Re: [dpdk-dev] [RFC 0/8] mbuf: structure reorganization

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

Hi Jan,

On Thu, 16 Feb 2017 18:26:39 +0100, Jan Blunck <jblunck@infradead.org>
wrote:
> On Thu, Feb 16, 2017 at 2:48 PM, Olivier Matz
> <olivier.matz@6wind.com> wrote:
> > On Mon, 6 Feb 2017 18:41:27 +0000, "Ananyev, Konstantin"
> > <konstantin.ananyev@intel.com> wrote:  
> >> >
> >> > The main changes are:
> >> > - reorder structure to increase vector performance on some non-ia
> >> >   platforms.
> >> > - add a 64bits timestamp field in the 1st cache line  
> >>
> >> Wonder why it deserves to be in first cache line?
> >> How it differs from seqn below (pure SW stuff right now).  
> >
> > In case the timestamp is set from a NIC value, it is set in the Rx
> > path. So that's why I think it deserve to be located in the 1st
> > cache line.
> >
> > As you said, the seqn is a pure sw stuff right: it is set in a lib,
> > not in a PMD rx path.
> >  
> 
> If we talk about setting the timestamp value in the RX path this
> implicitly means software timestamps. Hardware timestamping usually
> works by letting the hardware inject sync events for coarse time
> tracking and additionally injecting fine granular per-packet ticks at
> a specific offset in the packet. Out of performance reasons I don't
> think it makes sense to extract this during the burst and write it
> into the mbuf again.

From what I've understand, at least it does not work like this for
mellanox NICs: timestamp is a metadata attached to a rx packet. But
maybe they (and other NIC vendors interrested in the feature) can
confirm or not.

> 
> The problem with timestamps is to get the abstraction right wrt the
> correction factors and the size of the tick vs. the timestamp in the
> events injected. From my perspective it would be better to extract the
> handling of timestamp data into a library with PMD specific
> implementation of the conversions. That way the normalized timestamp
> values can get extracted if they are present. The mbuf itself would
> only indicate the presence of timestamp metadata in that case.

I agree however that we need to properly define the meaning of this
field. My idea is:

- the timestamp is in nanosecond
- the reference is always the same for a given path: if the timestamp is
  set in a PMD, all the packets for this PMD will have the same
  reference, but for 2 different PMDs (or a sw lib), the reference
  would not be the same.

I think it's enough for many use cases.
We can later add helpers to compare timestamps with different
references.


Regards,
Olivier