[USRP-users] Underflows problem but limited CPU usage

Michael West michael.west at ettus.com
Tue Dec 9 19:20:19 EST 2014

That sounds like an OS issue to me.  A few things to try:

- Enable Receive Side Scaling (if not already enabled)
- Increase the size of the receive socket buffer
- Disable all power management (may be throttling CPU cores)
- Use a single Ethernet connection (reduce the number of network interrupts)

Hardware interrupts trump all software, so changing priority will not
matter if there are more hardware interrupts to process than can be
processed (i.e. from the NIC and mouse).  A good test is to see how much
kernel activity there is on each core when shaking the console window and
running UHD.  If there is a concentrated kernel load on a single core, that
is a good indication that all interrupts are being handled by a single core
and that is why packet processing is being delayed.  Enabling Receive Side
Scaling will spread the network interrupts among the cores and increasing
the socket buffer will give more time to process the interrupts.

Michael E. West

On Tue, Dec 9, 2014 at 2:59 PM, Damien Serant via USRP-users <
usrp-users at lists.ettus.com> wrote:

> Hi list,
> I'm trying to push the USRPN200 to the limit in throughput and I
> experienced a quite strange underflow behavior.
> Here is my configuration :
>    - 2USRPN200 with Mimo cable, 25MHz and 8 bit on each with 2 Ethernet
>    to avoid saturation of the link
>    - Intel Xeon processor with 6 core (12 Threads)
>    - Windows 7 Pro 64 bit
> My UHD program is only reading samples without any processing (not even
> disk writing). If i do nothing else on the PC, the program work without any
> packet loss with a very limited CPU charge of about 5% (about 20% on two
> cores) according to the task manager and for at least 30 minutes (the
> longest test that i made). As soon as i start soliciting a bit the CPU
> (e.g. opening another application, or simply by moving the scroll bar of
> the console up and down or shaking the console window) underflows and
> packet losses arise. I tried to play with the process priority but the
> result is the same.
> The underflows are apparently due to an increased CPU charge but with only
> 5% of CPU usage for my UHD program i should not expect that, no ?
> Does anyone already experienced that ?
> Is there some tricks in the OS configuration or in UHD configuration to
> avoid that ?
> Thanks in advance,
> Damien
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> USRP-users at lists.ettus.com
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