I've upgraded to 3.5rc0. The same thing happened. I got some more details:
When I ran benchmark_tx on 1 machine, at low bitrate (0.1Mbps or 0.2MSps - I'm using bpsk) the CPU utilization is roughly 9%.
But the receiver, running benchmark_rx showed 110% CPU utilization.
the transmitter showed 90% CPU utilization so it scaled linearly.
the receiver starts out showing 110% CPU utilization, so I guess it processed noise also, which makes sense because there's no carrier-sense. It did nothing for a while (~30 sec), then started showing correct receptions. Until a point, it started to show overrun. Here's part of the output on the receiver:
ok = True pktno = 859 n_rcvd = 859 n_right = 859
ok = True pktno = 860 n_rcvd = 860 n_right = 860
ok = True pktno = 861 n_rcvd = 861 n_right = 861
ok = True pktno = 862 n_rcvd = 862 n_right = 862
ok = True pktno = 863 n_rcvd = 863 n_right = 863
ok = True pktno = 864 n_rcvd = 864 n_right = 864
ok = True pktno = 865 n_rcvd = 865 n_right = 865
ok = True pktno = 866 n_rcvd = 866 n_right = 866
ok = True pktno = 867 n_rcvd = 867 n_right = 867
Ook = True pktno = 868 n_rcvd = 868 n_right = 868
OOOok = False pktno = 869 n_rcvd = 869 n_right = 868
OOOOOOOok = False pktno = 879 n_rcvd = 870 n_right = 868
OOOOOOOOOok = False pktno = 902 n_rcvd = 871 n_right = 868
OOOOOOOok = False pktno = 914 n_rcvd = 872 n_right = 868
OOOOOOOOok = False pktno = 929 n_rcvd = 873 n_right = 868
OOOOOOOOOOOok = False pktno = 950 n_rcvd = 874 n_right = 868
OOOOOOOOOOOOOOOok = False pktno = 970 n_rcvd = 875 n_right = 868
OOOOOOOOOOOOOOOOOOOOOOOOOOok = False pktno = 983 n_rcvd = 876 n_right = 868
OOOOOOOok = False pktno = 987 n_rcvd = 877 n_right = 868
OOOOOOok = False pktno = 990 n_rcvd = 878 n_right = 868
OOOOOOok = False pktno = 993 n_rcvd = 879 n_right = 868
OOOOOOok = False pktno = 996 n_rcvd = 880 n_right = 868
Another thing is when I started the transmitter first, then the receiver started to show received samples right away. If I started the receiver first, then it had the 30 sec delay mentioned above.
Before I used to run these code on the old gnuradio version (3.2.2) and Ethernet driver and didn't have this problem. Could this be related to the new implementation of gnuradio and/or the UHD driver? Or is there any other possible explanation?