Re: [dpdk-dev] Accessing 2nd cacheline in rte_pktmbuf_prefree_seg()

[Yongseok Koh <yskoh@mellanox.com>]

> On Feb 13, 2018, at 2:45 PM, Yongseok Koh <yskoh@mellanox.com> wrote:
> 
> Hi Olivier
> 
> I'm wondering why rte_pktmbuf_prefree_seg() checks m->next instead of
> m->nb_segs? As 'next' is in the 2nd cacheline, checking nb_segs seems beneficial
> to the cases where almost mbufs have single segment.
> 
> A customer reported high rate of cache misses in the code and I thought the
> following patch could be helpful. I haven't had them try it yet but just wanted
> to hear from you.
> 
> I'd appreciate if you can review this idea.
> 
> diff --git a/lib/librte_mbuf/rte_mbuf.h b/lib/librte_mbuf/rte_mbuf.h
> index 62740254d..96edbcb9e 100644
> --- a/lib/librte_mbuf/rte_mbuf.h
> +++ b/lib/librte_mbuf/rte_mbuf.h
> @@ -1398,7 +1398,7 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
>                if (RTE_MBUF_INDIRECT(m))
>                        rte_pktmbuf_detach(m);
> 
> -               if (m->next != NULL) {
> +               if (m->nb_segs > 1) {
>                        m->next = NULL;
>                        m->nb_segs = 1;
>                }
> @@ -1410,7 +1410,7 @@ rte_pktmbuf_prefree_seg(struct rte_mbuf *m)
>                if (RTE_MBUF_INDIRECT(m))
>                        rte_pktmbuf_detach(m);
> 
> -               if (m->next != NULL) {
> +               if (m->nb_segs > 1) {
>                        m->next = NULL;
>                        m->nb_segs = 1;
>                }

Well, m->pool in the 2nd cacheline has to be accessed anyway in order to put it back to the mempool.
It looks like the cache miss is unavoidable.

Thanks
Yongseok