Hi, Derek.Thanks for detailed answer.I'm using ubuntu 14.04 and kernel version
4.2.0-42-generic and USRP X300 with two UBX-160 which is connected to the host
PC via 1G ethernet.In my application, the lowest delta value that works
reliably is 20ms. The definition of reliability means that there is no 'L' for
one minute during the execution of the application.I want the delta value to be
3~4 ms.so, since the problem is latency in OS side, I cannot improve the delta
value even if I upgrade my host PC to good one. Am I right?the number of thread
in the application may affect the latency? My application use two thread only.
Since there are 4 cores in my host PC, I believe it barely affects the
latency.To sum it up, to achieve 3~4 ms delta value (based on your delta
value(5~20ms), I believe 3~4ms delta can be achieved), I should change the OS
in the host PC to low latency linux kernel.Please correct me if I'm wrong.
Thanks again!
-----------------------Original Message-----------------------From: Derek Kozel
<derek.ko...@ettus.com>To: Hojoon Yang <omnibu...@kaist.ac.kr>CC: usrp-users
<usrp-users@lists.ettus.com>Sent date: 2018-07-05 22:11:03 GMT +0900
(ROK)Subject: Re: [USRP-users] timed transmission LATE issue
Hello Hojoon,
You will have to profile your system to determine the minimum delta that
works reliably on your system, as well as choosing a definition for reliable.
Networking stacks nearly always have stochastic delays due to the various
context switches and scheduling issues with transferring data to and from a
user program and the actual network hardware. On the USRP side the FPGA
provides a much more deterministic behavior because the logic is exclusively
serving the network traffic. The DRAM buffer on the transmit side is there to
absorb some of the variability of the host computer network stack, but it can
only do that if there is enough time delay between samples being sent by the
program and when the DAC must be producing the associated analog signal.
In practice I have always used a few (5-20) ms and found it to be reliable
for transmissions lasting up to an hour, but you will have to test your
specific setup. Different OS network stacks have different options for tuning
to reduce both the average and peak latency of the network drivers. I'm not an
expert in them but there are a variety of good resources online. In
applications demanding both small latency and high reliability moving to a
realtime Linux environment can help in meeting those hard deadlines. I believe
that some of the LTE and GSM projects have been successful with that approach.
Regards,
Derek
On Thu, Jul 5, 2018 at 12:34 PM, Hojoon Yang via USRP-users
<usrp-users@lists.ettus.com> wrote:
Hi,
I am using USRP X310 with two UBX-160 daughterboards.
In my application, I am using timed transmission, for example,
start transmission at (usrp_get_time_now() + delta)
My question is, what is the smallest delta value that can be successful in
the *real world*? Theoretically, the delta can be any value, if the host can
send the packet to the USRP fast enough.
Suppose we have a PC which have i7-8700k CPU and the PC only runs this
application. when I set the delta 1ms, Can I avoid late 'L'?
Best regards,
Hojoon
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