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

[Richardson, Bruce]

> -----Original Message-----
> From: Thomas Monjalon [mailto:thomas.monjalon at 6wind.com]
> Sent: Wednesday, April 5, 2017 10:38 AM
> To: Olivier Matz <olivier.matz at 6wind.com>
> Cc: dev at dpdk.org; Ananyev, Konstantin <konstantin.ananyev at intel.com>;
> Richardson, Bruce <bruce.richardson at intel.com>; mb at smartsharesystems.com;
> Chilikin, Andrey <andrey.chilikin at intel.com>; jblunck at infradead.org;
> nelio.laranjeiro at 6wind.com; arybchenko at solarflare.com;
> jerin.jacob at caviumnetworks.com
> Subject: Re: [PATCH v2 0/8] mbuf: structure reorganization
> 
> 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
> 
+1

> We need to add a patch to bump ABIVER and document the changes.
> 
> 
> > 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?

Any drivers that are already setting these fields directly may get a perf bump by not doing so. However, I'm not sure there is a compelling need to move them down just yet. Let's try and avoid breaking the mbuf again for a few releases.

> 
> > 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.

We indeed plan to do so!

/Bruce