> From: Varghese, Vipin [mailto:vipin.vargh...@amd.com]
> Sent: Friday, 27 June 2025 14.07
>
> [Public]
>
> Hi Morten,
>
> We have tested the effect of the patch using func-latency and PPs via
> testpmd.
> Please find our observations below
>
> - DPDK tag: 25.07-rc1
> - compiler: gcc 14.2
> - platform: AMD EPYC 8534P 64core 2.3GHz
> - app cmd:
> -- One port: ` sudo build/app/dpdk-testpmd -l 15,16 --vdev=net_null1 -
> -no-pci -- --nb-cores=1 --nb-ports=1 --txq=1 --rxq=1 --txd=2048 --
> rxd=2048 -a --forward-mode=io --stats-period=1`
> -- Two port: ` sudo build/app/dpdk-testpmd -l 15,16,17 --
> vdev=net_null1 --vdev=net_null2 --no-pci -- --nb-cores=2 --nb-ports=2 -
> -txq=1 --rxq=1 --txd=2048 --rxd=2048 -a --forward-mode=io --stats-
> period=1`
>
> Result 1 port:
> - Before patch: TX MPPs 117.61, RX-PPs 117.67, Func-latency TX: 1918ns,
> Func-latency free-bulk: 2667ns
> - After patch: TX MPPs 117.55, RX-PPs 117.54, Func-latency TX: 1921ns,
> Func-latency free-bulk: 2660ns
>
> Result 2 port:
> - Before patch: TX MPPs 117.61, RX-PPs 117.67, Func-latency TX: 1942ns,
> Func-latency free-bulk: 2557ns
> - After patch: TX MPPs 117.54, RX-PPs 117.54, Func-latency TX: 1946ns,
> Func-latency free-bulk: 2740ns
>
> Perf Top: diff before vs after shows 13.84% vs 13.79%
>
> Reviewed-by: Thiyagarjan P <thiyagaraja...@amd.com>
> Tested-by: Vipin Varghese <vipin.vargh...@amd.com>
Thank you for reviewing and testing.
>
> Clarification request: `with fast-mbuf-free on single port we see free-
> bulk reduction by -7ns. But null_tx increase by +3ns. TX PPs reduction
> by 0.07 Mpps. Is this anomaly of null_net PMD?`
I have finally found the bug in my patch:
It announces device-level capability for FAST_FREE, but ignores device-level
FAST_FREE configuration, and uses queue-level FAST_FREE configuration instead.
Due to this bug, your testing probably shows the performance of the
non-FAST_FREE code path.
The added comparison for FAST_FREE (code path not taken) might explain the
null_tx +3ns increase.
I will send a v2 patch.
>
> > >
> > > On Tue, 24 Jun 2025 18:14:16 +0000
> > > Morten Brørup <m...@smartsharesystems.com> wrote:
> > >
> > > > Added fast mbuf release, re-using the existing mbuf pool pointer
> in
> > > > the queue structure.
> > > >
> > > > Signed-off-by: Morten Brørup <m...@smartsharesystems.com>
> > >
> > > Makes sense.
> > >
> > > > ---
> > > > drivers/net/null/rte_eth_null.c | 30 +++++++++++++++++++++++++++-
> --
> > > > 1 file changed, 27 insertions(+), 3 deletions(-)
> > > >
> > > > diff --git a/drivers/net/null/rte_eth_null.c
> > > b/drivers/net/null/rte_eth_null.c
> > > > index 8a9b74a03b..12c0d8d1ff 100644
> > > > --- a/drivers/net/null/rte_eth_null.c
> > > > +++ b/drivers/net/null/rte_eth_null.c
> > > > @@ -34,6 +34,17 @@ struct pmd_internals; struct null_queue {
> > > > struct pmd_internals *internals;
> > > >
> > > > + /**
> > > > + * For RX queue:
> > > > + * Mempool to allocate mbufs from.
> > > > + *
> > > > + * For TX queue:
> > > > + * Mempool to free mbufs to, if fast release of mbufs is
> enabled.
> > > > + * UINTPTR_MAX if the mempool for fast release of mbufs has
> not
> > > yet been detected.
> > > > + * NULL if fast release of mbufs is not enabled.
> > > > + *
> > > > + * @see RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE
> > > > + */
> > > > struct rte_mempool *mb_pool;
> > >
> > > Do all drivers to it this way?
> >
> > No, I think most drivers have separate structures for rx and tx
> queues. This driver
> > doesn't so I'm reusing the existing mempool pointer.
> > Also, they don't cache the mempool pointer, but look at mbuf[0].pool
> at every burst;
> > so their tx queue structure doesn't have a mempool pointer field.
> > And they check an offload flag (either the bit in the raw offload
> field, or a shadow
> > variable for the relevant offload flag), instead of checking the
> mempool pointer.
> >
> > Other drivers can be improved, and I have submitted an optimization
> patch for the
> > i40e driver with some of the things I do in this patch:
> > https://inbox.dpdk.org/dev/20250624061238.89259-1-
> > m...@smartsharesystems.com/
> >
> > > Is it documented in ethdev?
> >
> > The RTE_ETH_TX_OFFLOAD_MBUF_FAST_FREE flag is documented.
> > How to implement it in the drivers is not.
> >
> > -Morten
