Re: [dpdk-dev] [PATCH v5 6/8] net/ice: support Rx AVX2 vector

[Maxime Coquelin <maxime.coquelin@redhat.com>]

On 3/27/19 1:56 AM, Lu, Wenzhuo wrote:
> Hi Maxime,
> 
>> -----Original Message-----
>> From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
>> Sent: Tuesday, March 26, 2019 5:29 PM
>> To: Lu, Wenzhuo <wenzhuo.lu@intel.com>; dev@dpdk.org
>> Subject: Re: [dpdk-dev] [PATCH v5 6/8] net/ice: support Rx AVX2 vector
>>
>> Hi,
>>
>> On 3/26/19 2:00 AM, Lu, Wenzhuo wrote:
>>> Hi Maxime,
>>>
>>>> -----Original Message-----
>>>> From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
>>>> Sent: Monday, March 25, 2019 4:26 PM
>>>> To: Lu, Wenzhuo <wenzhuo.lu@intel.com>; dev@dpdk.org
>>>> Subject: Re: [dpdk-dev] [PATCH v5 6/8] net/ice: support Rx AVX2
>>>> vector
>>>>
>>>> Hi,
>>>>
>>>> On 3/25/19 3:22 AM, Lu, Wenzhuo wrote:
>>>>> Hi Maxime,
>>>>>
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: Maxime Coquelin [mailto:maxime.coquelin@redhat.com]
>>>>>> Sent: Friday, March 22, 2019 6:12 PM
>>>>>> To: Lu, Wenzhuo <wenzhuo.lu@intel.com>; dev@dpdk.org
>>>>>> Subject: Re: [dpdk-dev] [PATCH v5 6/8] net/ice: support Rx AVX2
>>>>>> vector
>>>>>
>>>>>
>>>>>>> +#ifndef RTE_LIBRTE_ICE_16BYTE_RX_DESC
>>>>>>
>>>>>> I see same is done for other Intel NICs, but I wonder what would be
>>>>>> the performance cost of making it dynamic, if any cost?
>>>>> Currently we don't have a good idea to make it dynamic. If we use
>>>>> pointer
>>>> to point to different functions for 16 byte and 32 byte, there's too
>>>> much duplicate code to make it hard to maintain. If we use the same
>>>> function, and check the configure in it. It impacts the performance.
>>>>
>>>> Have you done some measurements, what would be the performance
>>>> impact?
>>> I mean if we check the configuration is 16 byte or 32 byte, this check will
>> consume extra CPU cycles.
>>> That why I think the better way is to have different paths for 16 byte and
>> 32 byte. We should choose the appropriate path at the beginning.
>>>
>>>>
>>>>> As HW does not support to change the configuration dynamically. The
>>>> device must be stopped and restarted if the configuration is changed.
>>>> It's not very helpful to make it a dynamic configuration. We assume
>>>> that the users can make their choice at the beginning and will not change
>> it.
>>>>
>>>> The problem is that the user has to recompile to switch between the
>>>> two configurations. And it may not be an option for the user if he
>>>> uses dpdk packaged by a distribution, for example.
>>>>
>>>> Maybe I was not clear, but I don't mean to be able to switch mode
>>>> while the port is started. I think it would be better to make it
>>>> possible to switch mode at application startup time.
>>> Yes, I understand the problem is the recompiling. But we think the users
>> will not change it after they made decision. That's why's acceptable in
>> previous drivers.
>>
>> The problem is that the user may not be able to change it, if he does not get
>> DPDK from source but from a distribution like Debian, Ubuntu or Red Hat.
>>
>> In this case, it means the user has no choice than sticking to 32 bytes
>> descriptors.
> Normally using 32 bytes is the default behavior and it's good to do that.
> But I have to say I don't quite understand the scenario. DPDK is open source, whatever OS that users are using, nothing prevents them going to dpdk website to get the code and customize it.

The user may prefer to use the distribution package for several reasons.
Like not loosing the support he pays to the distributor by recompiling
the package, or also not benefiting from the validation done by the
distributor on the pre-built package.

For example, would it make sense to fix the queue size at build time
instead of using the --txd/--rxd run-time paramaters to save a few
cycles here and there? I think not.

> 
>>
>>> Agree it's better to remove all the compile configuration. Looks like that's
>> what we're trying to do. We'd like to think about how to optimize it later.
>>
>> My suggestion would be a devarg, so that you can have a per-port policy
>> (which is another advantage of doing so).
> We're thinking about moving some configuration from per port to per queue.
> To my opinion, it's also a case that maybe it’s better to make it a queue's parameter.
> Obviously it’s an API change. So we have to be slow and careful :)

Having it per queue would be even better, but yes, it would certainly
mean an API change.

>>
>>>
>>>
>>>>
>>>>>
>>>>>>
>>>>>> Having it dynamic (as a dev arg for instance) would make it
>>>>>> possible to change the value when the user is using dpdk from a
>>>>>> distro. It would also help testing coverage.
>>>>>>
>>>>>> Btw, how do you select this option with meson build system?
>>>>> Not very familiar with meson. As I know, we can change the
>>>>> meson.build
>>>> to add the configure.
>>>>>
>>>>
>>>> Ok, then please try to do it, because the legacy build system is
>>>> going to be deprecated.