> -----Original Message----- > From: Richardson, Bruce > Sent: Wednesday, February 14, 2018 12:12 PM > To: Ananyev, Konstantin <konstantin.anan...@intel.com> > Cc: Yongseok Koh <ys...@mellanox.com>; Olivier Matz <olivier.m...@6wind.com>; > dev@dpdk.org > Subject: Re: [dpdk-dev] Accessing 2nd cacheline in rte_pktmbuf_prefree_seg() > > On Wed, Feb 14, 2018 at 12:03:55PM +0000, Ananyev, Konstantin wrote: > > > > > > > -----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 > > 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.
In multi-seg case PMD frees segments (not packets). It could happen that first segment would be already freed while the second still not. > It means that your idea of storing the pool pointer for each > mbuf becomes useful for single-segment packets. But then we'll have to support 2 different flavors of prefree_seg(). Alternative would be to change all PMDs multi-seg TX so when first segment is going to be freed we update nb_segs for the second and so on. Both options seems like too much hassle. Konstantin
