I am also seeing this warning:
[WARNING] [RFNOC::GRAPH::DETAIL] Cannot forward action tx_event from
0/Radio#0:INPUT_EDGE:0, no neighbour found!
________________________________
From: Eugene Grayver <[email protected]>
Sent: Friday, March 6, 2026 1:17 PM
To: usrp-users <[email protected]>
Subject: X440 X4_200 image TX underflows
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I am trying to send out 16 channels of 40 Msps data from two X440s.
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Getting many underflows (~1/s or more)
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As far as I can tell the X440 are synchronized correctly
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Set time to 0 on next PPS (external)
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Start TX at a future time (5s) using a time_spec in TX metadata
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Each TX streamer is in a separate thread
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Each thread pinned to a different core
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CPU utilization is about 20%
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I am NOT reading anything from disk — just sending the same buffer over and over
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wmem_max size is 25 MB
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MTU is 9000
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UHD 4.9.0
What could be wrong?
Eugene Grayver, Ph.D.
Principal Engineer
310-336-1274
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