How send message with when message is less than sample rate?

[sp]

When every 3 seconds I call msqq->inserttail, messageTempI is only 64 bytes
but the sample rate is 500khz...
I know that 64-byte string is more than 500khz samples....but usrp and uhd
in gnuradio send it....!!!
my question is how message is sent to USRP and how does USRP send it? when
samplerate is not completed?!!!
it waits to receive 500k samples or it sends it with fault...
Sending is correct? using message source in this structure true?

            message_source =
gr::blocks::message_source::make(sizeof(gr_complex));

            this->tb->connect(message_source, 0, multiplyControl, 0);

            this->tb->connect(multiplyControl, 0, this->sinkUHD, 0);

            //every 3 second i call insert tail....

            
message_source->msgq()->insert_tail(gr::message::make_from_string(messageTempI));