Re: [PATCH v8 3/3] mbuf: optimize reset of reinitialized mbufs

[Thomas Monjalon <thomas@monjalon.net>]

09/10/2025 19:35, Morten Brørup:
> > From: Bruce Richardson [mailto:bruce.richardson@intel.com]
> > > +		m->pkt_len = 0;
> > > +		m->tx_offload = 0;
> > > +		m->vlan_tci = 0;
> > > +		m->vlan_tci_outer = 0;
> > > +		m->port = RTE_MBUF_PORT_INVALID;
> > 
> > Have you considered doing all initialization using 64-bit stores? It's
> > generally cheaper to do a single 64-bit store than e.g. set of 16-bit
> > ones.
> 
> The code is basically copy-paste from rte_pktmbuf_reset().
> I kept it the same way for readability.
> 
> > This also means that we could remove the restriction on having refcnt
> > and
> > nb_segs already set. As in PMDs, a single store can init data_off,
> > ref_cnt,
> > nb_segs and port.
> 
> Yes, I have given the concept a lot of thought already.
> If we didn't require mbufs residing in the mempool to have any fields initialized, specifically "next" and "nb_segs", it would improve performance for drivers freeing mbufs back to the mempool, because writing to the mbufs would no longer be required at that point; the mbufs could simply be freed back to the mempool. Instead, we would require the driver to initialize these fields - which it probably does on RX anyway, if it supports segmented packets.
> But I consider this concept a major API change, also affecting applications assuming that these fields are initialized when allocating raw mbufs from the mempool. So I haven't pursued it.
> 
> > 
> > Similarly for packet_type and pkt_len, and data_len/vlan_tci and rss
> > fields
> > etc. For max performance, the whole of the mbuf cleared here can be
> > done in
> > 40 bytes, or 5 64-bit stores. If we do the stores in order, possibly
> > the
> > compiler can even opportunistically coalesce more stores, so we could
> > even
> > end up getting 128-bit or larger stores depending on the ISA compiled
> > for.
> > [Maybe the compiler will do this even if they are not in order, but I'd
> > like to maximize my chances here! :-)]

Morten, you didn't reply to this.
Can we optimize more with big stores?