[USRP-users] X3x0 PCIe ExpressCard benchmark_rate transmit test rate limit

Stan Vitebsky svite at lgsinnovations.com
Wed Sep 17 09:52:37 EDT 2014


I am trying to setup communication with X300 using ExpressCard interface 
from a laptop running CentOS 6 (kernel version 
3.10.33-rt32.45el6rt.x86_64, real-time patch from RedHat MRG).
I am running uhd v3.7.2-49 and PCIe interface from Ettus based on NI 
The PCIe bridge is configured with x1 Width and ASPM is disabled.
The tests were executed both with performance governor (powersave 
disabled) and with cpuspeed service completely disabled with the same 
The tests were reproduced on two different X300s (but with the same 
host, uhd, cable and ExpressCard) with the same results.
There is no problem in setting up and communicating with X300 using both 
1G Ethernet and ExpressCard (i.e. uhd_usrp_probe works fine for both 

However, when I try to use benchmark_rate program supplied with uhd, 
there are significant differences:

  - For 1GE interface, the benchmark transmit test runs fine and an 
expected amount of data is transmitted (~125Ms for 12.5Msps over 10s) 
without any underruns

  - For ExpressCard, any transmit test with a sampling rate >~ 1.4Msps 
allows only about 14.5Ms over 10s with 98-99 overruns (i.e. the transmit 
rate is always limited to about 1.4Msps)
The result is the same for different data rates from 1.4Msps to 25Msps 
(including 12.5Msps case above)

The laptop has plenty of CPU resources  (Xeon 2690 v2 10 core) and CPU 
utilization on any core never exceeds 15% during the test.
The Rx benchmark test happily executes up to 25Msps using the same 
ExpressCard with no problem

I would really appreciate any suggestions on how to get some further 
insight into the root cause of this limitation.

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