In case with syn flood you should take into account return syn-ack traffic, which generates PCIe DLLP's from NIC to host, thus pcie bandwith exceeds faster. And don't forget about DLLP's generated by rx traffic, which saturates host-to-NIC bus.
2015-07-01 16:05 GMT+03:00 Pavel Odintsov <pavel.odintsov at gmail.com>: > Yes, Bruce, we understand this. But we are working with huge SYN > attacks processing and they are 64byte only :( > > On Wed, Jul 1, 2015 at 3:59 PM, Bruce Richardson > <bruce.richardson at intel.com> wrote: > > 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 at 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 > >> > > >> > > > > > -- > Sincerely yours, Pavel Odintsov >