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diff --git a/host/docs/transport.rst b/host/docs/transport.rst new file mode 100644 index 000000000..2371d2497 --- /dev/null +++ b/host/docs/transport.rst @@ -0,0 +1,98 @@ +======================================================================== +UHD - Transport Application Notes +======================================================================== + +.. contents:: Table of Contents + +------------------------------------------------------------------------ +Introduction +------------------------------------------------------------------------ +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: + +------------------------------------------------------------------------ +UDP transport (sockets) +------------------------------------------------------------------------ +The UDP transport is implemented with standard user-space/Berkeley sockets. + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Transport parameters +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +The following parameters can be used to alter the transport's default behavior: + +* **recv_frame_size:** The size of a single receive buffer in bytes +* **num_recv_frames:** The number of receive buffers to allocate +* **send_frame_size:** The size of a single send buffer in bytes +* **num_send_frames:** The number of send buffers to allocate + +**Note1:** num_recv_frames and num_send_frames do not affect performance. + +**Note2:** 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. + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Flow control parameters +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +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: + +* **ups_per_fifo:** The number of update packets for each FIFO's worth of bytes sent into the device +* **ups_per_sec:** The number of update packets per second (disabled by default) + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Resize socket buffers +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +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: + +* **recv_buff_size:** The desired size of the receive buffer in bytes +* **send_buff_size:** The desired size of the send buffer in bytes + +**Note:** Large send buffers tend to decrease transmit performance. + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Linux specific notes +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +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* + +------------------------------------------------------------------------ +USB transport (libusb) +------------------------------------------------------------------------ +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. + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Transport parameters +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +The following parameters can be used to alter the transport's default behavior: + +* **recv_frame_size:** The size of a single receive transfers in bytes +* **num_recv_frames:** The number of simultaneous receive transfers +* **send_frame_size:** The size of a single send transfers in bytes +* **num_send_frames:** The number of simultaneous send transfers +* **concurrency_hint:** The number of threads to run the event handler |