Re: rte_mbuf.next in 2nd cacheline

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

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.

Olivier