[dpdk-dev] [PATCH v12 0/6] add Tx preparation

Thomas Monjalon thomas.monjalon at 6wind.com
Wed Nov 30 09:50:02 CET 2016

2016-11-30 08:40, Adrien Mazarguil:
> I understand tx_prep() automates this process, however I'm wondering why
> isn't the TX burst function doing that itself. Using nb_mtu_seg_max as an
> example, tx_prep() has an extra check in case of TSO that the TX burst
> function does not perform. This ends up being much more expensive to
> applications due to the additional loop doing redundant testing on each
> mbuf.
> If, say as a performance improvement, we decided to leave the validation
> part to the TX burst function; what remains in tx_prep() is basically heavy
> "preparation" requiring mbuf changes (i.e. erasing checksums, for now).
> Following the same logic, why can't such a thing be made part of the TX
> burst function as well (through a direct call to rte_phdr_cksum_fix()
> whenever necessary). From an application standpoint, what are the advantages
> of having to:
>  if (tx_prep()) // iterate and update mbufs as needed
>      tx_burst(); // iterate and send
> Compared to:
>  tx_burst(); // iterate, update as needed and send
> Note that PMDs could still provide different TX callbacks depending on the
> set of enabled offloads so performance is not unnecessarily impacted.
> In my opinion the second approach is both faster to applications and more
> friendly from a usability perspective, am I missing something obvious?

I think it was not clearly explained in this patchset, but this is
my understanding:
tx_prepare and tx_burst can be called at different stages of a pipeline,
on different cores.

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