Re: [dpdk-dev] rte_mbuf.next in 2nd cacheline

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

On 06/15/2015 04:52 PM, Ananyev, Konstantin wrote:
> Hi Olivier,
> 
>> -----Original Message-----
>> From: Olivier MATZ [mailto:olivier.matz@6wind.com]
>> Sent: Monday, June 15, 2015 3:31 PM
>> To: Richardson, Bruce
>> Cc: Ananyev, Konstantin; dev@dpdk.org; Damjan Marion (damarion)
>> Subject: Re: [dpdk-dev] rte_mbuf.next in 2nd cacheline
>>
>>
>>
>> On 06/15/2015 04:12 PM, Bruce Richardson wrote:
>>> On Mon, Jun 15, 2015 at 04:05:05PM +0200, Olivier MATZ wrote:
>>>> Hi,
>>>>
>>>> On 06/15/2015 03:54 PM, Ananyev, Konstantin wrote:
>>>>>
>>>>>
>>>>>> -----Original Message-----
>>>>>> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Bruce Richardson
>>>>>> Sent: Monday, June 15, 2015 2:44 PM
>>>>>> To: Olivier MATZ
>>>>>> Cc: dev@dpdk.org; Damjan Marion (damarion)
>>>>>> Subject: Re: [dpdk-dev] rte_mbuf.next in 2nd cacheline
>>>>>>
>>>>>> On Mon, Jun 15, 2015 at 03:20:22PM +0200, Olivier MATZ wrote:
>>>>>>> Hi Damjan,
>>>>>>>
>>>>>>> On 06/10/2015 11:47 PM, Damjan Marion (damarion) wrote:
>>>>>>>>
>>>>>>>> Hi,
>>>>>>>>
>>>>>>>> We noticed 7% performance improvement by simply moving rte_mbuf.next field to the 1st cache line.
>>>>>>>>
>>>>>>>> Currently, it falls under /* second cache line - fields only used in slow path or on TX */
>>>>>>>> but it is actually used at several places in rx fast path. (e.g.: i40e_rx_alloc_bufs() is setting that field to NULL).
>>>>>>>>
>>>>>>>> Is there anything we can do here (stop using next field, or move it to 1st cache line)?
>>>>>>>
>>>>>>> Agree, this is also something I noticed, see:
>>>>>>> http://dpdk.org/ml/archives/dev/2015-February/014400.html
>>>>>>>
>>>>>>> I did not have the time to do performance testing, but it's something
>>>>>>> I'd like to do as soon as I can. I don't see any obvious reason not to
>>>>>>> do it.
>>>>>>>
>>>>>>> It seems we currently just have enough room to do it (8 bytes are
>>>>>>> remaining in the first cache line when compiled in 64 bits).
>>>>>>
>>>>>> This, to me, is the obvious reason not to do it! It prevents us from taking in
>>>>>> any other offload fields in the RX fast-path into the mbuf.
>>>>>>
>>>>>> That being said, I can see why we might want to look to move it - but from the
>>>>>> work done in the ixgbe driver, I'd be hopeful we can get as much performance with
>>>>>> it on the second cache line for most cases, through judicious use of prefetching,
>>>>>> or otherwise.
>>>>>>
>>>>>> It took a lot of work and investigation to get free space in the mbuf - especially
>>>>>> in cache line 0, and I'd like to avoid just filling the cache line up again as
>>>>>> long as we possibly can!
>>>>>
>>>>> Yep, agree with Bruce here.
>>>>> Plus, with packet_type going to be 4B and vlan_tci_outer,
>>>>> we just don't have 8 free bytes at the first cache line any more.
>>>>
>>>> I don't understand why m->next would not be a better candidate than
>>>> rx offload fields to be in the first cache line. For instance, m->next
>>>> is mandatory and must be initialized when allocating a mbuf (to be
>>>> compared with m->seqn for instance, which is also in the first cache
>>>> line). So if we want to do some room in the first cache line, I
>>>> think we can.
>>>>
>>>> To me, the only reason for not doing it now is because we don't
>>>> have a full performance test report (several use-cases, drivers, ...)
>>>> that shows it's better.
>>>>
>>> Because the "next" field is not mandatory to be set on initialization. It can
>>> instead be set only when needed, and cleared on free if it is used.
>>>
>>> The next pointers always start out as NULL when the mbuf pool is created. The
>>> only time it is set to non-NULL is when we have chained mbufs. If we never have
>>> any chained mbufs, we never need to touch the next field, or even read it - since
>>> we have the num-segments count in the first cache line. If we do have a multi-segment
>>> mbuf, it's likely to be a big packet, so we have more processing time available
>>> and we can then take the hit of setting the next pointer. Whenever we go to
>>> free that mbuf for that packet, the code to do the freeing obviously needs to
>>> read the next pointer so as to free all the buffers in the chain, and so it can
>>> also reset the next pointer to NULL when doing so.
>>>
>>> In this way, we can ensure that the next pointer on cache line 1 is not a problem
>>> in our fast path.
>>
>> This is a good idea, but looking at the drivers, it seems that today
>> they all set m->next to NULL in the rx function. What you are suggesting
>> is to remove all of them, and document somewhere that all mbufs in a
>> pool are supposed to have their m->next set to NULL, correct?
>>
>> I think what you are describing could also apply to reference counter
>> (set to 1 by default), right?
> 
> We probably can reset next to NULL at __rte_pktmbuf_prefree_seg(),
> at the same time we do reset refcnt to 0.
> Is that what you suggesting?  

Yes, I can give it a try.



> 
> Konstantin
> 
>>
>>
>> Olivier
>>
>>
>>>
>>> /Bruce
>>>