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

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

[Yongseok Koh]

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;
                }

Thanks,
Yongseok

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

[Yongseok Koh]

> 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.

Thanks
Yongseok

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

[Ananyev, Konstantin]

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.
Konstantin

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

[Ananyev, Konstantin]

> -----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

Re: [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@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

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

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

[Ananyev, Konstantin]

> -----Original Message-----
> From: Richardson, Bruce
> Sent: Wednesday, February 14, 2018 12:12 PM
> To: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> Cc: Yongseok Koh <yskoh@mellanox.com>; Olivier Matz <olivier.matz@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-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
> 
> 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

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

[Ananyev, Konstantin]

> -----Original Message-----
> From: dev [mailto:dev-bounces@dpdk.org] On Behalf Of Ananyev, Konstantin
> Sent: Wednesday, February 14, 2018 12:35 PM
> To: Richardson, Bruce <bruce.richardson@intel.com>
> Cc: Yongseok Koh <yskoh@mellanox.com>; Olivier Matz <olivier.matz@6wind.com>; dev@dpdk.org
> Subject: Re: [dpdk-dev] Accessing 2nd cacheline in rte_pktmbuf_prefree_seg()
> 
> 
> 
> > -----Original Message-----
> > From: Richardson, Bruce
> > Sent: Wednesday, February 14, 2018 12:12 PM
> > To: Ananyev, Konstantin <konstantin.ananyev@intel.com>
> > Cc: Yongseok Koh <yskoh@mellanox.com>; Olivier Matz <olivier.matz@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-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
> >
> > 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.
> 

As  a side thought what probably can be  done to minimize access
to 2-nd mbuf's cache line at PMD tx free:
Introduce something like that:
static __rte_always_inline struct rte_mepool *
xxx_prefree_seg(struct rte_mbuf *m)
{
        if (rte_mbuf_refcnt_read(m) == 1 && RTE_MBUF_DIRECT(m)) {
                if (m->next != NULL) {
                        m->next = NULL;
                        m->nb_segs = 1;
                }
                return m->pool;
       }
       return NULL;
}

Then at tx_burst() before doing actual TX PMD can call that function
and store it's return value along with mbuf:
..
m[x] = pkt;
pool[x] = xxx_prefree_seg(m[x]);

Then at free time, we can do something ilike:
If (pool[x] != NULL) 
   rte_mempool_put(pool[x], m[x]);
else
    rte_pktmbuf_free_seg(m[x]);

We still access m->next but doing that before actual TX is done.
Hopefully there would be more chances that m->next
is still in the cache at that moment.
In theory, that might help for most common case when
we have direct mbufs with refcnt==1.
Though for indirect/refcnt>1 mbufs there would be extra overhead.
Konstantin