> -----Original Message----- > From: dev [mailto:dev-boun...@dpdk.org] On Behalf Of Ananyev, Konstantin > Sent: Wednesday, February 14, 2018 11:48 AM > To: Yongseok Koh <ys...@mellanox.com>; Olivier Matz <olivier.m...@6wind.com> > Cc: dev@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 <ys...@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
