Re: mbuf fast-free requirements analysis

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Jan 14, 2026 at 04:31:31PM +0100, Morten Brørup wrote:
> > > If I'm not mistaken, the mbuf library is not a barrier for fast-
> > freeing
> > > segmented packet mbufs, and thus fast-free of jumbo frames is
> > possible.
> > >
> > > We need a driver developer to confirm that my suggested approach -
> > > resetting the mbuf fields, incl. 'm->nb_segs' and 'm->next', when
> > > preparing the Tx descriptor - is viable.
> > >
> > Excellent analysis, Morten. If I get a chance some time this release
> > cycle,
> > I will try implementing this change in our drivers, see if any
> > difference
> > is made.
> 
> Bruce,
> 
> Have you had a chance to look into the driver change requirements?
> If not, could you please try scratching the surface, to build a gut feeling.
> 
Prototype implementation done and posted as a patch to this thread[1]. (It
applies on top of my "combine multiple Intel scalar Tx paths" patchset[2]).

The logic is not terribly complicated, and running a quick perf test with
scattered Tx enabled (to force use of scalar path, this optimization
doesn't apply to vector path*) shows a small perf increase from it using
testpmd iofwd.

/Bruce

[1] https://patches.dpdk.org/project/dpdk/patch/20260123112032.2174361-1-bruce.richardson@intel.com/
[2] https://patches.dpdk.org/project/dpdk/list/?series=37033

* Vector path already has support for fast-free, and disallows use of
  scattered packets, so there is nothing to optimize for here.