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

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

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?


Olivier


> 
> /Bruce
>