Re: [PATCH 0/9] mbuf: structure reorganization

["Morten Brørup" <mb@smartsharesystems.com>]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ananyev,
> Konstantin
> Sent: Friday, March 31, 2017 3:01 AM
> 
> After applying the patch below got nearly original numbers (though not
> quite) on my box.
> dpdk.org mainline:           50.8
> with Olivier patch:           47.8
> with patch below:            50.4
> What I tried to do in it - avoid unnecessary updates of mbuf inside
> rte_pktmbuf_prefree_seg().
> For one segment per packet it seems to help.
> Though so far I didn't try it on i40e and didn't do any testing for
> multi-seg scenario.
> Konstantin
> 
> $ cat patch.mod4
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index d7af852..558233f 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -1283,12 +1283,28 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)  {
>         __rte_mbuf_sanity_check(m, 0);
> 
> -       if (likely(rte_mbuf_refcnt_update(m, -1) == 0)) {
> +       if (likely(rte_mbuf_refcnt_read(m) == 1)) {
> +
> +               if (m->next != NULL) {
> +                       m->next = NULL;
> +                       m->nb_segs = 1;
> +               }
> +
> +               if (RTE_MBUF_INDIRECT(m))
> +                       rte_pktmbuf_detach(m);
> +
> +               return m;
> +
> +       } else if (rte_atomic16_add_return(&m->refcnt_atomic, -1) == 0)
> + {
> +
>                 if (RTE_MBUF_INDIRECT(m))
>                         rte_pktmbuf_detach(m);
> 
> -               m->next = NULL;
> -               m->nb_segs = 1;
> +               if (m->next != NULL) {
> +                       m->next = NULL;
> +                       m->nb_segs = 1;
> +               }
> +
>                 rte_mbuf_refcnt_set(m, 1);
> 
>                 return m;

Maybe the access to the second cache line (for single-segment packets) can be avoided altogether in rte_pktmbuf_prefree_seg() by adding a multi-segment indication flag to the first cache line, and using this flag instead of the test for m->next != NULL.

Med venlig hilsen / kind regards
- Morten Brørup