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From: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
To: Olivier Matz <olivier.matz@6wind.com>
CC: Jan Blunck <jblunck@infradead.org>, "Richardson, Bruce"
<bruce.richardson@intel.com>, "dev@dpdk.org" <dev@dpdk.org>
Thread-Topic: [dpdk-dev] [RFC 0/8] mbuf: structure reorganization
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Date: Tue, 28 Feb 2017 10:29:41 +0000
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Subject: Re: [dpdk-dev] [RFC 0/8] mbuf: structure reorganization
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>=20
> Hi,
>=20
> On Tue, 28 Feb 2017 09:05:07 +0000, "Ananyev, Konstantin"
> <konstantin.ananyev@intel.com> wrote:
> > Hi everyone,
> >
> > > >
> > > > In my opinion, if we have the room in the first cache line, we
> > > > should put it there. The only argument I see against is "we may
> > > > find something more important in the future, and we won't have
> > > > room for it in the first cache line". I don't feel we should
> > > > penalize today's use cases for hypothetic future use cases.
> > > >
> > > >
> > > >
> > > >> 2. timestamp normalization point
> > > >> inside PMD RX vs somewhere later as user needs it (extra
> > > >> function in dev_ops?).
> > > >
> > > > This point could be changed. My initial proposition tries to
> > > > provide a generic API for timestamp. Let me remind it here:
> > > >
> > > > a- the timestamp is in nanosecond
> > > > b- 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.
> > > >
> > > > We may remove a-, and just have:
> > > > - the reference and the unit are 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 and unit, but for 2 different
> > > > PMDs (or a sw lib), they would not be the same.
> > > >
> > > > In both cases, we would need a conversion code (maybe in a
> > > > library) if the application wants to work with timestamps from
> > > > several sources. The second solution removes the normalization
> > > > code in the PMD when not needed, it is probably better.
> > >
> > > I agree.
> >
> > One question - does that mean that application would need to
> > keep a track from what PMD each particular packet came to do the
> > normalization? Konstantin
>=20
> I'd say yes. It does not look very difficult to do, since the mbuf
> contains the input port id.
>=20
I understand that we can use mbuf->port here, but it means that we'll
introduce new implicit dependency between timestamp and port values.
>>From my point that introduces new implications:
1. all PMDs that do set a timestamp would also have to set port value too.
Probably not a big deal as most of PMDs do set port value anyway right =
now,
but it means it would be hard to get rid/change mbuf->port in future.
2. Applications would not allowed to change mbuf->port value before normali=
zation is done
(from what I heard some apps do update mbuf->port to store routing deci=
sions). =20
BTW, how the app would keep track which mbufs were already normalized, =
and which were not?
3. In theory with eth_dev_detach() - mbuf->port value might be not valid at=
the point when application
would decide to do normalization.=20
So to me all that approach with delayed normalization seems unnecessary ove=
rcomplicated.
Original one suggested by Olivier, when normalization is done in PMD at RX =
look
much cleaner and more manageable.=20
Konstantin