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 - TransferPCIwhere:
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.
Example
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