Shaping based on priority
The bandwidth available for transfer of frames from the SDRAM and into the host memory is allocated among the host buffers according to the greatest need.
Time to fill the SDRAM
If the capture load exceeds the transfer over the PCIe bus, the SDRAM will fill up according to this equation:
SizeSDRAM × 8 × F tfull = ––––––––––––––––––––––– for Loadcapture > TransferPCI Loadcapture - TransferPCI
tfull is the time in seconds to fill the SDRAM.
SizeSDRAM is the size of the SDRAM in GB.
F is a factor normally close to 1 that reflects the dependency on number of host buffers, length of packet descriptor used and packet size.
Loadcapture is the network capture load in Gbit/s.
TransferPCI is the data transfer in Gbit/s over the PCIe bus.
With a packet descriptor of 20 bytes, which equals the Preamble, Start Frame Delimiter and minimum Inter-Frame Gap fields of an Ethernet frame, the factor F only depends on the number of host buffers, getting slightly smaller than 1 for high numbers of host buffers. A packet descriptor shorter than 20 bytes will increase F slightly especially for short packets while a packet descriptor longer than 20 bytes will decrease F also especially for short packets.
As an example, consider an NT200A02 running at 2 × 100 Gbit/s and with full capture load with 300-byte packets, using a standard packet descriptor and transferring 100 Gbit/s of data over the PCIe bus:
The size of the SDRAM is 12.288 GB. There is no considerable host buffer contribution to the factor F, and the contribution from the packet descriptor is approximately 320/316. This means that the host buffer runs full in:
12.288 × 8 × 1.01 Gbit ––––––––––––––––––––––--– = 1.0 s 200 Gbit/s - 100 Gbit/s