Re: [dpdk-dev] [PATCH v6 1/2] mbuf: provide rte_pktmbuf_alloc_bulk API

["Xie, Huawei" <huawei.xie@intel.com>]

On 2/24/2016 9:23 PM, Ananyev, Konstantin wrote:
> Hi Panu,
>
>> -----Original Message-----
>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Panu Matilainen
>> Sent: Wednesday, February 24, 2016 12:12 PM
>> To: Xie, Huawei; Olivier MATZ; dev@dpdk.org
>> Cc: dprovan@bivio.net
>> Subject: Re: [dpdk-dev] [PATCH v6 1/2] mbuf: provide rte_pktmbuf_alloc_bulk API
>>
>> On 02/23/2016 07:35 AM, Xie, Huawei wrote:
>>> On 2/22/2016 10:52 PM, Xie, Huawei wrote:
>>>> On 2/4/2016 1:24 AM, Olivier MATZ wrote:
>>>>> Hi,
>>>>>
>>>>> On 01/27/2016 02:56 PM, Panu Matilainen wrote:
>>>>>> Since rte_pktmbuf_alloc_bulk() is an inline function, it is not part of
>>>>>> the library ABI and should not be listed in the version map.
>>>>>>
>>>>>> I assume its inline for performance reasons, but then you lose the
>>>>>> benefits of dynamic linking such as ability to fix bugs and/or improve
>>>>>> itby just updating the library. Since the point of having a bulk API is
>>>>>> to improve performance by reducing the number of calls required, does it
>>>>>> really have to be inline? As in, have you actually measured the
>>>>>> difference between inline and non-inline and decided its worth all the
>>>>>> downsides?
>>>>> Agree with Panu. It would be interesting to compare the performance
>>>>> between inline and non inline to decide whether inlining it or not.
>>>> Will update after i gathered more data. inline could show obvious
>>>> performance difference in some cases.
>>> Panu and Oliver:
>>> I write a simple benchmark. This benchmark run 10M rounds, in each round
>>> 8 mbufs are allocated through bulk API, and then freed.
>>> These are the CPU cycles measured(Intel(R) Xeon(R) CPU E5-2680 0 @
>>> 2.70GHz, CPU isolated, timer interrupt disabled, rcu offloaded).
>>> Btw, i have removed some exceptional data, the frequency of which is
>>> like 1/10. Sometimes observed user usage suddenly disappeared, no clue
>>> what happened.
>>>
>>> With 8 mbufs allocated, there is about 6% performance increase using inline.
>> [...]
>>> With 16 mbufs allocated, we could still observe obvious performance
>>> difference, though only 1%-2%
>>>
>> [...]
>>> With 32/64 mbufs allocated, the deviation of the data itself would hide
>>> the performance difference.
>>> So we prefer using inline for performance.
>> At least I was more after real-world performance in a real-world
>> use-case rather than CPU cycles in a microbenchmark, we know function
>> calls have a cost but the benefits tend to outweight the cons.

It depends on what could be called the real world case. It could be
argued. I think the case Konstantin mentioned could be called a real
world one.
If your opinion on whether use benchmark or real-world use case is not
specific to this bulk API, then i have different opinion. For example,
for kernel virtio optimization, people use vring bench. We couldn't
guarantee each small optimization could bring obvious performance gain
in some big workload. The gain could be hided if bottleneck is
elsewhere, so i also plan to build such kind of virtio bench in DPDK.

Finally, i am open to inline or not, but currently priority better goes
with performance. If we make it an API now, we couldn't easily step back
in future; But we could change otherwise, after we have more confidence.
We could even check every inline "API", whether it should be inline or
be in the lib.

>>
>> Inline functions have their place and they're far less evil in project
>> internal use, but in library public API they are BAD and should be ...
>> well, not banned because there are exceptions to every rule, but highly
>> discouraged.
> Why is that?
> As you can see right now we have all mbuf alloc/free routines as static inline.
> And I think we would like to keep it like that.
> So why that particular function should be different?
> After all that function is nothing more than a wrapper 
> around rte_mempool_get_bulk()  unrolled by 4 loop {rte_pktmbuf_reset()}
> So unless mempool get/put API would change, I can hardly see there could be any ABI
> breakages in future. 
> About 'real world' performance gain - it was a 'real world' performance problem,
> that we tried to solve by introducing that function:
> http://dpdk.org/ml/archives/dev/2015-May/017633.html
>
> And according to the user feedback, it does help:  
> http://dpdk.org/ml/archives/dev/2016-February/033203.html
>
> Konstantin
>
>> 	- Panu -
>>