RFNoC Frequently Asked Questions

Configuring the Stream Endpoint Buffer Size in RFNoC

What is the SEP buffer size?

Each stream endpoint (SEP) has an ingress buffer to store data received from others stream endpoints. This size of this buffer affects the data transfer rate that can be achieved when streaming to that endpoint. A larger ingress buffer in the stream endpoint means that there is more space to put data, minimizing idle time on the network. Additionally, streamers can queue up data before it is needed, reducing the chance of a buffer underflow.

How do I set the SEP buffer size?

The stream endpoint buffer size is set by adding a parameter under the endpoint you want to configure in the RFNoC image core YAML file. There are two parameters you can use to set the stream endpoint ingress buffer size in your RFNoC image core YAML file.

• buff_size: Buffer size in CHDR words. The size in bytes depends on the CHDR width. For example, if the chdr_width parameter for the device is 64, then each CHDR word is 8 bytes. So a buff size of 32768 would be 262,144 bytes or 256 KiB. See here for an example.
• buff_size_bytes:  Buffer size in bytes. See here for an example.

To what value should I set the SEP buffer size?

The buffer size should be a power of two in size to make optimal use of FPGA RAM resources. The default FPGA bitstreams typically set them to the largest size the FPGA can fit in order to maximize performance. Here are some general recommendations:

• Set to 0 if you don't need to send data to that SEP.
• Set to 8192 bytes (8 KiB = 1 MTU) minimum in order to stream data packets.
• Set to 32768 bytes (32 KiB = 4 MTU) in order to stream at maximum rates between SEPs on the same FPGA.
• Set to 262144 bytes (256 KiB = 32 MTU) or lager for high performance streaming between a host computer and the FPGA.

Note that the requirements are application-dependent, so optimal sizes for your application may be different. MTU refers to the maximum transmission unit, which is the largest CHDR packet supported by the FPGA.

If you need to free up FPGA resources (particularly block RAM) for your application, you can reduce the SEP buffer sizes. Just keep in mind that the maximum streaming rate may be affected.