On Sun, Oct 19, 2025 at 01:45:45PM -0700, Stephen Hemminger wrote: > On Thu, 9 Oct 2025 18:15:12 +0100 > Bruce Richardson <[email protected]> wrote: > > > On Sat, Aug 23, 2025 at 06:30:02AM +0000, Morten Brørup wrote: > > > An optimized function for resetting a bulk of newly allocated > > > reinitialized mbufs (a.k.a. raw mbufs) was added. > > > > > > Compared to the normal packet mbuf reset function, it takes advantage of > > > the following two details: > > > 1. The 'next' and 'nb_segs' fields are already reset, so resetting them > > > has been omitted. > > > 2. When resetting the mbuf, the 'ol_flags' field must indicate whether the > > > mbuf uses an external buffer, and the 'data_off' field must not exceed the > > > data room size when resetting the data offset to include the default > > > headroom. > > > Unlike the normal packet mbuf reset function, which reads the mbuf itself > > > to get the information required for resetting these two fields, this > > > function gets the information from the mempool. > > > > > > This makes the function write-only of the mbuf, unlike the normal packet > > > mbuf reset function, which is read-modify-write of the mbuf. > > > > > > Signed-off-by: Morten Brørup <[email protected]> > > > --- > > > lib/mbuf/rte_mbuf.h | 74 ++++++++++++++++++++++++++++----------------- > > > 1 file changed, 46 insertions(+), 28 deletions(-) > > > > > > diff --git a/lib/mbuf/rte_mbuf.h b/lib/mbuf/rte_mbuf.h > > > index 49c93ab356..6f37a2e91e 100644 > > > --- a/lib/mbuf/rte_mbuf.h > > > +++ b/lib/mbuf/rte_mbuf.h > > > @@ -954,6 +954,50 @@ static inline void rte_pktmbuf_reset_headroom(struct > > > rte_mbuf *m) > > > (uint16_t)m->buf_len); > > > } > > > > > > +/** > > > + * Reset the fields of a bulk of packet mbufs to their default values. > > > + * > > > + * The caller must ensure that the mbufs come from the specified mempool, > > > + * are direct and properly reinitialized (refcnt=1, next=NULL, > > > nb_segs=1), > > > + * as done by rte_pktmbuf_prefree_seg(). > > > + * > > > + * This function should be used with care, when optimization is required. > > > + * For standard needs, prefer rte_pktmbuf_reset(). > > > + * > > > + * @param mp > > > + * The mempool to which the mbuf belongs. > > > + * @param mbufs > > > + * Array of pointers to packet mbufs. > > > + * The array must not contain NULL pointers. > > > + * @param count > > > + * Array size. > > > + */ > > > +static inline void > > > +rte_mbuf_raw_reset_bulk(struct rte_mempool *mp, struct rte_mbuf **mbufs, > > > unsigned int count) > > > +{ > > > + uint64_t ol_flags = (rte_pktmbuf_priv_flags(mp) & > > > RTE_PKTMBUF_POOL_F_PINNED_EXT_BUF) ? > > > + RTE_MBUF_F_EXTERNAL : 0; > > > + uint16_t data_off = RTE_MIN_T(RTE_PKTMBUF_HEADROOM, > > > rte_pktmbuf_data_room_size(mp), > > > + uint16_t); > > > + > > > + for (unsigned int idx = 0; idx < count; idx++) { > > > + struct rte_mbuf *m = mbufs[idx]; > > > + > > > + 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. > > 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. > > > > 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! :-)] > > > > /Bruce > > Although it is possible to use less CPU instructions, the performance > limiting factor is which fields are in cache.
Yes, the cache presence of the target of the stores has a massive effect on how well the code will perform. However, the number of stores can make a difference too - especially if you are in store-heavy code. Consider the number of store operations which would be generated by storing field-by-field to a burst of 32 packets. With the previous work we have done on our PMDs, and vectorizing them, we got a noticible benefit from doing larger vector stores compared to smaller ones! /Bruce
