Source use of the time.sleep function causes USRP 'U'

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Hi,

     I'm using USRP to send and receive data, with two host PCs and two USRPs as senders and receivers respectively. I am using the ofdm example in GNURadio. The source of the file on the sending side is an out-of-tree module I wrote myself, which will output N sets of data, and I want to output the next set of data after a 10-second interval after each set of data, so I use the "time.sleep(10)" function, at this time the problem occurs as follows.

     1??Except for the first group of data which was received normally, all other groups of data were lost??Terminal print "U"??underrun??PC not providing data quickly??

     2??When I remove the "time.sleep" statement, all data is received normally

     Why does this happen? How can I fix it?

     Thanks in advance??

Best,

Regards.