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

On 2026-04-24 17:27, Marcus Müller wrote: > 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 Gah!  I should have paid more attention to the filenames. Another parameter that can be tweaked in the device arguments, num_send_frames, is described here, and MIGHT help (or might not): https://files.ettus.com/manual/page_transport.html > > 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 >> > >