Re: advice to pmd maintainters: do not touch mbuf initialized fields

[Thomas Monjalon <thomas@monjalon.net>]

08/06/2017 11:06, Olivier Matz:
> Hi,
> 
> In the structure reorganization patchset [1], some work was done
> to ensure that the values of the following fields are always initialized
> for mbuf that are inside the mbuf pool (not yet allocated).
> 
> In other words, calling rte_mbuf_raw_alloc() or rte_mempool_get() return
> mbufs that are initialized as following:
> 
>   m->next = NULL
>   m->nb_segs = 1
>   m->refcnt = 1
> 
> This property can be used in the Rx path of PMDs where the mbuf
> structure is filled from the hardware descriptors: the initialization of
> these fields can be skipped to gain few cycles per packet, and to avoid
> an access to the second cache line (m->next) before it is prefetched.
> 
> It is advised to do this optimization in the PMDs for this reason, and
> also because we could want to move some of these fields to the second
> mbuf cache line in the future: accessing them in the Rx path would cost.
> 
> This work has already been done at several places:
> 
>   http://dpdk.org/browse/dpdk/commit/?id=ebb7bcabb
>   http://dpdk.org/browse/next/dpdk-next-net/commit/?id=e211a7569
>   http://dpdk.org/browse/next/dpdk-next-net/commit/?id=a37cf9005
>   http://dpdk.org/browse/next/dpdk-next-net/commit/?id=fb83a23ee
>   ...
> 
> Feel free to ask if you have any question.

Please Ferruh, could you check which drivers are not yet updated
regarding this optimization?

Thanks