Re: [dpdk-dev] [PATCH 1/3] mbuf: embedding timestamp into the packet

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

On 10/13/2016 04:35 PM, Oleg Kuporosov wrote:
> The hard requirement of financial services industry is accurate
> timestamping aligned with the packet itself. This patch is to satisfy
> this requirement:
> 
> - include uint64_t timestamp field into rte_mbuf with minimal impact to
>   throughput/latency. Keep it just simple uint64_t in ns (more than 580
>   years) would be enough for immediate needs while using full
>   struct timespec with twice bigger size would have much stronger
>   performance impact as missed cacheline0.
> 
> - it is possible as there is 6-bytes gap in 1st cacheline (fast path)
>   and moving uint16_t vlan_tci_outer field to 2nd cacheline.
> 
> - such move will only impact for pretty rare usable VLAN RX stripping
>   mode for outer TCI (it used only for one NIC i40e from the whole set and
>   allows to keep minimal performance impact for RX/TX timestamps.

This argument is difficult to accept. One can say you are adding
a field for a pretty rare case used by only one NIC :)

Honestly, I'm not able to judge whether timestamp is more important than
vlan_tci_outer. As room is tight in the first cache line, your patch
submission is the occasion to raise the question: how to decide what
should be in the first part of the mbuf? There are also some other
candidates for moving: m->seqn is only used in librte_reorder and it
is not set in the RX part of a driver.

About the timestamp, it would be valuable to have other opinions,
not only about the placement of the field in the structure, but also
to check that this API is also usable for other NICs.

Have you measured the impact of having the timestamp in the second part
of the mbuf?

Changing the mbuf structure should happen as rarely as possible, and we
have to make sure we take the correct decisions. I think we will
discuss this at dpdk userland 2016.


Apart from that, I wonder if an ol_flag should be added to tell that
the timestamp field is valid in the mbuf.

Regards,
Olivier