> From: Bruce Richardson [mailto:bruce.richard...@intel.com] > Sent: Tuesday, November 3, 2020 1:26 PM > > On Tue, Nov 03, 2020 at 01:10:05PM +0100, Morten Brørup wrote: > > > From: Thomas Monjalon [mailto:tho...@monjalon.net] > > > Sent: Monday, November 2, 2020 4:58 PM > > > > > > +Cc techboard > > > > > > We need benchmark numbers in order to take a decision. > > > Please all, prepare some arguments and numbers so we can discuss > > > the mbuf layout in the next techboard meeting. > > > > I propose that the techboard considers this from two angels: > > > > 1. Long term goals and their relative priority. I.e. what can be > > achieved with wide-ranging modifications, requiring yet another ABI > > break and due notices. > > > > 2. Short term goals, i.e. what can be achieved for this release. > > > > > > My suggestions follow... > > > > 1. Regarding long term goals: > > > > I have argued that simple forwarding of non-segmented packets using > > only the first mbuf cache line can be achieved by making three > > modifications: > > > > a) Move m->tx_offload to the first cache line. > > b) Use an 8 bit pktmbuf mempool index in the first cache line, > > instead of the 64 bit m->pool pointer in the second cache line. > > c) Do not access m->next when we know that it is NULL. > > We can use m->nb_segs == 1 or some other invariant as the gate. > > It can be implemented by adding an m->next accessor function: > > struct rte_mbuf * rte_mbuf_next(struct rte_mbuf * m) > > { > > return m->nb_segs == 1 ? NULL : m->next; > > } > > > > Regarding the priority of this goal, I guess that simple forwarding > > of non-segmented packets is probably the path taken by the majority > > of packets handled by DPDK. > > > > > > An alternative goal could be: > > Do not touch the second cache line during RX. > > A comment in the mbuf structure says so, but it is not true anymore. > > > > The comment should be true for non-scattered RX, I believe.
You are correct. My suggestion was unclear: Extend this remark to include segmented packets. This could be a priority if the techboard considers RX segmented packets more important than my suggestion for single cache line forwarding of non-segmented packets. > I'm not aware of any use of second cacheline for the fast-path RXs for many > drivers. > Am I missing something that has changed recently here? Check out eth_igb_recv_pkts() in the E1000 driver: rxm->next = NULL; Or pmd_rx_burst() in the TAP driver: new_tail->next = seg->next; Perhaps the documentation should describe best practices for implementing RX and TX functions in drivers, including allocating/freeing mbufs. Or an example dummy Ethernet driver could do it.
