[dpdk-dev] [PATCH v2 0/8] mbuf: structure reorganization

Olivier MATZ olivier.matz at 6wind.com
Wed Apr 5 11:46:08 CEST 2017

On Wed, 05 Apr 2017 11:37:39 +0200, Thomas Monjalon <thomas.monjalon at 6wind.com> wrote:
> 2017-04-04 18:27, Olivier Matz:
> > Based on discussions done in [1] and in this thread, this patchset reorganizes
> > the mbuf.
> > 
> > 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. This timestamp
> >   is not normalized, i.e. no unit or time reference is enforced. A
> >   library may be added to do this job in the future.
> > - m->next, m->nb_segs, and m->refcnt are always initialized for mbufs
> >   in the pool, avoiding the need of setting m->next (located in the
> >   2nd cache line) in the Rx path for mono-segment packets.
> > - change port and nb_segs to 16 bits
> > - move seqn in the 2nd cache line  
> Applied, thanks for the long work
> We need to add a patch to bump ABIVER and document the changes.

Thanks Thomas. I'm on it, I will send it ASAP.

> > Things discussed but not done in the patchset:
> > - move refcnt and nb_segs to the 2nd cache line: many drivers sets
> >   them in the Rx path, so it could introduce a performance regression, or
> >   it would require to change all the drivers, which is not an easy task.  
> If it is worth to move these fields in 2nd cache line,
> can we plan to rework drivers for not setting them in Rx?

I think it's worth doing the driver modification, it may gain some
cycles. Once it's done, it becomes easy to see the impact of moving the
fields... except if it breaks a vector code ;)

I think this move should only occur if we need more room in the first
cache line.

> > Once this patchset is pushed, the Rx path of drivers could be optimized a bit,
> > by removing writes to m->next, m->nb_segs and m->refcnt. The patch 4/8 gives an
> > idea of what could be done.  
> Yes drivers patches are welcome :)
> Please target RC2 for these changes.

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