> Commit dfaf00e started using the result of dpdk_buf_size() to calculate
> the available size on each mbuf, as opposed to using the previous
> MBUF_SIZE macro. However, this was calculating the mbuf size by adding up
> the MTU with RTE_PKTMBUF_HEADROOM and only then aligning to
> NETDEV_DPDK_MBUF_ALIGN. Instead, the accounting for the
> RTE_PKTMBUF_HEADROOM should only happen after alignment, as per below.
>
> Before alignment:
> ROUNDUP(MTU(1500) + RTE_PKTMBUF_HEADROOM(128), 1024) = 2048
>
> After aligment:
> ROUNDUP(MTU(1500), 1024) + 128 = 2176
>
> This might seem insignificant, however, it might have performance
> implications in DPDK, where each mbuf is expected to have 2k +
> RTE_PKTMBUF_HEADROOM of available space. This is because not only some
> NICs have course grained alignments of 1k, they will also take
> RTE_PKTMBUF_HEADROOM bytes from the overall available space in an mbuf
> when setting up their Rx requirements. Thus, only the "After alignment"
> case above would guarantee a 2k of available room, as the "Before
> alignment" would report only 1920B.
>
> Some extra information can be found at:
> https://mails.dpdk.org/archives/dev/2018-November/119219.html
>
> Note: This has been found by Ian Stokes while going through some af_packet
> checks.
>
Hi Tiago,, thanks for this, just a query below.
> Reported-by: Ian Stokes <ian.sto...@intel.com>
> Fixes: dfaf00e ("netdev-dpdk: fix mbuf sizing")
> Signed-off-by: Tiago Lam <tiago....@intel.com>
> ---
> Documentation/topics/dpdk/memory.rst | 20 ++++++++++----------
> lib/netdev-dpdk.c | 6 ++++--
> 2 files changed, 14 insertions(+), 12 deletions(-)
>
> diff --git a/Documentation/topics/dpdk/memory.rst
> b/Documentation/topics/dpdk/memory.rst
> index c9b739f..3c4ee17 100644
> --- a/Documentation/topics/dpdk/memory.rst
> +++ b/Documentation/topics/dpdk/memory.rst
> @@ -107,8 +107,8 @@ Example 1
>
> MTU = 1500 Bytes
> Number of mbufs = 262144
> - Mbuf size = 2752 Bytes
> - Memory required = 262144 * 2752 = 721 MB
> + Mbuf size = 2880 Bytes
> + Memory required = 262144 * 2880 = 755 MB
These measurements don't deem to take the header and trailer into account for
total size when calculating the memory requirements? Is there any particular
reason?
total_elt_sz = mp->header_size + mp->elt_size + mp->trailer_size;
I would expect above to be 262144 * 3008 = 788 MB.
Ian
>
> Example 2
> +++++++++
> @@ -116,8 +116,8 @@ Example 2
>
> MTU = 1800 Bytes
> Number of mbufs = 262144
> - Mbuf size = 2752 Bytes
> - Memory required = 262144 * 2752 = 721 MB
> + Mbuf size = 2880 Bytes
> + Memory required = 262144 * 2880 = 755 MB
>
> .. note::
>
> @@ -130,8 +130,8 @@ Example 3
>
> MTU = 6000 Bytes
> Number of mbufs = 262144
> - Mbuf size = 8000 Bytes
> - Memory required = 262144 * 8000 = 2097 MB
> + Mbuf size = 6976 Bytes
> + Memory required = 262144 * 6976 = 1829 MB
>
> Example 4
> +++++++++
> @@ -194,8 +194,8 @@ Example 1: (1 rxq, 1 PMD, 1500 MTU)
>
> MTU = 1500
> Number of mbufs = (1 * 2048) + (2 * 2048) + (1 * 32) + (16384) = 22560
> - Mbuf size = 2752 Bytes
> - Memory required = 22560 * 2752 = 62 MB
> + Mbuf size = 2880 Bytes
> + Memory required = 22560 * 2880 = 65 MB
>
> Example 2: (1 rxq, 2 PMD, 6000 MTU)
> +++++++++++++++++++++++++++++++++++
> @@ -203,8 +203,8 @@ Example 2: (1 rxq, 2 PMD, 6000 MTU)
>
> MTU = 6000
> Number of mbufs = (1 * 2048) + (3 * 2048) + (1 * 32) + (16384) = 24608
> - Mbuf size = 8000 Bytes
> - Memory required = 24608 * 8000 = 196 MB
> + Mbuf size = 6976 Bytes
> + Memory required = 24608 * 6976 = 171 MB
>
> Example 3: (2 rxq, 2 PMD, 9000 MTU)
> +++++++++++++++++++++++++++++++++++
> diff --git a/lib/netdev-dpdk.c b/lib/netdev-dpdk.c index e8618a6..a871743
> 100644
> --- a/lib/netdev-dpdk.c
> +++ b/lib/netdev-dpdk.c
> @@ -521,8 +521,8 @@ is_dpdk_class(const struct netdev_class *class)
> static uint32_t dpdk_buf_size(int mtu) {
> - return ROUND_UP((MTU_TO_MAX_FRAME_LEN(mtu) + RTE_PKTMBUF_HEADROOM),
> - NETDEV_DPDK_MBUF_ALIGN);
> + return ROUND_UP(MTU_TO_MAX_FRAME_LEN(mtu), NETDEV_DPDK_MBUF_ALIGN)
> + + RTE_PKTMBUF_HEADROOM;
> }
>
> /* Allocates an area of 'sz' bytes from DPDK. The memory is zero'ed.
> @@ -681,6 +681,8 @@ dpdk_mp_create(struct netdev_dpdk *dev, int mtu, bool
> per_port_mp)
> dev->requested_n_rxq, dev->requested_n_txq,
> RTE_CACHE_LINE_SIZE);
>
> + /* The size of the mbuf's private area (i.e. area that holds OvS'
> + * dp_packet data)*/
> mbuf_priv_data_len = sizeof(struct dp_packet) -
> sizeof(struct rte_mbuf);
> /* The size of the entire dp_packet. */
> --
> 2.7.4
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