Just to be clear and to complete the previous answers. It happens to me
same problem, i.e., the fft GUI is blocked (shadowed in grey color and
responsive). In my case, the problem occurs when pressing the “Peak Hold
value” when using a Q6600 @ 2.40GHz with 4 GB RAM and NVIDIA GeForce
SE/7200 GS. I checked it with UN210 and USRP1. Also, under a slower
computer (core duo and 2 gb ram) the problem is increased. However, in
cases the CPU use is not at 100 %.
If i execute this command line the problem seems to dissapear (checked
the faster computer and with XCRV daughterboard).
uhd_fft.py -s 1M --fft-rate 15 -f 2.4123G --averaging
Hope it helps,
PS: i have installed the latest UHD version from the master branch.