Re: [dpdk-dev] [PATCH] mbuf: make rearm_data address naturally aligned

[Olivier MATZ <olivier.matz@6wind.com>]

Hi Jerin,

On 07/04/2016 02:45 PM, Jerin Jacob wrote:
> On Mon, May 23, 2016 at 01:19:46PM +0200, Olivier Matz wrote:
>> Hi,
>>
>> On 05/19/2016 05:50 PM, Thomas Monjalon wrote:
>>> 2016-05-19 19:05, Jerin Jacob:
>>>> On Thu, May 19, 2016 at 12:18:57PM +0000, Ananyev, Konstantin wrote:
>>>>>> On Thu, May 19, 2016 at 12:20:16AM +0530, Jerin Jacob wrote:
>>>>>>> On Wed, May 18, 2016 at 05:43:00PM +0100, Bruce Richardson wrote:
>>>>>>>> On Wed, May 18, 2016 at 07:27:43PM +0530, Jerin Jacob wrote:
>>>>> I wonder does anyone really use mbuf port field?
>>>>> My though was - could we to drop it completely?
>>>>> Actually, after discussing it with Bruce offline, an interesting idea came out:
>>>>> if we'll drop port and make mbuf_prefree() to reset nb_segs=1, then
>>>>> we can reduce RX rearm_data to 4B. So with that layout:
>>>>>
>>>>> struct rte_mbuf {
>>>>>
>>>>>          MARKER cacheline0;
>>>>>
>>>>>         void *buf_addr;           
>>>>>         phys_addr_t buf_physaddr; 
>>>>>         uint16_t buf_len;
>>>>>         uint8_t nb_segs;
>>>>>         uint8_t reserved_1byte;   /* former port */
>>>>>         
>>>>>         MARKER32 rearm_data;
>>>>>         uint16_t data_off;
>>>>>        uint16_t refcnt;
>>>>>        
>>>>>         uint64_t ol_flags;
>>>>>         ...
>>>>>
>>>>> We can keep buf_len at its place and avoid 2B gap, while making rearm_data
>>>>> 4B long and 4B aligned.
>>>>
>>>> Couple of comments,
>>>> - IMO, It is good if nb_segs can move under rearm_data, as some
>>>> drivers(not in ixgbe may be) can write nb_segs in one shot also
>>>> in segmented rx handler case
>>>> - I think, it makes sense to keep port in mbuf so that application
>>>> can make use of it(Not sure what real application developers think of
>>>> this)
>>>
>>> I agree we could try to remove the port id from mbuf.
>>> These mbuf data are a common base to pass infos between drivers and apps.
>>> If you need to store some data which are read and write from the app only,
>>> you can have use the private zone (see rte_pktmbuf_priv_size).
>>
>> At the first read, I was in favor of keeping the port_id in the
>> mbuf. But after checking the examples and applications, I'm not
>> opposed to remove it. Indeed, this information could go in an
>> application-specific part or it could be an additional function
>> parameter in the application processing function.
>>
>> The same question could be raised for nb_seg: it seems this info
>> is not used a lot, and having a 8 bits value here also prevents from
>> having long chains (ex: for socket buffer in a tcp stack).
>>
>> Just an idea thrown in the air: if these 2 fields are removed, it
>> brings some room for the m->next field to go in the first cache line.
>> This was mentioned several times (at least [1]).
>>
>> [1] http://dpdk.org/ml/archives/dev/2015-June/019182.html
> 
> 
> Can we come to some consensus on this for this release. The original problem was
> mbuf->rearm_data not being natually aligned and the assosiated performacnce
> issues with ARM architecture for non naturally aligned access.
> I believe that is fixing in this patch without any performance degradation on IA.
> I believe that is a good progress. Can we make further mbuff improvements as
> a additional problem to solve.
> 
> Thoughts ?

Changing the mbuf topology is something that should happen as rarely as
possible, so I think we should group all mbuf modifications in one version.

Your issue (mbuf->rearm alignment), the removing of uneeded fields (port
id, maybe nb_segs), and possibly other things should be addressed for
next version (16.11). I'll send a deprecation notice before the 16.07 is
out if there is no opposition.

Regards,
Olivier