RE: mbuf fast-free requirements analysis

Morten Brørup
2026-01-14T17:31:30Z

> From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> Sent: Wednesday, 14 January 2026 18.01
>
> On Mon, Dec 15, 2025 at 12:06:38PM +0100, Morten Brørup wrote:
> > Executive Summary:
> >
> > My analysis shows that the mbuf library is not a barrier for fast-
> freeing
> > segmented packet mbufs, and thus fast-free of jumbo frames is
> possible.
> >
> >
> > Detailed Analysis:
> >
> > The purpose of the mbuf fast-free Tx optimization is to reduce
> > rte_pktmbuf_free_seg() to something much simpler in the ethdev
> drivers, by
> > eliminating the code path related to indirect mbufs.
> > Optimally, we want to simplify the ethdev driver's function that
> frees the
> > transmitted mbufs, so it can free them directly to their mempool
> without
> > accessing the mbufs themselves.
> >
> > If the driver cannot access the mbuf itself, it cannot determine
> which
> > mempool it belongs to.
> > We don't want the driver to access every mbuf being freed; but if all
> > mbufs of a Tx queue belong to the same mempool, the driver can
> determine
> > which mempool by looking into just one of the mbufs.
> >
>
> <snip>
>
> >
> > 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.
> >
>
> Just to make sure I understand things correctly here, the suggestion to
> prototype is:
>
> - When FAST_FREE flag is set:
> - reset the m->nb_segs and m->next fields (if necessary) when
> accessing
> the mbuf to write the descriptor
> - skip calling pre-free seg on cleanup and instead
> - just free all buffers directly to the mempool
>
> Is that correct?

Yes.
If this can be done with multi-segment packets, we should be able to eliminate the single-segment requirement to FAST_FREE.
(Unless something in the code that writes the descriptor requires single-segment to be super performant, as I suspected of vectorization.)