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

[Ananyev, Konstantin]

> -----Original Message-----
> From: dev [mailto:dev-bounces at dpdk.org] On Behalf Of Ananyev, Konstantin
> Sent: Wednesday, February 14, 2018 11:48 AM
> To: Yongseok Koh <yskoh at mellanox.com>; Olivier Matz <olivier.matz at 6wind.com>
> Cc: dev at dpdk.org
> Subject: Re: [dpdk-dev] Accessing 2nd cacheline in rte_pktmbuf_prefree_seg()
> 
> Hi Yongseok,
> 
> > > On Feb 13, 2018, at 2:45 PM, Yongseok Koh <yskoh at 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.
> 
> As a thought: in theory PMD can store pool pointer together with each mbuf it has to free,
> then it could be something like:
> 
> if (rte_pktmbuf_prefree_seg(m[x] != NULL)
>    rte_mempool_put(pool[x], m[x]);
> 
> Then what you suggested above might help.

After another thought - we have to check m->next not m->nb_segs.
There could be a situations where nb_segs==1, but m->next != NULL
(2-nd segment of the 3 segment packet for example).
So probably we have to keep it as it is.
Sorry for the noise
Konstantin