Re: [dpdk-dev] Could not achieve wire speed for 40GE with any DPDK version on XL710 NIC's

[Bruce Richardson <bruce.richardson@intel.com>]

On Wed, Jul 01, 2015 at 03:44:57PM +0300, Pavel Odintsov wrote:
> Thanks for answer, Vladimir! So we need look for x16 NIC if we want
> achieve 40GE line rate...
> 
Note that this would only apply for your minimal i.e. 64-byte, packet sizes.
Once you go up to larger e.g. 128B packets, your PCI bandwidth requirements
are lower and you can easier achieve line rate.

/Bruce

> On Wed, Jul 1, 2015 at 3:06 PM, Vladimir Medvedkin <medvedkinv@gmail.com> wrote:
> > Hi Pavel,
> >
> > Looks like you ran into pcie bottleneck. So let's calculate xl710 rx only
> > case.
> > Assume we have 32byte descriptors (if we want more offload).
> > DMA makes one pcie transaction with packet payload, one descriptor writeback
> > and one memory request for free descriptors for every 4 packets. For
> > Transaction Layer Packet (TLP) there is 30 bytes overhead (4 PHY + 6 DLL +
> > 16 header + 4 ECRC). So for 1 rx packet dma sends 30 + 64(packet itself) +
> > 30 + 32 (writeback descriptor) + (16 / 4) (read request for new
> > descriptors). Note that we do not take into account PCIe ACK/NACK/FC Update
> > DLLP. So we have 160 bytes per packet. One lane PCIe 3.0 transmits 1 byte in
> > 1 ns, so x8 transmits 8 bytes  in 1 ns. 1 packet transmits in 20 ns.  Thus
> > in theory pcie 3.0 x8 may transfer not more than 50mpps.
> > Correct me if I'm wrong.
> >
> > Regards,
> > Vladimir
> >
> >