Hello --
I've got some questions re: latency with the x300 over the 10GigE
interface.
If I use the latency_test example operating at a rate of 50 MSPS, I have no
issues with a latency of 1ms. The latency test receives data, examines the
time stamp, and transmits a single packet.
I have an application where I'd like to run the transmitter continuously,
and I got curious about the latency involved in that operation. My
application is similar to the benchmark_rate example. I added the
following lines to the benchmark_rate example at line 256 after the line.
md.has_time_spec = false;
====
if ( (num_tx_samps % 50000000) < 4*max_samps_per_packet )
{
uhd::time_spec_t expectedTime = startTime + (double) ( num_tx_samps )
/ (double)usrp->get_tx_rate();
uhd::time_spec_t timeAtLog = usrp->get_time_now();
timeAtLog = usrp->get_time_now();
std::cerr << "==== Actual time ====" << std::endl;
std::cerr << " " << timeAtLog.get_full_secs() << " / "
<< timeAtLog.get_frac_secs() << std::endl;
std::cerr << "==== Expected time ====" << std::endl;
std::cerr << " " << expectedTime.get_full_secs() << " / "
<< expectedTime.get_frac_secs() << std::endl;
}
====
The intent of this insertion is to log the time at which we return from
tx_stream->send() and the time at which the first sample of that sent data
should be transmitted -- at approximately once per second when running at
50 MSPS.
After the first second, I consistently saw the following results:
==== Actual time ====
1 / 0.10517
==== Expected time ====
1 / 0.27253
==== Actual time ====
1 / 0.105419
==== Expected time ====
1 / 0.27255
Which indicates to me that there is a latency of approximately 167ms when
transmitting data. That is, the send() function is returning 167ms earlier
than I expect the data to actually be transmitted. If I halve the sample
rate to 25 MSPS, the latency doubles.
What is the source of the latency when running in a continuous mode?
Initially, I had thought that this might be related to the
send_buffer_size, but making changes to send_buffer_size seem to not have
an effect. FWIW, 167ms at 50 MSPS is suspiciously close to the value for
wmem_max (33554432) suggested in the x300 system configuration ... but
neither changing that value or send_buffer_size seems to make a difference.
Is this latency tunable?
Thank you for your help!
Best Regards,
Doug Blackburn
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