Dear SangHyuk,
On 07.04.2016 08:25, SangHyuk Kim
wrote:
Hi all,
I'm trying to solve Rx overflow problem.
I am using USRP N210 and CBX daughter board. As I know,
my machine's maximum sample rate is up to 25 MSps.
No, that is the maximum number of 16bit complex samples you can get
over Gigabit ethernet; I think I've done the math before, but here
it goes again :)
and because the USRP can only give you integer fractions of its
master clock rate 100MHz as sampling rate, 25MS/s is the maximum you
get.
Yes, it works well at Tx (ex ./benchmark -f 2.5G -r
25000000).
However, when I use USRP to Rx mode (ex ./benchmark -f
2.5G -r 25000000), letter 'D' (overflow) be occurred.
Rx overflow 'D' is happened when host cannot consume packet
fast enough.
... and that to a degree that the network stack, not UHD, decides to
drop packets (not to "O"verflow). That is a very bad sign.
I observed incoming packet from USRP to host using
wireshark tool.
Which will pose a significant additional load on your CPU, something
that will influence your measurement.
After Rx command be launched (ex ./benchmark -f 2.5G -r
25000000), USRP sends many packet to host very fast.
Of course.
So, I checked cpu utilization at those moments. As a
result, cpu utilization is over 200% (PC has 2.7GHz
quad-cores). I couldn't believe it.
Why? How is that surprising? More than two cores under full load
sounds like a reasonable load here.
However, I found "interrupt coalescing".
Incoming packet occurs interrupt to host and interrupt
coalescing adjust how long/many packets make one interrupt to
PC.
So, I changed these value using ethtool -C eth0 rx-usecs
1000 rx-frames 200.
But, it doesn't any effect for my case.
Well, then these values simply don't help; honestly, your PC is
overwhelmed, and that not only by the interrupt load.
I'm using tg3 network driver and my setting like below:
Supported ports: [ TP ]
Supported link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Supported pause frame use: No
Supports auto-negotiation: Yes
Advertised link modes: 10baseT/Half 10baseT/Full
100baseT/Half 100baseT/Full
1000baseT/Half 1000baseT/Full
Advertised pause frame use: Symmetric
Advertised auto-negotiation: Yes
Link partner advertised link modes: 1000baseT/Half
1000baseT/Full
Link partner advertised pause frame use: Transmit-only
Link partner advertised auto-negotiation: Yes
Speed: 1000Mb/s
Duplex: Full
Port: Twisted Pair
PHYAD: 1
Transceiver: internal
Auto-negotiation: on
MDI-X: on
Supports Wake-on: g
Wake-on: g
Current message level: 0x000000ff (255)
drv probe link timer ifdown ifup rx_err
tx_err
Link detected: yes
Did I miss something ?
How can I solve Rx overflow problem at high sample rate ?
Does UHD's
benchmark_rate --rx_rate 25e6
work? If that's the case, the receiver flowgraph is simply too
difficult for your PC to handle in real time. Nothing you can really
do about that but get a faster PC, or design a less complex
receiver.
Best regards,
Marcus
|