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From: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>
To: "Ananyev, Konstantin" <konstantin.ananyev@intel.com>,
	Yongseok Koh <yskoh@mellanox.com>,
	Olivier Matz <olivier.matz@6wind.com>
Cc: "dev@dpdk.org" <dev@dpdk.org>
Subject: Re: [dpdk-dev] Accessing 2nd cacheline in rte_pktmbuf_prefree_seg()
Date: Wed, 14 Feb 2018 12:03:55 +0000	[thread overview]
Message-ID: <2601191342CEEE43887BDE71AB97725890572EC6@irsmsx105.ger.corp.intel.com> (raw)
In-Reply-To: <2601191342CEEE43887BDE71AB97725890572EA2@irsmsx105.ger.corp.intel.com>



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

  reply	other threads:[~2018-02-14 12:03 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-02-13 22:45 Yongseok Koh
2018-02-14  3:16 ` Yongseok Koh
2018-02-14 11:48   ` Ananyev, Konstantin
2018-02-14 12:03     ` Ananyev, Konstantin [this message]
2018-02-14 12:11       ` Bruce Richardson
2018-02-14 12:35         ` Ananyev, Konstantin
2018-02-14 14:16           ` Ananyev, Konstantin

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