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Diffstat (limited to 'host/docs')
| -rw-r--r-- | host/docs/transport.rst | 42 |
1 files changed, 39 insertions, 3 deletions
diff --git a/host/docs/transport.rst b/host/docs/transport.rst index e7c2f1885..b601cd8ff 100644 --- a/host/docs/transport.rst +++ b/host/docs/transport.rst @@ -19,7 +19,10 @@ 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. +The UDP transport is implemented with user-space sockets: + +* **UNIX:** standard Berkeley sockets API using send()/recv() +* **Windows:** Windows Sockets API (WSA) using overlapped IO ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Transport parameters @@ -31,9 +34,14 @@ The following parameters can be used to alter the transport's default behavior: * **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. +**Note1:** +num_recv_frames does not affect performance (all platforms). + +**Note2:** +num_send_frames does not affect performance (UNIX only). -**Note2:** recv_frame_size and send_frame_size can be used to +**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. @@ -65,6 +73,25 @@ The following parameters can be used to alter socket's buffer sizes: **Note:** Large send buffers tend to decrease transmit performance. ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Latency Optimization +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +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: + +* http://publib.boulder.ibm.com/infocenter/pseries/v5r3/index.jsp?topic=/com.ibm.aix.prftungd/doc/prftungd/interrupt_coal.htm + +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ Linux specific notes ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ On linux, the maximum buffer sizes are capped by the sysctl values @@ -77,6 +104,15 @@ To change the maximum values, run the following commands: Set the values permanently by editing */etc/sysctl.conf* +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +Windows specific notes +^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ +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: + +* http://www.microsoft.com/windows/windowsmedia/howto/articles/optimize_web.aspx#appendix_e + ------------------------------------------------------------------------ USB transport (libusb) ------------------------------------------------------------------------ |