Table of Contents
A transport is the layer between the packet interface and a device IO interface. The advanced user can pass optional parameters into the underlying transport layer through the device address. These optional parameters control how the transport object allocates memory, resizes kernel buffers, spawns threads, etc. When not spcified, the transport layer will use values for these parameters that are known to perform well on a variety of systems. The transport parameters are defined below for the various transports in the UHD:
The UDP transport is implemented with user-space sockets:
The following parameters can be used to alter the transport's default behavior:
Note1: num_recv_frames does not affect performance (all platforms).
Note2: num_send_frames does not affect performance (UNIX only).
Note3: recv_frame_size and send_frame_size can be used to increase or decrease the maximum number of samples per packet. The frame sizes default to an MTU of 1472 bytes per IP/UDP packet, and may be increased if permitted by your network hardware.
The host-based flow control expects periodic update packets from the device. These update packets inform the host of the last packet consumed by the device, which allows the host to determine throttling conditions for the transmission of packets. The following mechanisms affect the transmission of periodic update packets:
It may be useful increase the size of the socket buffers to move the burden of buffering samples into the kernel, or to buffer incoming samples faster than they can be processed. However, if your application cannot process samples fast enough, no amount of buffering can save you. The following parameters can be used to alter socket's buffer sizes:
Note: Large send buffers tend to decrease transmit performance.
Latency is a measurement of the time it takes a sample to travel between the host and device. Most computer hardware and software is bandwidth optimized which may negatively affect latency. If your application has strict latency requirements, please consider the following notes:
Note1: The time taken by the device to populate a packet is proportional to the sample rate. Therefore, to improve receive latency, configure the transport for a smaller frame size.
Note2: For overall latency improvements, look for "Interrupt Coalescing" settings for your OS and ethernet chipset. It seems the Intel ethernet chipsets offer fine-grained control in Linux. Also, consult:
On linux, the maximum buffer sizes are capped by the sysctl values net.core.rmem_max and net.core.wmem_max. To change the maximum values, run the following commands:
sudo sysctl -w net.core.rmem_max=<new value> sudo sysctl -w net.core.wmem_max=<new value>
Set the values permanently by editing /etc/sysctl.conf
On Windows, it is important to change the default UDP behavior such that 1500 byte packets still travel through the fast path of the sockets stack. FastSendDatagramThreshold registry key to change documented here:
The USB transport is implemented with libusb. Libusb provides an asynchronous API for USB bulk transfers. The transport implementation allocates a number of buffers and submits asynchronous requests through libusb. Event handler threads run in the background to process these requests.
The following parameters can be used to alter the transport's default behavior: