vladimir" <vladimir.medvedkin@intel.com>]

On 01/07/2019 07:44, Ruifeng Wang (Arm Technology China) wrote:
> Hi Medvedkin,
>
>> -----Original Message-----
>> From: Stephen Hemminger <stephen@networkplumber.org>
>> Sent: Friday, June 28, 2019 23:35
>> To: Medvedkin, Vladimir <vladimir.medvedkin@intel.com>
>> Cc: Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>; Ruifeng Wang
>> (Arm Technology China) <Ruifeng.Wang@arm.com>;
>> bruce.richardson@intel.com; dev@dpdk.org; Gavin Hu (Arm Technology
>> China) <Gavin.Hu@arm.com>; nd <nd@arm.com>
>> Subject: Re: [dpdk-dev] [PATCH v3 1/3] lib/lpm: not inline unnecessary
>> functions
>>
>> On Fri, 28 Jun 2019 15:16:30 +0100
>> "Medvedkin, Vladimir" <vladimir.medvedkin@intel.com> wrote:
>>
>>> Hi Honnappa,
>>>
>>> On 28/06/2019 14:57, Honnappa Nagarahalli wrote:
>>>>> Hi all,
>>>>>
>>>>> On 28/06/2019 05:34, Stephen Hemminger wrote:
>>>>>> On Fri, 28 Jun 2019 02:44:54 +0000 "Ruifeng Wang (Arm Technology
>>>>>> China)"<Ruifeng.Wang@arm.com>  wrote:
>>>>>>
>>>>>>>>> Tests showed that the function inlining caused performance drop
>>>>>>>>> on some x86 platforms with the memory ordering patches applied.
>>>>>>>>> By force no-inline functions, the performance was better than
>>>>>>>>> before on x86 and no impact to arm64 platforms.
>>>>>>>>>
>>>>>>>>> Suggested-by: Medvedkin
>> Vladimir<vladimir.medvedkin@intel.com>
>>>>>>>>> Signed-off-by: Ruifeng Wang<ruifeng.wang@arm.com>
>>>>>>>>> Reviewed-by: Gavin Hu<gavin.hu@arm.com>
>>>>>>>>     {
>>>>>>>>
>>>>>>>> Do you actually need to force noinline or is just taking of inline
>> enough?
>>>>>>>> In general, letting compiler decide is often best practice.
>>>>>>> The force noinline is an optimization for x86 platforms to keep
>>>>>>> rte_lpm_add() API performance with memory ordering applied.
>>>>>> I don't think you answered my question. What does a recent version
>>>>>> of GCC do if you drop the inline.
>>>>>>
>>>>>> Actually all the functions in rte_lpm should drop inline.
>>>>> I'm agree with Stephen. If it is not a fastpath and size of
>>>>> function is not minimal it is good to remove inline qualifier for
>>>>> other control plane functions such as rule_add/delete/find/etc and
>>>>> let the compiler decide to inline it (unless it affects performance).
>>>> IMO, the rule needs to be simple. If it is control plane function, we should
>> leave it to the compiler to decide. I do not think we need to worry too much
>> about performance for control plane functions.
>>> Control plane is not as important as data plane speed but it is still
>>> important. For lpm we are talking not about initialization, but
>>> runtime routes add/del related functions. If it is very slow the
>>> library will be totally unusable because after it receives a route
>>> update it will be blocked for a long time and route update queue would
>> overflow.
>>
>> Control plane performance is more impacted by algorithmic choice.
>> The original LPM had terrible (n^2?) control path. Current code is better.
>> I had a patch using RB tree, but it was rejected because it used the
>> /usr/include/bsd/sys/tree.h which added a dependency.
> Based on current discussion, I'd like to drop this single patch from the patch set.
> Since it is not directly related to memory ordering changes in this library.
> We can remove inlines in a follow up patch.
I think this patch is indirectly related to changes. I can't accept a 
memory ordering patch series _before_ this patch because a repository 
state will appear in which the performance of LPM add/delete has 
dropped. So if it could be avoided it have to be avoided.

-- 
Regards,
Vladimir