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

[Bruce Richardson]

On Wed, Feb 14, 2018 at 12:03:55PM +0000, Ananyev, Konstantin wrote:
> 
> 
> > -----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

It's still worth considering as an option. We could check nb_segs for
the first segment of a packet and thereafter iterate using the next
pointer. It means that your idea of storing the pool pointer for each
mbuf becomes useful for single-segment packets.

/Bruce