Re: USRP B210 Dual-Channel 30.72 Msps: Underflows in GRC, but benchmark_rate works fine

Dear Yunxi Chen, UHD's benchmark rate doesn't try to read data from a file, copy it through buffers and put it on the line; it's really a lower CPU load problem than what you showed here. Still, what you want typically works. You would want to make sure you have enabled "realtime scheduling" in the "Options" (pseudo-)Block in GRC. Other than that, hah! I didn't realize that /dev/shm is still a thing :) nowadays, it's just a regular tmpfs mount, so make sure your two files actually fit fully in your RAM, and you're not "Swapping" RAM to disk. 2 files of 10 ms of 30.72 MSam/s with each sample being 8 B, that's 2 · 10·10⁻³ s · 30.72·10⁶ Sam/s · 8 B/Sam = 4915200 B = 4.7 MB, so that's probably not the problem, unless you have much other things in RAM. Check with `free -m` how many megabytes are currently "used" in "Swap". Best regards, Marcus On 2026-04-24 4:33 PM, Yunxi Chen wrote: > Hi, > > I am working on a 5G PRS localization project and encountering persistent Underflow issues > when attempting to transmit at 30.72 Msps using both channels on a USRP B210. > > *The Setup:* > > * *Hardware:*USRP B210 connected via USB 3.0. > * *Host PC:*Dell G15 Laptop (13th Gen Intel(R) Core(TM) i7-13650HX (20 cores), 64 GB RAM). > * *OS:*Ubuntu 22.04. > * *Software:*GNU Radio 3.10.7.0, UHD 4.1.0.5. > > *The Experiment:*I am transmitting a 5G PRS signal (mu=1, 30kHz SCS) with a slot-level TDM > pattern (alternating between Ant A and Ant B). The frames are pre-calculated and zero- > padded to exactly 307,200 samples (10ms @ 30.72 MHz) to ensure perfect timing alignment. > > *The Flowgraph:* > > *The Problem:*Despite these optimizations, I get immediate and constant Underflows. > However, when I run the official UHD benchmark tool: |benchmark_rate --args="type=b200" -- > channels="0,1" --tx_rate 30.72e6 | It completes successfully without a single dropped packet. > > *My Questions:*Since |benchmark_rate| works, it seems the hardware/USB controller is > physically capable of this rate. Why does the GNU Radio flowgraph fail even with a simple > File Source -> USRP Sink architecture? Are there specific buffer or scheduler tunings in > GRC that I am missing to handle 2x2 MIMO at the 30.72 Msps limit? > > Any advice or insights would be greatly appreciated. > > Best regards, > > Yunxi Chen >